Elizabeth A Tibbetts
email: tibbetts@umich.edu

Juanita Pardo-Sanchez

Julliana Ramirez-Matias

Aurore Avarguès-Weber
email: aurore.avargues-weber@univ-tlse3.fr

Individual recognition is associated with holistic face processing in Polistes paper wasps in a species-specific way

Keywords: Behaviour Subject Areas: behaviour, cognition configural processing, visual cognition, insect cognition, Polistes dominula, Polistes fuscatus

Most recognition is based on identifying features, but specialization for face recognition in primates relies on a different mechanism, termed 'holistic processing' where facial features are bound together into a gestalt which is more than the sum of its parts. Here, we test whether individual face recognition in paper wasps also involved holistic processing using a modification of the classic part-whole test in two related paper wasp species: Polistes fuscatus, which use facial patterns to individually identify conspecifics, and Polistes dominula, which lacks individual recognition. We show that P. fuscatus use holistic processing to discriminate between P. fuscatus face images but not P. dominula face images. By contrast, P. dominula do not rely on holistic processing to discriminate between conspecific or heterospecific face images. Therefore, P. fuscatus wasps have evolved holistic face processing, but this ability is highly specific and shaped by species-specific and stimuli-specific selective pressures. Convergence toward holistic face processing in distant taxa ( primates, wasps) as well as divergence among closely related taxa with different recognition behaviour (P. dominula, P. fuscatus) suggests that holistic processing may be a universal adaptive strategy to facilitate expertize in face recognition.

Introduction

Primates are particularly efficient at learning and remembering the unique faces of conspecifics [START_REF] Maurer | The many faces of configural processing[END_REF][START_REF] Parr | The evolution of face processing in primates[END_REF]. For example, humans can learn an individual face in seconds, remember the face for years and recognize it again even if the face is shown from a new perspective. The remarkable capacity for face recognition has been attributed to configural processing of faces [START_REF] Maurer | The many faces of configural processing[END_REF][START_REF] Tanaka | Parts and wholes in face recognition[END_REF][START_REF] Richler | A meta-analysis and review of holistic face processing[END_REF]. During configural processing, both the features and the spatial relationships among features influence recognition. For example, face recognition involves recognizing components of the face (e.g. big nose, brown eyes) and relationships among these components (e.g. wide-set eyes). There are three levels of configural face processing [START_REF] Maurer | The many faces of configural processing[END_REF]. The first type is sensitivity to first-order relationships that define a face (i.e. two eyes above a nose and mouth). The second type is termed 'holistic processing' and implies that the features are bound together into a gestalt which is more than the sum of its parts. The third type is sensitivity to second-order relationships, which means that distances between facial features are perceived and used for discrimination.

Humans primarily rely on the first level of configural processing for categorizing visual objects, but use all three types of configural processing to identify human faces [START_REF] Maurer | The many faces of configural processing[END_REF][START_REF] Tanaka | Parts and wholes in face recognition[END_REF]. Incorporating the relationships among features into the visual representation of faces is thought to facilitate accurate face recognition [START_REF] Richler | Holistic processing predicts face recognition[END_REF]. Configural processing also allows humans to consistently identify individuals despite changes in viewpoint (e.g. front view versus rotated face) [START_REF] Mckone | Configural processing and face viewpoint[END_REF]. In particular, holistic processing increases sensitivity to subtle variations so humans can detect small differences in second-order relations between facial features (e.g. distance between eyes) [START_REF] Richler | A meta-analysis and review of holistic face processing[END_REF]. There is much debate about how frequently holistic processing is used to recognize non-face objects [START_REF] Gauthier | The development of face expertise[END_REF][START_REF] Kanwisher | Functional specificity in the human brain: a window into the functional architecture of the mind[END_REF][START_REF] Mckone | Can generic expertise explain special processing for faces?[END_REF][START_REF] Kanwisher | Domain specificity in face perception[END_REF]. While most non-face objects are not processed holistically, holistic processing may be used to help recognition of similar objects such as birds, dogs or cars [START_REF] Kanwisher | Domain specificity in face perception[END_REF]. Importantly, the viewer needs extensive experience identifying fine variation in the objects for holistic processing to occur [START_REF] Gauthier | Expertise for cars and birds recruits brain areas involved in face recognition[END_REF]. Thus, the mechanisms originally evolved for face recognition may be mobilized to identify specific, non-face objects. Interestingly, neuronal networks thought to be specialized for face processing are also activated during recognition of non-face objects when the viewer has extensive experience with those objects [START_REF] Gauthier | Expertise for cars and birds recruits brain areas involved in face recognition[END_REF].

Holistic processing is typically tested by assessing recognition of experimentally manipulated stimuli. For example, human face recognition is dramatically reduced when part of the face is presented in isolation rather than in the context of the whole face ( part-whole effect). Humans easily discriminate between two noses when the noses are presented in a whole face, but have difficulty discriminating between the same noses when the noses are presented in isolation [START_REF] Tanaka | Parts and wholes in face recognition[END_REF]. Similarly, holistic processing disrupts feature extraction, so it is more difficult to recognize that facial features are identical when the features are presented in a context of a different face (composite-face effect) [START_REF] Rossion | The 'Parts and Wholes' of face recognition: a review of the literature[END_REF]. Many primates were shown to rely on holistic processing to identify faces, as they are sensitive to both the composite and the partwhole effect specifically when discriminating faces [START_REF] Parr | The evolution of face processing in primates[END_REF][START_REF] Dahl | Humans and macaques employ similar faceprocessing strategies[END_REF][START_REF] Burke | The evolution of holistic processing of faces[END_REF].

Although diverse species recognize conspecifics using facial features, little previous work has tested whether nonprimates use holistic processing during face recognition. Conspecific face recognition has been found in primates, sheep, dogs, fishes and paper wasps [START_REF] Parr | The evolution of face processing in primates[END_REF][START_REF] Wang | Individual recognition and the 'face inversion effect' in medaka fish (Oryzias latipes)[END_REF][START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF][START_REF] Kendrick | Sheep don't forget a face[END_REF][START_REF] Racca | Discrimination of human and dog faces and inversion responses in domestic dogs (Canis familiaris)[END_REF], but there is currently no evidence that holistic processing is used for conspecific face recognition in these taxa. Non-primates may use holistic processing to recognize heterospecific human faces with extensive experience. Dogs are sensitive to the partwhole effect with human faces [START_REF] Racca | Discrimination of human and dog faces and inversion responses in domestic dogs (Canis familiaris)[END_REF] as are honeybees [START_REF] Avarguès-Weber | The forest or the trees: preference for global over local image processing is reversed by prior experience in honeybees[END_REF][START_REF] Avarguès-Weber | Does holistic processing require a large brain? Insights from honeybees and wasps in fine visual recognition tasks[END_REF][START_REF] Avarguès-Weber | Configural processing enables discrimination and categorization of face-like stimuli in honeybees[END_REF] and Vespula vulgaris wasps [START_REF] Avarguès-Weber | Does holistic processing require a large brain? Insights from honeybees and wasps in fine visual recognition tasks[END_REF][START_REF] Avarguès-Weber | Recognition of human face images by the free flying wasp Vespula vulgaris[END_REF]. The research in honeybees and Vespula wasps is notable because it suggests that even small-brained insects can use sophisticated configural processing with sufficient experience. However, holistic processing in honeybees and wasps is not linked to individual recognition but rather to visual object expertize. Honeybees and Vespula wasps also do not naturally recognize human faces, so these species provide limited information about how face recognition strategies evolve.

Polistes paper wasps are ideal models for studying the evolution of configural face processing because some Polistes species use individual face recognition, while other species do not, despite similar visual abilities. Polistes fuscatus wasps recognize individual conspecifics based on facial pattern variation [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF]. During individual recognition, P. fuscatus learn the unique facial features of conspecifics, associate the facial features with specific information about their rival, then recall both the face and the rival-related information during subsequent interactions. Previous work indicates that P. fuscatus may process conspecific faces differently than other visual stimuli, as P. fuscatus learn conspecific faces faster and more accurately than other images. Even minor alteration of face images (e.g. rearranging features or removing antennae) reduces face learning [START_REF] Sheehan | Specialized face learning is associated with individual recognition in paper wasps[END_REF], which already suggested the involvement of configural processing. By contrast, other Polistes species, including Polistes dominula, do not learn and remember unique conspecific faces during social interactions [START_REF] Sheehan | Selection for individual recognition and the evolution of polymorphic identity signals in Polistes paper wasps[END_REF]. Instead, P. dominula have facial patterns that signal fighting ability [START_REF] Tibbetts | A socially enforced signal of quality in a paper wasp[END_REF][START_REF] Tibbetts | The Function, Development, and Q2 Evolutionary Stability of Conventional Signals of Fighting Ability[END_REF]. Strong wasps have different facial patterns than weak wasps. Although P. dominula are attuneded to conspecific faces, they assess rival fighting ability without learning and remembering specific conspecific facial patterns [START_REF] Tibbetts | The Function, Development, and Q2 Evolutionary Stability of Conventional Signals of Fighting Ability[END_REF]. Previous work also suggests P. dominula do not use specialized mechanisms for learning conspecific faces [START_REF] Tibbetts | The development and evolution of specialized face learning in paper wasps[END_REF], (but see [START_REF] Sheehan | Specialized visual learning of facial signals of quality in the paper wasp, Polistes dominula[END_REF]). The natural variation in face learning and face specialization makes paper wasps a perfect system to understand the evolution of configural face processing independently of the primate brain evolution.

Here, we test holistic face processing for conspecific and heterospecific face images in two Polistes species: Polistes fuscatus, which have individual face recognition, and Polistes dominula, which do not have individual face recognition. If holistic face processing is an adaptation to facilitate individual face recognition, we predict P. fuscatus will use holistic face processing for conspecific face images, while P. dominula will not. We also test the specificity of holistic face processing by assessing holistic processing for heterospecific face images. We predict that holistic face processing is highly specific, so neither species will use holistic processing for heterospecific face images.

We use an adaptation of the classical part-whole method that accounts for the wasp's visual acuity to test configural processing. The part-whole method provides evidence of holistic face processing when features (e.g. eyes, nose) are recognized more accurately in the context of a face than in isolation [START_REF] Maurer | The many faces of configural processing[END_REF][START_REF] Rossion | The 'Parts and Wholes' of face recognition: a review of the literature[END_REF]. We trained wasps to discriminate between wasp face images that were experimentally altered so the outer part of the face (antennae, background, legs) were identical and the inner part of the face (clypeus, frons, inner eye) had natural levels of variation (figure 1). If wasps use featural mechanisms for face recognition, they will be equally adept at identifying the whole face and central face because all the featural differences between the two face images occur in the central part of the face. If wasps use holistic mechanisms for face recognition, they will identify the whole face more accurately than the central face. [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF][START_REF] Injaian | Cognition across castes: individual recognition in worker Polistes fuscatus wasps[END_REF]. Wasps use individual recognition to minimize conflict in groups of cooperating foundresses [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF][START_REF] Sheehan | Evolution of identity signals: Frequency-dependent benefits of distinctive phenotypes used for individual recognition[END_REF] and to assess potential rivals via social eavesdropping [START_REF] Tibbetts | Wasps use social eavesdropping to learn about individual rivals[END_REF]. P. fuscatus often nest in large cooperative groups where each foundress has a unique social role [START_REF] Jandt | Polistes paper wasps: a model genus for the study of social dominance hierarchies[END_REF]. Individual recognition allows foundresses to keep track of individual social relationships, including division of aggression, food and work on nests [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF]. Outside nests, individual recognition is used to assess rivals via social eavesdropping and memory of past interactions [START_REF] Tibbetts | Wasps use social eavesdropping to learn about individual rivals[END_REF].

Polistes dominula also have visual signals, but these signals convey information about fighting ability rather than individual identity. Strong P. dominula have more broken black facial patterns than weaker P. dominula [START_REF] Tibbetts | A socially enforced signal of quality in a paper wasp[END_REF]. Signals of fighting ability are used to minimize the costs of conflict when animals interact with unfamiliar rivals [START_REF] Tibbetts | The Function, Development, and Q2 Evolutionary Stability of Conventional Signals of Fighting Ability[END_REF]. Signals of fighting ability are quite different from signals of individual identity. First, the facial patterns that signal fighting ability have insufficient variation for individual recognition [START_REF] Sheehan | Selection for individual recognition and the evolution of polymorphic identity signals in Polistes paper wasps[END_REF]. Second, signals of individual identity require wasps to learn and remember unique faces, while signals of fighting ability do not require learning and memory. For example, during individual recognition, wasps learn the unique facial features of conspecifics, associate the facial features with specific information about their rival, then recall both the face and the rival-related information during subsequent interactions. Effective signals of fighting ability only require wasps to assess whether the signal of an unknown rival indicates that the rival is strong or weak, not to learn or remember the faces of social partners [START_REF] Tibbetts | The Function, Development, and Q2 Evolutionary Stability of Conventional Signals of Fighting Ability[END_REF]. As a result, having a signal of fighting ability is unlikely to influence their ability to learn and remember faces, as quality signalling does not involve face learning and previous experiments have shown that P. dominula are not capable of individual face recognition in any sensory modality [START_REF] Sheehan | Selection for individual recognition and the evolution of polymorphic identity signals in Polistes paper wasps[END_REF].

(b) Collection

P. fuscatus and P. dominula wasps used in the experiment were collected from areas surrounding Ann Arbor, MI. After collection, wasps and their nests were housed in individual containers under a natural day/night cycle and fed ad lib with sugar and caterpillars. All experiments were performed with nest-founding queens prior to worker emergence.

(c) Stimuli

Wasps from both species were trained to discriminate between pairs of P. fuscatus or P. dominula face images species and then tested for their recognition abilities both with the whole faces or with the inner part only (figure 1). The particular face was chosen as the correct stimulus was swapped across wasps. Face images were photographs of wasps from Michigan, USA. Face images were altered in Adobe Photoshop so all the differences between the two face images occurred in the central inner part of the face. The outer part of the faces (antennae, background, legs) was identical between both faces, while the inner part of the face (colours on the clypeus, frons, inner eye) had natural levels of variation. All images were printed at life size (face approx. 3.5 mm wide) using a commercially available Sony Picture Station photo printer that uses ink cartridges.

(d) Training and testing procedure

Wasps were initially individually trained to discriminate a pair of whole face images (figure 1) using the same setup used in [START_REF] Tibbetts | The development and evolution of specialized face learning in paper wasps[END_REF][START_REF] Tibbetts | Social isolation prevents the development of individual face recognition in paper wasps[END_REF] (electronic supplementary material, figure S1). During training, wasps were placed in a 2.5 wide × 4 long × 0.7 height cm wood and plexiglass box with six identical stimuli glued onto the inside walls. Two longer sides had two face pictures each, while two shorter sides had one face picture each. In half of the bouts, the wasp was placed in the box with only incorrect face stimuli (CS+) and received a mild electric shock from an electrified pad for 2 min. The entire floor was electrified. The electrified pad was made of anti-static conductive foam electrified by two copper wires connected to a Variac transformer, which provided continuous 0.4 volt AC current. The boxes were shallow so that wasps could not escape the shock by flying or climbing the walls. The mild electric shock is aversive but not harmful to the wasp. In the other half of the bouts, the wasp was placed in a box with only the correct face stimuli (CS-) for 2 min while the pad was not electrified. Between each bout, the wasp was given a 1 min break in a holding container. This sequence of one CS+ and one CS-bouts was repeated three times per wasp, so wasps experienced three CS+ and three CS-bouts in total. After training, the wasp was given a 45 min break in a holding container with access to sugar and water.

Each wasp was then tested twice: once using whole face stimuli and once using inner face stimuli. The testing order was randomized so that half the wasps were initially tested on whole faces and half were initially tested on inner faces. Whether a stimulus was tested first or second did not influence performance (see results). Learning performance was assessed by measuring whether the wasp approached the correct (CS-) or incorrect (CS+) image over 10 trials separated by 1 min breaks. After the first bout of 10 testing trials, each wasp was given a 45 min break, then tested for 10 trials on the second set of stimuli. Testing occurred in a 3 × 10 × 0.7 cm rectangular box. One end of the rectangle displayed the correct stimulus (CS-) while the other end of the rectangle displayed the incorrect stimulus (CS+). The entire floor of the rectangle was electrified except the 2.25 cm zone closest to the correct stimulus, the 'safety zone'. The correct choice was associated with safety to ensure learned preferences from the initial training were not extinguished during the 10 trials test. Receiving a shock while choosing a preferred stimulus can rapidly extinguish learned preferences.

The centre of the rectangle had two removable, clear partitions that confined the wasp. At the beginning of each trial, the wasp was placed in the centre of the rectangle between the clear partitions, the electric shock was turned on for five seconds, both partitions were removed simultaneously, and the wasp was free to walk through the rectangle. Providing an electric shock ensured wasps' motivation to look for the correct, 'safe', stimulus. Wasps who learned to discriminate the faces during the training phase typically turn toward the correct stimulus while confined in the centre and quickly walk toward it as soon as the partitions were removed. A choice was scored when the wasp's head entered one of the 2.5 cm zone closest to each stimulus, at each end of the rectangle. The stimuli subtended a visual angle of approximately 30°at the 2.5 cm distance. Wasps are scored as making a choice before they reach the non-shocking safety zone to ensure wasps made choices based on learned stimuli preferences rather than directly assessing the presence or absence of shock. Wasps made quick choices (typically less than 5 s). After a wasp made a choice, it was removed from the testing arena and given a 1 min break in a holding container. The placement of the correct (CS-) and incorrect (CS+) stimuli (right or left side) was determined randomly and changed between trials. This ensures that wasps did not associate a particular direction with correct choices.

(e) Statistical analysis

Twenty-three P. dominula were trained on P. dominula faces. Twenty-three P. dominula were trained on P. fuscatus faces.

royalsocietypublishing.org/journal/rspb Proc. R. Soc. B 20203010 Twenty-one P. fuscatus were trained on P. dominula faces. Fifteen P. fuscatus were trained on P. fuscatus faces. Each wasp was tested on both whole and inner face stimuli.

The data were analysed using generalized mixed linear models (GLMM) with a binomial error structure-logit-link function (glmer function of R package lme4) [START_REF] Bates | Fitting Q3 linear mixed-effects models using lme4[END_REF]. The dependent variable was the number of correct choices out of 10. The independent variables (fixed factors) were Species trained (categorical: P. fuscatus or P. dominula), stimuli species (categorical: P. fuscatus or P. dominula), whole versus inner face (called test, categorical: whole or inner) and order (continuous: first or second stimuli tested). Wasp ID was included as a random factor to account for the repeated-measure design as each wasp was tested on both whole and inner faces. Several models were run by testing interactions between factors and by dropping each factor subsequently to select the significant model with the highest explanatory power (i.e. the lowest AIC value) (See statistical tables in electronic supplementary material).

Tukey post hoc tests using the ghlt function (multicomp R package) [START_REF] Hothorn | Simultaneous inference in general parametric models[END_REF] were run on the best models after splitting the data between species to compare tests performance between conditions. A Wilcoxon test was used to compare directly the performance of both species for the P. fuscatus faces.

We finally compared performance in each test to the 50 : 50 random expectation using a binomial test. The binomial test provides an exact test of whether the number of correct versus incorrect choices differs from the 50 : 50 random expectation.

All statistical analyses were performed with R 3.4.2 (R Development Core Team, 2016).

Results

There are significant differences in how P. fuscatus and P. dominula process faces (figures 2 and 3). Face recognition was influenced by the three-way interaction between species trained, stimulus species and whole versus inner face (χ 2 = 14.35, p = 0.006; electronic supplementary material, table S1). However, discrimination performance was not influenced by the order of the tests (χ 2 = 0.04, p = 0.85; electronic supplementary material, table S1). We further analysed the data by examining the performance of each species separately.

P. fuscatus process conspecific faces holistically but do not use holistic processing for heterospecific faces (figure 2). Indeed, there was a significant interaction between the stimuli species and the whole or inner face stimuli tests (χ 2 = 7.94, p = 0.005; electronic supplementary material, table S2). The significant interaction occurs because P. fuscatus recognize whole P. fuscatus faces more accurately than they recognize inner P. fuscatus faces (figure 2, Z = 4.05, p < 0.001). By contrast, P. fuscatus recognize whole and inner P. dominula faces with similar accuracy (figure 2, Z = 0.50, p = 0.96). Indeed, P. fuscatus succeeded in discriminating whole P. fuscatus faces (0.69 ± 0.03% of correct choices (mean ± s.e.m.); p = 0.006) but not partial P. fuscatus faces (0.46 ± 0.02%; p = 0.36). P. fuscatus also discriminated both whole and inner P. dominula faces better than expected by random chance (whole: 0.61 ± 0.03%, p = 0.01; inner: 0.59 ± 0.03%, p = 0.01).

P. dominula do not process either conspecific or heterospecific faces holistically and face processing consequently did not differ across stimuli. Performance was not influenced by the interaction between stimulus species and whole versus inner face (figure 3, χ 2 = 0.06, p = 0.81; electronic supplementary material, table S3), by the stimuli species (χ 2 = 0.005, p = 0.95), the type of test (whole versus inner face, χ 2 = 0.11, p = 0.74) or the order of the tests (χ 2 = 0.13, p = 0.72). The lack of significant interaction illustrates that P. dominula use the same processing mechanisms for learning conspecific and heterospecific faces. P. dominula discriminated all types of stimuli: P. dominula faces (whole: 0.59 ± 0.03%, p = 0.01, inner: 0.60 ± 0.03%, p = 0.002) and P. fuscatus faces (whole: 0.60 ± 0.02%, p = 0.006, inner: 0.60 ± 0.02%, p = 0.004).

Finally, P. fuscatus learned to discriminate whole conspecific faces more accurately than P. dominula discriminate the same stimuli (W = 250, p = 0.02), consistent with holistic face processing providing a learning advantage. P. fuscatus wasps use holistic processing to identify conspecific, but not heterospecific faces. P. fuscatus easily differentiated whole conspecific face images. However, P. fuscatus were unable to differentiate faces when only the inner part of the face was available. The inability to discriminate inner face images was somewhat surprising because the images were experimentally manipulated to ensure the inner face images contained the same amount of information as whole face images because the outer part of the face was identical across stimuli. Further, there is nothing inherently difficult about recognizing inner face images, as P. fuscatus recognized whole and inner heterospecific faces with similar accuracy. Therefore, results of part-whole face discrimination tests suggest that P. fuscatus wasps use holistic processing to identify conspecific but not heterospecific faces. P. fuscatus encode conspecific facial features as integral parts of the face rather than as separate features. As a result, wasps do not identify conspecific facial features outside of the context of a face.

Although many taxa may use first-order configural information during object recognition [START_REF] Parr | The evolution of face processing in primates[END_REF][START_REF] Avarguès-Weber | Configural processing enables discrimination and categorization of face-like stimuli in honeybees[END_REF][START_REF] Peirce | Configurational coding, familiarity and the right hemisphere advantage for face recognition in sheep[END_REF], this study provides the first evidence to our knowledge of holistic processing of conspecific faces in a non-primate species. Holistic face processing is key to fast, accurate individual face recognition in humans [START_REF] Maurer | The many faces of configural processing[END_REF][START_REF] Richler | Holistic processing predicts face recognition[END_REF] and likely serves a similar function in paper wasps. Individual face recognition is indeed an important aspect of the social life of P. fuscatus, as wasps learn and remember the unique facial patterns of many individuals on and off nests [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF][START_REF] Tibbetts | Wasps use social eavesdropping to learn about individual rivals[END_REF]. The species specificity of holistic face processing in P. fuscatus matches previous work in primates [START_REF] Parr | The evolution of face processing in primates[END_REF]. Like wasps, humans naturally use holistic face processing to identify conspecific faces, but do not naturally use holistic processing for heterospecific faces [START_REF] Dufour | Face processing limitation to own species in primates: a comparative study in brown capuchins, Tonkean macaques and humans[END_REF].

Our results also demonstrate that P. dominula, a related species of P. fuscatus, do not use holistic processing to identify conspecific or heterospecific faces (figure 3). Instead, P. dominula use featural processing to identify all face images. The lack of holistic processing in P. dominula is consistent with the hypothesis that holistic processing is an adaptation to facilitate individual face discrimination. P. dominula provide a particularly interesting comparison to P. fuscatus because they are closely related and both species use facial patterns for social interactions. Nevertheless, the species differ in holistic processing for conspecific faces, likely because P. fuscatus learn and remember the unique faces of many conspecifics, while P. dominula do not. Instead, facial patterns in P. dominula are status badges that signal fighting ability. Strong wasps have different facial patterns than weak wasps [START_REF] Tibbetts | A socially enforced signal of quality in a paper wasp[END_REF][START_REF] Tibbetts | The Function, Development, and Q2 Evolutionary Stability of Conventional Signals of Fighting Ability[END_REF]. Wasps gain information about fighting ability without learning and remembering specific conspecific facial patterns. Consequently, holistic face processing likely provides no evolutionary benefit to P. dominula wasps, in contrast with P. fuscatus.

Importantly, the difference in performance between species and face stimuli probably do not arise from the characteristics of the face stimuli. Both P. fuscatus and P. dominula faces have general characteristics that are thought to facilitate configural processing. Previous work suggests that features are more likely to be processed using configural mechanisms when features are predictably arranged with variation in specific areas [START_REF] Avarguès-Weber | Configural processing enables discrimination and categorization of face-like stimuli in honeybees[END_REF]. For example, human faces have a predictable arrangement (eyes above nose above mouth). Face variation occurs in the size and shape of features as well as their spacing. In P. dominula, wasps vary in the size and shape of black clypeus spots [START_REF] Tibbetts | A socially enforced signal of quality in a paper wasp[END_REF]. In P. fuscatus, faces vary in the brown, black, and yellow coloration in four facial areas (clypeus, frons, inner eye and outer eye) [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF]. The predictable variation facilitates categorization and visual search because it is straightforward for receivers to focus on the most informative visual features. As a result, stimuli that have predictable configurations may favour the evolution of specialized mechanisms to extract relevant information [START_REF] Yang | Visual search is guided to categorically defined targets[END_REF].

This study thus adds to previous work in wasps suggesting that individual recognition is linked to specific face processing mechanisms. For example, P. fuscatus learn normal conspecific faces faster and more accurately than altered faces ( jumbled facial features, antennae digitally removed), other biologically relevant visual objects such as prey items or simple geometrical forms [START_REF] Sheehan | Specialized face learning is associated with individual recognition in paper wasps[END_REF]. Less work has examined face processing in P. dominula, though P. dominula seem also to have difficulty in learning face stimuli when the antennae are removed [START_REF] Sheehan | Specialized visual learning of facial signals of quality in the paper wasp, Polistes dominula[END_REF]. The current study used controlled stimuli with identical antennae and body position, finding no evidence of specialized face processing in P. dominula.

Paper wasps also allow us to study the role of experience in the development of specialized face processing. Testing the role of experience in face specialization is difficult in primates [START_REF] Gauthier | The development of face expertise[END_REF][START_REF] Hoehl | The early development of face processing -What makes faces special?[END_REF][START_REF] Sugita | Face perception in monkeys reared with no exposure to faces[END_REF]. Previous studies have shown that P. fuscatus wasps raised in isolation without social contact lost their facility for individual face recognition [START_REF] Tibbetts | Social isolation prevents the development of individual face recognition in paper wasps[END_REF]. Additionally, raising P. fuscatus wasps with P. dominula prevented the development of specialized conspecific face processing [START_REF] Tibbetts | The development and evolution of specialized face learning in paper wasps[END_REF]. In parallel, P. dominula individuals raised with P. fuscatus did not show specialized processing for P. fuscatus faces but did improve P. fuscatus face learning performance [START_REF] Tibbetts | The development and evolution of specialized face learning in paper wasps[END_REF]. Therefore, face specialization is influenced by both experience and innate, species-specific differences.

In conclusion, this study illustrates that species-specific evolutionary adaptation plays a key role in the development of holistic processing. Selection for efficient individual recognition has favoured the adaptive evolution of holistic face processing in P. fuscatus. Our results also confirm that holistic processing did not emerge with the development of large, sophisticated brain in primates. Instead, holistic face processing provides a remarkable example of convergent evolution between wasps and mammals. Although mammals and wasps have different sensory and neural structures, holistic processing has arisen independently in both groups suggesting the efficiency of such mechanism for accurately learning a large number of similar visual objects. Future work should explore whether face-processing specialization in wasps relies on the development of specific neuronal networks as found in primates.

Ethics. Our protocols comply with standard welfare practice in our field.

Figure 1 .

 1 Figure 1. Images of Polistes dominula (a,b) and Polistes fuscatus (c,d ) used for training and testing. All variation is in the inner part of the faces. The outer parts (legs, body position, antennae, background) are identical between the pair of faces from each species. (Online version in colour.) royalsocietypublishing.org/journal/rspb Proc. R. Soc. B 20203010

Figure 2 .Figure 3 .

 23 Figure 2. P. fuscatus face discrimination when wasps are tested on P. dominula and P. fuscatus whole and inner face stimuli after being trained with the whole faces. Box: 1Q, mean, 3Q. Whiskers: min/max. Dotted line represents the 50 : 50 random choice expectation. Stars reflect significant difference from random choice *p < 0.05, **p < 0.01.

rspb20203010-18/12/20-23:21-Copy Edited by: Not Mentioned

Funding. The project was supported by the Fyssen foundation and by the National Science Foundation under grant number IOS-1557564.

Data accessibility. The datasets supporting this article have been uploaded as part of the electronic supplementary material.

Authors' contributions.


V Rat

M Bienia

G D Dhamale

F Mavier

C Ruelle

S Goutier

Relationships between arc plasma jet properties and plasma/liquid interaction mechanisms for the deposition of nanostructured ceramic coatings

Keywords: thermal plasma, plasma torches, arc instabilities, liquid injection, ceramic coatings growth

Relationships between arc plasma jet properties and plasma/liquid interaction mechanisms for the deposition of nanostructured ceramic coatings

Introduction

The development of plasma spraying of liquid feedstocks has started at the end of the 90's with the first papers describing the formation of nanostructured coatings or the synthesis of nanosized particles in thermal plasma jets [START_REF] Bouyer | Parametric study of suspension plasma sprayed hydroxyapatite[END_REF][START_REF] Gitzhofer | Suspension plasm spray deposition[END_REF][START_REF] Karthikeyan | Plasma spray synthesis of nanomaterial powders and deposits[END_REF]. The improvement of properties of plasma sprayed coatings imparting new features at nanoscale underlay the motivations of such an original research. Thereafter, two main trends have emerged depending on the nature of liquid feedstocks injected in the plasma. On the one hand, Suspension Plasma Spraying (SPS) consists in forming a suspension of submicron particles dispersed within a solvent. On the other hand, Solution Precursor Plasma Spraying (SPPS) offers the possibility to inject a precursor solution which the metallic element aimed at being sprayed is dissolved as a salt in a solvent. These deposition methods have turned out to be flexible enough to consider the association of the different elements of a solid oxide fuel cell in a single step process with the control of the chemical composition and the coating architecture [START_REF] Fauchais | Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs[END_REF][START_REF] Kassner | Application of Suspension Plasma Spraying (SPS) for Manufacture of Ceramic Coatings[END_REF]. The use of SPS and SPPS for manufacturing Thermal Barrier Coatings have been also extensively studied and reviewed especially for the columnar microstructures of coatings, the control of the porosity content and size, and segmentation cracks [START_REF] Jordan | Superior thermal barrier coatings using solution precursor plasma spray[END_REF][START_REF] Vanevery | Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties[END_REF][START_REF] Seshadri | Characterizing Suspension Plasma Spray Coating Formation Dynamics through Curvature Measurements[END_REF][START_REF] Bernard | Columnar suspension plasma sprayed coating microstructural control for thermal barrier coating application[END_REF][START_REF] Jordan | The Solution Precursor Plasma Spray (SPPS) Process: A Review with Energy Considerations[END_REF]. Moreover, the typical coating thicknesses can be varied between a few tens and a few hundreds of micrometers bridging the gap between the thin films and the coatings constructed by conventional plasma spraying.

The understanding and the control of coating properties are obviously deeply related to the complex and intricate phenomena governing the liquid precursor treatment by the plasma jet from the early stages of the liquid injection to the impact of sprayed materials onto the substrate. They encompass the injection of liquid as a droplets or liquid jet, the liquid fragmentation and its evaporation, the solid particles formation (or in-fight agglomeration) competing with their possible evaporation during their transport in the plasma jet until the coating growth which can be affected by gas phase combustion reactions resulting from the solvent nature especially in the case of SPPS [START_REF] Fauchais | Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs[END_REF][START_REF] Jordan | The Solution Precursor Plasma Spray (SPPS) Process: A Review with Energy Considerations[END_REF][START_REF] Fazilleau | Phenomena Involved in Suspension Plasma Spraying Part 1: Suspension Injection and Behavior[END_REF]. Even though the liquid has a basic function of a carrying medium for particles in SPS giving them enough momentum to penetrate the plasma jet, the liquid properties not only affect profoundly the entire history of solid materials in their interaction with the plasma but also the coating growth. Consequently, one of the major challenges is to control the plasma properties (enthalpy, speed, flow regime of the plasma jet) and therefore the dynamics of the arc which in turn strongly interacts with the plasma torch design.

This paper gives an overview of the main challenges and current issues encountered in plasma spraying of liquid feedstocks by focusing on the inter-dependent key parts in the deposition process. Section 1 presents the typical arc plasma torches currently used in SPS and SPPS.

Classification of torches is made according to their specific enthalpy but also to their stability.

The properties of the liquid injected as ceramic ink in the plasma are described and the in-flight phenomena are described in section 2. Before concluding, the last section presents the typical features of as-sprayed coatings and discuss the key parameters to form nanostructures.

Current non-transferred plasma torches 2.1 Energetic features

A plasma torch is a conversion tool that permits to change the electrical energy into thermal energy in order to melt and spray materials. The efficiency of this conversion is deeply dependent upon the torch design (geometry of the electrodes, relative position of cathode/anode) and the experimental parameters such as the arc current, the chemical composition of the gas flow rates and their mass flow rates. As depicted in Figure 1, the Conventional non-transferred Torch (CT) are equipped with a thoriated (or lanthanum doped) rod-cathode and a water-cooled anode nozzle that form an inter-electrode space where the arc is confined and burns. The electrical power 𝑃 𝑒 is measured as 𝑃 𝑒 = 𝑉 ̅ 𝐼 where 𝑉 ̅ and I are respectively the mean arc voltage and the arc current.

The specific enthalpy available to treat materials results from an energy balance that obeys to the following equation at steady state:

𝜌𝑢 𝜕ℎ 𝜕𝑧 = 𝜎𝐸 2 -𝛁. 𝒒 -𝑞 𝑟 (1)
where , u and h are respectively the plasma density, the axial plasma speed and the specific enthalpy.

The first term on the right-hand side is the Joule heating where  and E are respectively the electrical conductivity and the electric field. The second and third terms on the right-hand side are the power densities lost by the arc respectively due to the thermal conduction where q is the heat flux, and due to radiation. processes so that the overall heat flux ranges between 10 6 to 10 8 W.m -2 . Actually, the heat losses due the arc attachment only, i.e. corresponding to the heat flux of electrical origin, can reach 10 9 -10 10 W.m -2 due to highly localized arc root. This intensified heat flux at the arc root is responsible for the anode erosion whose the characteristic time 𝜏 𝑚 of melting can be evaluated under the simplified assumption of a semi-infinite anode wall exposed to a heat flux 𝜑 𝑒𝑙𝑒𝑐 , i.e.

𝜏 𝑚 = 𝜋(𝜅 𝑎 (𝑇 𝑚 -𝑇 𝑤 )) 2 𝛼(2𝜑 𝑒𝑙𝑒𝑐 ) 2 ⁄
where a, , Tm are respectively the thermal conductivity, the thermal diffusivity and the melting temperature of the anode, and Tw is the temperature of the anode wall [START_REF] Coudert | From transferred arc to plasma torches[END_REF]. In the case of a typical copper anode, the time 𝜏 𝑚 is roughly 50 µs for a heat flux of 5x10 9 W.m -2 . It means that the arc root cannot be fixed longer than 𝜏 𝑚 otherwise a rapid destruction of the anode wall can be expected. Consequently, despite the harmful influence of the arc instabilities on the plasma jet at nozzle exit, they do allow the anode wall to sustain the heat load caused by the arc attachment. The thermal efficiency of the plasma torch, i.e. its capability in converting the electrical energy into thermal energy, is given by 𝜂 = where ℎ ̅ (𝑟) is the time-averaged radial enthalpy profile and 𝑑 the inner nozzle diameter. The enthalpy is linked to temperature thanks to thermodynamic data that permits to evaluate the mean plasma temperature at nozzle exit under the LTE assumption.

By considering that the plasma speed increases from the anode wall to the center of the arc whereas the plasma density has a reverse dependence, the density of mass flux is roughly constant so that 𝜌𝑢 ̅ = 4𝑚̇𝜋𝑑 2 ⁄ where 𝑢 ̅ is the mean plasma speed at nozzle exit. This simplified relationship highlights the dependence of the speed on the mass flow rate of plasma forming and the mean temperature through the plasma density. This intricate dependence is also reported in the correlation equation given by Planche et al. [START_REF] Planche | Velocity Measurements for Arc Jets Produced by a DC Plasma Spray Torch[END_REF] where the centerline speed 𝑣 𝑚𝑎𝑥 of speed profile is written as 𝑣 𝑚𝑎𝑥 = 𝐾𝑚̇0 .21 𝐼 0.44 𝑑 1.96 ⁄ where K is correlation constant depending on plasma forming gases composition. Another approach gives 𝑢 ̅ = ℎ ̅ 0 𝑚̇(𝛾 -1) 𝑃 𝑎 𝑆𝛾 ⁄ where 𝛾, S and 𝑃 𝑎 are the isentropic exponent, the nozzle cross section, the pressure at nozzle exit, and establishes a straightforward dependence on the mean specific enthalpy [START_REF] Rat | A simplified analytical model for dc plasma spray torch: influence of gas properties and experimental conditions[END_REF]. Consequently, mean thermal energy and plasma momentum can be in principle described by measuring of the specific enthalpy which can be used as a criterion of torch classification.

Different DC plasma torches families are currently used in plasma spraying of liquid feedstocks, namely the CT torches, but also the Segmented Torches (ST, sometimes named cascade torch)

and the Axial Multi-electrode Torches (AMT). In the ST torches (Figure 1b), a stack of copper rings electrically insulated from each other, called neutrodes or inter-electrode insert, permits to increase the arc length, and hence the arc voltage and electrical power, but also improves the arc stability. The thickness of a copper ring must satisfy the condition ∫ 𝐸𝑑𝑧 < 𝑉 𝑎 + 𝑉 𝑐 𝑒 0 where E, z, 𝑉 𝑎 and 𝑉 𝑐 are respectively the electric field of the arc column, the axial coordinate, the anode and cathode drop voltages, meaning that the arc cannot exist if the column drop voltage is lower than the electrode drop voltages, which can be estimated between 10 and 20 V for typical plasma forming gases in plasma spraying. Zhukov and Zasypkin have reviewed the socalled linear plasma torches dating back to the 80's and permitting to promote a fixed arc length either by means of a ledge in the anode wall or by the use of an inter-electrode insert [START_REF] Zhukov | Thermal plasma torches: design, characteristics, applications[END_REF].

Several commercial versions for research and industrial purposes are used today [START_REF] Fauchais | Thermal Spray Fundamentals[END_REF][START_REF] Schein | Tomographic Investigation of Plasma Jets Produced by Multielectrode Plasma Torches[END_REF][START_REF] Zhukovskii | Predicted Anode Arc Attachment by LTE (Local Thermodynamic Equilibrium) and 2-T (Two-Temperature) Arc Models in a Cascaded-Anode DC Plasma Spray Torch[END_REF][START_REF] Ghorui | Unique Aspects of Thermal Plasma Torches and Reactor Design for Process Applications[END_REF]. The addition of cathodes properly arranged is aimed at mitigating the impact of electrode erosion by reducing the arc current of each cathode while maintaining high electrical power. This torch belongs to the Multi-electrode Torch (MT) family. The axial injection of materials usually permits better material processing which implies the injection in the region of merging plasma jet generated by several CT torches combined within a unique torch body. This torch family refers to Axial Multi-electrode Torch (AMT) in Figure 1c. Note that Figure 1c only depicts two cathodes for legibility reason but three arc plasma jets usually merge together to form the main high-temperature flow where materials are plasma sprayed. Note that Figure 2a shows the current/voltage characteristics of the above-mentioned current plasma torches families and their corresponding mean specific enthalpy ℎ ̅ 0 dependence on the electrical power in Figure 2b. The purpose of Figure 2b is to point out the particular features of different plasma torches families from some reported results. It must be underlined that scarce studies presents complete set of measurements of plasma torches used in plasma spraying, especially for ST and AMT torches.

The labels CT, ST, MT and AMT respectively refers to the torches F4, SinplexPro, TriplexPro-210 (Oerlikon Metco) and Axial III (Mettech) in experiments. Different plasma forming gases, mixtures and flow rates (Ar, H2, N2, He) are used. For ST torch, three arc voltages are obtained for the same current because of changes in mass flow rates in Ar-H2 mixtures which affect the enthalpy through the arc voltage and the total gas mass flow rate. For the AMT, the specific enthalpy was evaluated assuming a thermal efficiency of 60% [START_REF] Zimmermann | Investigation and new physical behaviour of specila DC-Plasma generator Mettech Axial III[END_REF].

The torches CT, ST and MT operate with arc current ranging between 50 and 600 A while the arc voltage usually does not exceed 350 V for typical argon, hydrogen, nitrogen and helium gas mixtures. The arc length fixation for ST and MT torches results in higher voltage and electrical power than for CT torches. The CT torches have a well-known decreasing feature of voltage as function of current for given plasma forming gases flow rate. In this case, two coincident effects are combined to lower the arc voltage despite the increase of the current. On the one hand, the electrical conduction of the arc is improved as the arc current is increased due to an increased arc diameter. On the other hand, the arc length is also reduced because the arc can reattach closer to the cathode at higher arc current. It can be mentioned that, at very high current, the electrical field of the arc column can increase to sustain the electrical discharge to give rise to an increasing feature of the arc voltage [START_REF] Zhukov | Thermal plasma torches: design, characteristics, applications[END_REF]. The ST torches have instead an increasing feature as shown in Figure 2a resulting mainly from the arc length more or less constant and a dominant effect of the increase current on the electric arc conduction. It means that the amplitude of arc voltage fluctuations compared to the mean voltage is weak, usually less than 10%. It is interesting to note that the AMT torch can operated at very high electrical power. Actually, it combines three torches, each working at 230 A [START_REF] Zimmermann | Investigation and new physical behaviour of specila DC-Plasma generator Mettech Axial III[END_REF] . The CT, ST, MT and AMT torches can reach moderate specific enthalpy below 40 MJ.kg -1 due to moderate electrical power but also high mass flow of plasma forming gases that require energy to be ionized. It is must be underlined that the torch classification proposed in Figure 2 does not allow inferring the properties profiles of the plasma jet at nozzle exit. For example, Planche et al. shows that the centerline speed 𝑣 𝑚𝑎𝑥 increases as 𝑚̇ increases but the speed profiles also become narrower [START_REF] Planche | Velocity Measurements for Arc Jets Produced by a DC Plasma Spray Torch[END_REF]. Moreover, it turns out the above plasma torches currently used in plasma spraying applications generate turbulent plasma jet downstream the nozzle exit leading strong air cooling and strong decrease of plasma temperature and velocity [START_REF] Spores | Flow structure of a turbulent thermal plasma jet[END_REF]. The turbulence is described from the formation of ring vortices around the jet close to the nozzle exit bringing about the surrounding air engulfment [START_REF] Spores | Flow structure of a turbulent thermal plasma jet[END_REF]. However, in the case of highly fluctuating torch (CT), the turbulence of the plasma jet originates mainly from the arc motion inside the torch [START_REF] Caruyer | Analysis of the unsteadiness of a plasma jet and the related turbulence[END_REF]. Consequently, an increasing number of studies are also devoted to the development of ST torches operating in laminar mode with lower arc current and much higher arc voltage, up to 350 V (26-29). Solonenko et al. interestingly discuss the advantages of plasma torches working at low Reynolds number [START_REF] Solonenko | Visualization of arc and plasma flow patterns for advanced material processing[END_REF]. They operate at low gas mass flow rate (<1 g/s) and low arc current (<200 A). The adjustment of the length to diameter ratio of the arc

Arc control for liquid injection: pulsed DC torch

The control of the arc motion in DC torch has been the subject of many studies over several decades. It becomes a more challenging task especially today for elaborating coatings with the desired multiple functionalities evidently linked to their complex chemical composition, their nano/microstructure including (nano)porosities, cracks and also their crystallographic structure.

Consequently, the current DC torches (ST, MT, AMT configurations) are innovative plasma systems where not only the variations in plasma properties downstream the nozzle exit are intended to be reduced, either by limiting the arc motion or by merging several arcjets, but also where the problem of the electrode erosion is considered by reducing the arc current or by using appropriate plasma forming gases. The latter affect the arc attachment constriction and therefore the current density through the Cold Boundary Layer (CBL) between the arc column and the anode wall [START_REF] Huang | Arc Root Motion in an Argon-Hydrogen DC Plasma Torch[END_REF][START_REF] Dorier | Time-resolved imaging of anodic arc root behavior during fluctuations of a DC plasma spraying torch[END_REF]. The anode attachment modes (diffuse or constricted) are expected to strongly affect the erosion. They are dependent on arc current and gases but also on the anode design, operating pressure and applied magnetic field [START_REF] Yang | Anode attachment modes and their formation in a high intensity argon arc[END_REF][START_REF] Pan | Comparative Observation of Ar, Ar-H 2 and Ar-N 2 DC Arc Plasma Jets and Their Arc Root Behaviour at Reduced Pressure[END_REF][START_REF] Pan | Sufficiently diffused attachment of nitrogen arc by gasdynamic action[END_REF].

Thus, a stabilized plasma implies a stable and continuous injection of liquid feedstock leading to reproducible plasma/liquid interactions. However, if the arc motion is a major source of instabilities, others sources may originate in the interaction with the column liquid of droplets due to particularly the injection mode (axial or radial), the liquid injection technology or the injection location as shown in the next section. Consequently, a dc pulsed torch was designed to cope with these requirements and was associated with a phased radial liquid injection driven by a piezoelectric dispenser and a time delay generator [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF]. While the arc motion in a DC torch is ruled by the unbalance between the Lorenz and hydrodynamic forces, the arc reattaches through the CBL when the arc voltage 𝑉 𝑎𝑟𝑐 (𝑧, 𝑡) reaches a critical arc voltage 𝑉 𝑏 = 𝑒 𝑧 𝐸 𝑏 depending on both the mean breakdown electric field 𝐸 𝑏 (a few tens of kV.m -1 ) and the thickness of the CBL 𝑒 𝑧 [START_REF] Krowka | Investigation and control of dc arc jet instabilities to obtain a self-sustained pulsed laminar arc jet[END_REF][START_REF] Duan | Arc Instabilities in a Plasma Spray Torch[END_REF]. Shao et al. have recently highlighted in details the dynamics of the restrike mode along with time-resolved temperature measurements [START_REF] Shao | Experimental Study on the Restrike Mode of a DC Arc Anode Attachment[END_REF].

They obtained typical saw-tooth shape signals (restrike signal) for the arc voltage 𝑉 𝑎𝑟𝑐 for which the frequency 𝑓 𝑟 and the amplitude 𝛿 𝑣 are linked together by 𝑓 𝑟 = (𝑑𝑉 𝑎𝑟𝑐 𝑑𝑡 ⁄ ) 𝛿 𝑣 ⁄ and are deeply linked to the arc current, plasma gases and more generally to the specific enthalpy as explained in the previous section. Indeed, the rate of increase of the arc voltage is related to the mean plasma speed by 𝑑𝑉 𝑎𝑟𝑐 𝑑𝑡 ⁄ ~𝐸𝑐 𝑢 ̅ where 𝐸 𝑐 is the mean electric field of the arc column. Between pictures 1 and 2, the arc has undergone a small amplitude voltage restrike jump to the opposite side while the arc current has low value just before restrike (#1). Between pictures 3 and 4 a large amplitude voltage jump is observed whereas the arc current is high just before restrike (#3). At last, in pictures 1 and 3, the arc is attached at nozzle exit with almost the same length but it is clearly observed a smaller arc attachment radius in case of picture 1.

Consequently, the current modulation tends to favor the arc reattachment on the one hand at high current due to high arc radius (low CBL thickness) and on the other hand at low current probably due to an electric field intensification to maintain the electrical discharge [START_REF] Mavier | Electric arc in a plasma spray torch under modulated current[END_REF]. It is worth underlining that the above interpretation is based on the existence of a single arc attachment observed close to the nozzle exit and should be backed by end-on arc images of the arc inside the torch to discard the assumption of possible multiple arc roots as shown by Dorier et al [START_REF] Dorier | Time-resolved imaging of anodic arc root behavior during fluctuations of a DC plasma spraying torch[END_REF], Ghorui et al [START_REF] Ghorui | Direct probing of anode arc root dynamics and voltage instability in a dc non-transferred arc plasma jet[END_REF], and Qiang et al [START_REF] Qiang | A novel experimental method of investigating anode-arc-root behaviors in a DC non-transferred arc plasma torch[END_REF].

The association of time-resolved temperature measurements and a simplified model of heat transfer in the torch gives the dynamics of the specific enthalpy at nozzle exit, i.e. its amplitude modulation considering the thermal efficiency of the torch 𝜂 and also the time-shift between voltage and enthalpy which actually corresponds to the residence of plasma in the arc channel (proportional to 𝑢 ̅), i.e. the duration needed by the torch to release the net thermal energy stored inside the arc channel between two consecutive restrike jumps.

The classification of torches according to the enthalpy is essential because it determines the thermal energy available and indirectly the plasma speed for momentum transfer. However, it is worth underlining that it does not account for the load effect due to the injection, i.e. the twoway heat and momentum transfers that alters the plasma temperature and velocity fields. The typical mass flow rates of liquids injected in plasma generated by CT torches (35-40kW) are close to 20-30 g.min -1 . They roughly linearly increase as the electrical power increases to reach about 90 g.min -1 for AMT torches. As insightfully discussed by Jordan et al. [START_REF] Jordan | The Solution Precursor Plasma Spray (SPPS) Process: A Review with Energy Considerations[END_REF], only 10% of the net torch power is transferred to the liquid and more than half of the energy is consumed by the evaporation of the liquid phase. As a result, plasma spraying of liquid feedstock requires substantial energy to evaporate the liquid so that the material flow rate contained in the liquid phase and used to form the coatings is lower than when spraying solid material alone (typically 4 times lower) [START_REF] Jordan | The Solution Precursor Plasma Spray (SPPS) Process: A Review with Energy Considerations[END_REF]. It results that the deposition rate of solid materials to form coatings is actually strongly limited by the liquid phase.

Consequently, attention must be paid to the decomposition mechanisms of the injected liquid, including liquid fragmentation, evaporation, in-flight solid material formation and transport.

Plasma liquid treatment

This section first focuses on the ceramic ink requirements used in piezoelectric printed head for liquid injection. Second, plasma-liquid interaction phenomena are briefly presented highlighting the differences when operating with CT or dc pulsed torch.

Ink formulation

A ceramic ink is a suspension of solid particles which the properties were made compatible with the piezoelectric injection of the printing device. Indeed, the ink formulation must meet several requirements in order to be ejected properly as calibrated droplets. First, the ejection ratio Z has to be considered as follows:

𝑍 = √𝑅𝜌𝜎 𝜂 = 1 𝑂ℎ = 𝑅𝑒 √𝑊𝑒 (2)
with R, 𝜌, 𝜎 and 𝜂 respectively the nozzle radius, ink density, surface tension and viscosity.

This number accounts for the relative contribution of inertial, viscous and interfacial forces. It permits to establish a criterion of formation of a reproducible single droplet and must fall into the range 1 < 𝑍 < 10. If 𝑍 is too low, the liquid filament ejected does not break due to viscous damping of instabilities. If it is too high, the breakup generates a spray of multiple satellite droplets [START_REF] Reis | Ink jet deposition of ceramic suspensions: Modelling and experiments of droplet formation[END_REF]. The typical breakup time is of the order of the µs, and the generated drops has a velocity of a few m.s -1 [START_REF] Bienia | Inkjet printing of ceramic colloidal suspensions: Filament growth and breakup[END_REF]. Moreover, depending on the printhead used, viscosity and surface tension may also have an operational range. For the former, the typical shear stress in the nozzle is of the order of 10 5 s -1 , so an ink too viscous cannot be pushed through. For the latter, pooling on the printhead must be avoided, and this parameter is less critical for the case of capillary nozzles. These properties can easily be tuned by adding viscous solvents and surfactant molecules. In the case of SPPS, Mavier et al. dissolved an aluminum nitrate nonahydrate (Al(NO3)3.9H2O) salt into deionized water with glycerol and BRIJ surfactant to respectively adjust the viscosity and surface tension. Ammonia was also used to increase the solution pH which affects the limit of solubility [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF]. In the case of SPS with particulate inks, additional properties need to be taken into account, namely 90% of the particle size distribution must be smaller than 𝑅 50 ⁄ to prevent from clogging where R is the diameter of the printhead. Since 𝑅 ≈ 50 µm, the typical maximum particle size must fall below 1 µm.

Moreover, to avoid the formation of large agglomerates and prevent sedimentation in the printhead reservoir, the suspension must be stable in time, i.e. the inter-particle interactions must be controlled. They are often described by the Derjaguin-Landau-Vervey-Overbeek (DLVO) potential involving the van der Waals attraction (𝑣𝑑𝑊) and the electrostatic interactions (ES), which can be attractive or repulsive, 𝑉(𝑟) = 𝑉 𝐸𝑆 (𝑟) + 𝑉 𝑣𝑑𝑊 (𝑟) 𝑟 where 𝑟 the distance between the particles [START_REF] Derjaguin | Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes[END_REF]. The mathematical description for the interaction potential is sometimes found in the literature as the sum of Lennard-Jones potential and Yukawa potential for electrostatic repulsion [START_REF] Liang | Interaction forces between colloidal particles in liquid: Theory and experiment[END_REF]. These embed all contributions, including depletion attraction and hard-core repulsion. The resulting potential may have different profiles:

1. No electrostatic interactions: strongly attractive with a minimum at 𝑟 = 𝑟 0 where 𝑟 0 is the particle diameter. Hard core repulsion may be present to avoid overlapping.

2. Mild electrostatic interactions resulting in a potential barrier lower than 10𝑘 𝐵 𝑇 and a shallow secondary minimum: weak aggregation, reversible, where 𝑘 𝐵 and T are respectively the Boltzmann constant and the temperature of the suspension.

3. Strong electrostatic repulsion resulting in a potential barrier larger than 10𝑘 𝐵 𝑇: the suspension is considered stable.

When oxides are dispersed in an aqueous medium, an electrical double layer is formed around the particle due to hydration reactions on the particle. Depending on the nature of the oxide and the pH value, 𝑂𝐻 -or 𝐻 + ions are formed on the surface. Here are the reactions for titanium dioxide in water used for SPS [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF]:

{ 2𝐻 2 𝑂 ⇌ 𝐻 3 𝑂 + + 𝑂𝐻 - 𝑇𝑖𝑂𝐻 + 𝐻 3 𝑂 + ⇌ 𝑇𝑖𝑂𝐻 2 + + 𝐻 2 𝑂 𝑇𝑖𝑂𝐻 + 𝑂𝐻 -⇌ 𝑇𝑖𝑂 -+ 𝐻 2 𝑂
A net charge thus appears on the particle, and the counter-ions are distributed around it within a typical distance called the Debye length 𝜅 -1 :

𝜅 -1 = √ 𝑘 𝐵 𝑇 𝜀 0 𝜀 𝑟 2𝑒 2 𝐼𝑁 𝐴 (3)
with 𝜀 0 𝜀 𝑟 the dielectric permittivity of the medium, 𝑒 the elementary charge, 𝑁 𝐴 the Avogadro number and I the ionic strength 𝐼 = For example, 𝐴𝑙 2 𝑂 3 is positively charged for pH<9 and negatively for pH>9. This zero-charge value is 5.5 for 𝑇𝑖𝑂 2 and 2 for 𝑆𝑖𝑂 2 .

The larger the surface charge and the Debye length, the more electrostatic stabilization occurs in the suspension. Surface charge can be tuned by varying the pH of the suspension. The Debye length depends on the concentration of the ionized species in the medium.

When the surface charge generation and electrostatic repulsion are not enough to counteract the van der Waals attraction, dispersing additives are used. They can have different actions:

• Steric repulsion: large polymers adsorbed on the particles ensure that the particles do not enter within a close range to each other. For non-polar solvents, only steric dispersants can be used.

• Electrostatic repulsion: ionized salts adsorb on the surfaces of the particles, leading to an increase of surface charge (for example: dibasic ammonium citrate (DBAC))

• Electro-steric repulsion: both effects may be combined for the case of large polymeric electrolytes which develop both surface charge and steric repulsion (for example:

Darvan C)

The amount of dispersant needs to be perfectly adjusted because lack or excess both lead to attraction, due to depletion forces. The specific surface area of the particles is a physical quantity to be considered to do so.

Plasma liquid interactions

Liquids are usually injected in the plasma jet as a liquid column by means of a mechanical injection or as droplets from an atomization spray. The comparison of the characteristic times of fragmentation and evaporation shows differences by several orders of magnitude in favour of fragmentation (tens of µs) which was experimentally and numerically shown [START_REF] Fazilleau | Phenomena Involved in Suspension Plasma Spraying Part 1: Suspension Injection and Behavior[END_REF][START_REF] Qiang | A novel experimental method of investigating anode-arc-root behaviors in a DC non-transferred arc plasma torch[END_REF].

As shown in Figure 5A, the liquid column (150µm in diameter) is primarily fragmented into large liquids ligaments and drops, and a secondary fragmentation results in droplets size distribution of a few tens of micrometres depending on many parameters like the liquid flow rate, the injector position with respect to the nozzle and its injection angle. Jaddidi et al. [START_REF] Jadidi | Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process[END_REF][START_REF] Jadidi | Heat Transfer in Suspension Plasma Spraying[END_REF] have exhaustively reviewed heat transfers in SPS process but they have also highlighted the classification of breakup phenomena of liquid columns for the primary fragmentation and of drops for the secondary one according to the plasma gas Weber number

(𝑊 𝑒 = 𝜌 𝐺 𝑈 𝐺 2 𝐷 𝜎 ⁄ ), the momentum flux ratio 𝑞 = 𝜌 𝐿 𝑈 𝐿 2 𝜌 𝐺 𝑈 𝐺 2 ⁄
, and the Ohnesorge number of drops 𝑂ℎ = 𝜇 𝐿 √𝜌 𝐿 𝑑 𝐿 𝜎 𝐿 ⁄ where (𝜌, 𝑈) 𝐺 and (𝜌, 𝑈) 𝐿 are respectively the (density, relative velocity) of the plasma gas and the liquid phase, 𝜎 𝐿 is the surface tension, 𝜇 𝐿 the dynamic viscosity of the liquid phase, D is the diameter of liquid column and 𝑑 𝐿 the diameter of the drop.

According to the considerations of the previous section related to the specific enthalpy and plasma speed, a mechanical fragmentation is expected due to 𝑊 𝑒 > 100 while the liquid column momentum flux, or its dynamic pressure, remains higher that of the plasma jet (q > 1). It results that the liquid penetration in the core of the plasma has to be adjusted following the plasma properties to fulfill q > 1. Due to the plasma cross-flow shearing the surface of the liquid column, surface waves grow, even outside the plasma as shown in Figure 5A, to produce ligaments or threads lengthening in the plasma flow direction and breaking up into liquid fragments. It is worth underlining that the dynamic pressure of the plasma is strongly nonuniform in the radial direction because of the temperature dependence of plasma density. As pointed out by Meillot et al. [START_REF] Meillot | Modelling the interactions between a thermal plasma flow and a continuous liquid jet in a suspension spraying process[END_REF], the Weber number may be high in the fringes of the plasma due to high velocity gradient and high plasma density.

When travelling the hot plasma core, ligaments and drops undergo catastrophic fragmentation at high Weber numbers (𝑊 𝑒 > 100) resulting into droplets ( tens of µm in diameter) due to Kelvin-Helmholtz and Rayleigh-Taylor fluid instabilities [START_REF] Jadidi | Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process[END_REF][START_REF] Dalir | Modeling of Suspension Plasma Spraying Process Including Arc Movement Inside the Torch[END_REF][START_REF] Dalir | Three-Dimensional Modeling of Suspension Plasma Spraying with Arc Voltage Fluctuations[END_REF]. Figure 5A displays that the liquid column, injected with a speed of about 30 m.s -1 , is actually fragmented at the neck of instabilities growing from its surface due to shearing stress. It is equivalent to individually inject drops of about 200 µm in diameter with a space of 0.9 mm between each of them. The typical injection frequency is therefore 33 kHz. When fragmented, the drops result in clouds composed of a dense head and a tail further downstream containing droplets and particles. The ycomponent of injection velocity is conserved during the whole fragmentation process because the y-distance between each cloud is roughly constant and the time for the liquid to reach the axis of the torch, i.e. radially travelling about 3 mm, is about 100 µs. The latter is evidently much higher than the characteristic time of fragmentation but also that of acceleration. Indeed, the x-distance between the heads of the first and third clouds is about 1 mm while it is 3 mm for the corresponding tails. Given the time between the first cloud and third cloud is 66µs, the x-velocity component of the head of the third cloud is 15 m.s -1 whereas that of its tail is 45 m.s - 1 .

The fragmentation processes are inherently related to fluid instabilities and hence difficult to control because of plasma fluctuations in time (10-250 µs) and of the radial gradients of plasma properties. The mechanical fragmentation can be delayed or even canceled out by changing the liquid properties (surface tension mainly) but also by handling smaller droplets leading to very low Weber number, favoring the smooth radial gradients with laminar jet and controlling the injection timing following the arc instabilities.

The dc pulsed torch presented in the previous section has been associated with a piezoelectric injection to perform a phased injection triggered by the torch itself according to a time delay  depicted in Figure 3. Figure 5B displays the injection of precursor solution droplets (80 µm in diameter) in N2 plasma jet. The fragmentation mechanism is completely different from that of Figure 5A because the Weber number is lower that unity owing the low plasma speed (70 m.s - 1 ). Moreover, the characteristic time of solvent vaporization is given by [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF]:

𝜏 𝑣𝑎𝑝 = 𝜌 𝐿 𝐻 𝑣𝑎𝑝 4𝑁 𝑢 𝑎 𝜑 ( 𝑑 2 ℎ ̅ 0 ) (1 -( 𝑑 𝑠 𝑑 𝐿 ) 2 ) (4)
where 𝐻 𝑣𝑎𝑝 , 𝑑 𝑠 , 𝑁 𝑢 and 𝑎 𝜑 are respectively the latent of solvent vaporization, the diameter of the solid phase after vaporization, the Nusselt number and the coefficient of heat potential [START_REF] Rat | A simplified analytical model for dc plasma spray torch: influence of gas properties and experimental conditions[END_REF].

In a laminar flow, the acceleration time for the droplet is 𝜏 𝑎𝑐𝑐 = 𝜌 𝐿 𝑑 𝐿 2 18𝜂 ⁄ , where 𝜂 is the dynamic viscosity of the plasma.

Considering the mean specific enthalpy given by Figure 2b, the vaporization time is about 100 µs while the acceleration time is higher than 2 ms. Note that the equation ( 4) is also related to the coefficient 𝑎 𝜑 , namely the proportionality constant between the specific enthalpy and the heat potential of the plasma gas, so that it can be 2.5 times higher in the case of Ar-H2 (75-25 vol%) than with pure N2. Consequently, it can be assumed that thermal transfers occur during the slow acceleration phase leading to, as shown in Figure 5B1, a thermal fragmentation. The latter generate condensed material fragments treated further downstream by the plasma. Figures 5B2 and 5B3 depict the trajectories of such fragments during the exposure time which axial component velocities is between 30 and 55 m.s -1 . It is interesting to observe that close to the substrate some fragments follow the plasma streamlines as expected when evaluating the Stokes number of particles which is below unity. However, it has been shown that submicron-sized solid particles are trapped within the coatings that is why the liquid evaporation in Figure 5B1 results in the solution precipitation, probably also in in-flight molten particles and in the formation of crystallized -Al2O3 solid particles within the coating [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF]. Analogous phenomena are highlighted when spraying a ceramic ink of TiO2 nanoparticles giving rise to micro-sized molten particles onto the substrate [START_REF] Mavier | Suspension and precursor solution plasma spraying by means of synchronous injection in a pulsed arc plasma[END_REF] suggesting an in-flight coalescence of nanoparticles. The coating deposition resulting from the piezoelectric phased droplet injection lead mostly to very porous cauliflower shaped microstructures composed of nanometric grains that suggests the vapor condensation onto the substrate [START_REF] Mavier | Plasma spraying of solution precursor in pulsed mode: In-flight phenomena and coating deposition[END_REF][START_REF] Mavier | Suspension and precursor solution plasma spraying by means of synchronous injection in a pulsed arc plasma[END_REF]. This growth mechanism is profoundly different from that usually highlighted in SPS and SPPS at much higher liquid flow rates and relies on the stacking of micron-sized lamella on the substrate as explained in the next section.

Coatings growth

Many studies including numerous review papers report that typical SPS and SPPS coatings are formed from lamella resulting from fine melted particles conveyed by the plasma [START_REF] Fazilleau | Phenomena Involved in Suspension Plasma Spraying Part 1: Suspension Injection and Behavior[END_REF][START_REF] Vaßen | Suspension Plasma Spraying: Process Characteristics and Applications[END_REF][START_REF] Killinger | Review of New Developments in Suspension and Solution Precursor Thermal Spray Processes[END_REF][START_REF] Unabia | Current Progress in Solution Precursor Plasma Spraying of Cermets: A Review[END_REF][START_REF] Fauchais | Specific Measurements of In-Flight Droplet and Particle Behavior and Coating Microstructure in Suspension and Solution Plasma Spraying[END_REF][START_REF] Dalir | Modeling the effect of droplet shape and solid concentration on the suspension plasma spraying[END_REF].

For SPS, just after solvent evaporation, agglomeration and sintering of submicron particles are assumed [START_REF] Fazilleau | Phenomena Involved in Suspension Plasma Spraying Part 1: Suspension Injection and Behavior[END_REF]. For SPPS, the solvent evaporation leads to consecutive complex transformations of the precursor solution, namely, precipitation, pyrolysis, sintering and melting (4). These transformation processes and their influences on coating features, especially its columnar structure and the porosity content, are dependent on solvent properties, material loads (%wt fraction of solid particles for SPS or molarity for SPPS), injection locations and modes (radial, axial), plasma torch properties, surface topography and shape of substrate. The coatings growth mechanisms are strongly influenced by the shadowing effect favoring a columnar growth [START_REF] Seshadri | Characterizing Suspension Plasma Spray Coating Formation Dynamics through Curvature Measurements[END_REF][START_REF] Bernard | Columnar suspension plasma sprayed coating microstructural control for thermal barrier coating application[END_REF].

They are conceptualized by means of the Stokes (St) number of fine particles which encompasses all the thermal and dynamic history of particles and which is evaluated closed to the stagnation point during deposition. For very low St number, the speed component of particles parallel to the substrate (𝑣 ∥ ) is much higher than perpendicular one (𝑣 ⊥ ). Particles follow the deviated plasma streamlines and give rise to wider column and smaller intercolumnar voids. Dense coatings can be reached by increasing the plasma enthalpy and speed by favoring the increase of 𝑣 ⊥ but also by improving the liquid fragmentation and evaporation [START_REF] Dolmaire | Benefits of Hydrogen in a Segmented-Anode Plasma Torch in Suspension Plasma Spraying[END_REF][START_REF] Chen | The Solution Precursor Plasma Spray Coatings: Influence of Solvent Type[END_REF].

In some of the applications such as photocatalysis, SPPS offers porous nano-structured films of higher specific surface area suitable for effective photocatalytic action. These materials An interesting observation on different morphologies of ZnO as a function of substrate distance from plasma jet, plasma composing species, substrate temperature and precursor injection mode (mixed or layered) has been reported. Study shows that the substrate distance from plasma is crucially playing role in deciding the morphology. Also, the molarity of the solution or the concentration of Zn in the precursor is also affecting the morphology.

Formation of different microstructures of the same material subjected to different operating regimes can be probed with the help of a nucleation and growth modelling tool.

During a vapour phase synthesis of particles in thermal plasma, the cooling rate of plasma jet gas (typically between 10 4 -10 6 K/s) and initial monomer concentration which is dependent on concentration of the injected precursor, affect the supersaturation of nucleating species. More is the monomer concentration, supersaturation ratio mounts quickly at higher temperature as can be seen from Figure 8 [START_REF] Dhamale | Nucleation and growth of Y2O3 nanoparticles in a RF-ICTP reactor: A discrete sectional study based on CFD simulation supported with experiments[END_REF]. With decrease in monomer concentration (or precursor feeding rate), the species are nucleating at relatively lower temperature with higher supersaturation ratio as shown in Figure 8 where the supersaturation of yttria vapours is calculated for different initial mass flow rate and cooling rates. Another important factor is the cooling rate experienced by the vapours as they are passing the plasma jet down towards the substrate. The cooling rate is affected by the plasma gas velocity and spatial temperature fall (or gradient) at the plasma tail.

Shaper is the temperature gradient or more is the plasma gas velocity, faster is the drop in the plasma temperature. This is arresting a further growth of the nucleated particles. This decides the final distribution of particle size. Faster is the quenching, particles will be ending up in a narrow size distribution. Thus, the parameters which affect the process of nucleation and growth can be listed as: degree of cooling of the saturated vapours, initial concentration of the vapours in the plasma, vapour pressure and surface tension of the nucleating species, and substrate temperature etc.

A rapid fall in the temperature stops the further growth of the particle while a constant heating with relatively low temperature near the substrate forces the particles to grow heterogeneously favouring the formation of nanostructures like NRs, NWs or a cauliflower like structure. In conclusion, the initial precursor concentration, effective heat flux by plasma at the substrate surface, governed by the plasma specific enthalpy and speed, the plasma cooling rate are deciding parameters for different nanostructure formation.

Conclusion

Plasma spraying of liquid feedstock has been studied for about two decades because of improved finely structured coatings properties resulting from the use of submicron particles, especially in the application fields of aeronautical, automotive and health. Many research studies have permitted to identify the relationships between the coating functionalities and the key operating parameters of the four main domains, i.e. 1) the plasma properties (linked to the plasma torch), 2) the thermo-physical properties of liquids containing particles or precursor solution, 3) the injection mode and 4) the deposition methodology.

However, the control of coating properties and the precise understanding of involved phenomena are still lacking because of, first, the intricate influences of the above mentioned four domains that requires each a close examination while considering their interdependence.

Second, a multiscale approach of processes description must encompass transient phenomena (arc instabilities, heat and momentum transfers, liquid fragmentation and evaporation, particle transport with change of the phase, in-flight solid formation and coating growth) which considerably challenge in developing appropriate diagnostics and models. Third, the fine description of phenomena must give rise to the control of coating properties at macroscopic level.

To this end, the improvement of the plasma source acts as a leverage by controlling the arc instabilities with proactive strategies like the use of segmented plasma torches or alternatively applying a modulated DC arc current to strengthen in a controlled way the arc fluctuations.

Moreover, the arc root attachment modes at the anode wall have to be considered in regards to the arc stability and the anode erosion due the possible co-existence of multiple arc roots. It is then expected that the flow regime of the resulting arc plasma jet affects the thermal decomposition of the liquid phase precursors, injected as suspensions or precursors solutions, depending on fragmentation processes. Especially for low Weber number, a thermal fragmentation has been shown to be strongly anisotropic sowing micron-sized material fragments in the plasma and generating strong material evaporation. In the particular case of aqueous suspension, during the solvent evaporation, the submicron particles trapped within a droplet tend to agglomerate due to the loss of electrostatic repulsion established by the ionized species of the water solvent. The coating growth of SPS and SPPS is being well understood from the velocity components of solid particles close to the substrate and their impact onto the substrate as fine lamella. Challenging efforts should be dedicated to develop diagnostics in the deposition region close to the substrate to not only establish a direct correlation between the particle impact velocity and the coating columnar features but also to elucidate the formation mechanisms of typical nanostructures like nanorods and nanowires obtained with SPPS.

  Rat, Marguerite Bienia, Gayatri Dasharath D Dhamale, Fabrice Mavier, Céline Ruelle, Simon Goutier To cite this version: Vincent Rat, Marguerite Bienia, Gayatri Dasharath D Dhamale, Fabrice Mavier, Céline Ruelle, et al.. Relationships between arc plasma jet properties and plasma/liquid interaction mechanisms for the deposition of nanostructured ceramic coatings. Plasma Physics and Controlled Fusion, In press, 10.1088/1361-6587/ac3c39. hal-03453757

Figure 1 :

 1 Figure 1: Current non-transferred plasma torches used for liquid feedstock spraying a) Conventional torch CT, b) Segmented plasma Torch ST, c) Axial Multi-electrode Torch AMT

Figure 2 :

 2 Figure 2: a) Current/voltage characteristics of main DC plasma torches families used in plasma spraying of liquid feedstocks, Conventional Torch (CT), Segmented Torch (ST), Multielectrode Torch (MT), Axial Multi-electrode Torch (AMT). b) Specific enthalpy dependence on electrical power[START_REF] Zimmermann | Investigation and new physical behaviour of specila DC-Plasma generator Mettech Axial III[END_REF][START_REF] Dolmaire | Benefits of Hydrogen in a Segmented-Anode Plasma Torch in Suspension Plasma Spraying[END_REF][START_REF] Cao | Experimental Study on the Design and Characteristics of a Laminar Plasma Torch With Medium Working Power and its Applications for Surface Hardening[END_REF][START_REF] Liu | Generation of Long Laminar Plasma Jets: Experimental and Numerical Analyses[END_REF].

2 .

 2 reach high voltage by elongation and constriction of the arc. It results that laminar torches attain quite high arc voltage and also high specific enthalpy as shown in Figure Temperature and velocity fields are also higher than in the case of CT torches[START_REF] Liu | Numerical simulation of the flow characteristics inside a novel plasma spray torch[END_REF] meaning that the decrease of mass flow rate in comparison with torches generating turbulent plasma jet is compensated by higher specific enthalpy. The association of neutrodes in the inter-electrode insert and an optimized nozzle with a backward-facing step allows reaching laminar plasma jets as long as 500 mm from the nozzle along with a very large extension of high specific enthalpy in the plasma jet. Depending on the aperture size and length of the step in the nozzle wall, the gas from the surroundings flows into the nozzle and interacts with the internal closed vortex created by the detachment of the dynamic boundary layer due to the stepped nozzle. It results in the suppression of the closed vortex and stabilizes the outflowing jet[START_REF] Solonenko | Visualization of arc and plasma flow patterns for advanced material processing[END_REF]. It is also been shown that a trumpet-like shape of the anode favors the expansion of arc column and reduces the compression effect of the cold boundary around the arc[START_REF] Liu | Numerical simulation of the flow characteristics inside a novel plasma spray torch[END_REF].A low powered DC torch has been developed to operate in a self-sustained pulsed mode producing a pulsed laminar plasma jet depicted in Figure2a. The purpose was to improve the plasma heat and momentum transfers to droplets by the assistance of a phased liquid injection[START_REF] Krowka | Investigation and control of dc arc jet instabilities to obtain a self-sustained pulsed laminar arc jet[END_REF]. This torch operates at very low current (<25 A) and provides specific enthalpy as high as that of CT torch (20 MJ.kg -1 ) because of low mass flow rate of nitrogen plasma forming gas (< 3 slm). It results that the plasma speed is more than one order of magnitude smaller than that of CT torches, i.e. close to 70 m.s -1[START_REF] Mavier | Electric arc in a plasma spray torch under modulated current[END_REF].

Figure 3 .

 3 Figure 3. Arc voltage signal of dc pulsed torch and imaging of pseudo laminar jet[START_REF] Krowka | Investigation and control of dc arc jet instabilities to obtain a self-sustained pulsed laminar arc jet[END_REF]

Figure 3

 3 Figure3displays a typical restrike signal fluctuating at 1.4 kHz with large amplitudes[START_REF] Krowka | Investigation and control of dc arc jet instabilities to obtain a self-sustained pulsed laminar arc jet[END_REF]. A pseudo-laminar pure nitrogen plasma jet is obtained as shown with imaging performed at low

Figure 4 .

 4 Figure 4. Time-resolved imaging of the attachment close to the nozzle exit of an arc under modulated current (31).

1 2∑

 1 𝑐 𝑖 𝑧 𝑖 2 .(𝑐 𝑖 being the molar concentration of the species in the solution and 𝑧 𝑖 its charge number).

Figure 5 .

 5 Figure 5. Time-resolved imaging of liquid injection in a dc arc plasma jet at nozzle exit (CT torch): A) Mechanical liquid column injection of a suspension of Yttria Stabilized Zirconia submicron particles, 10µs exposure time, 26.6 m.s -1 injection speed, Ar-H2 plasma (45-15 slm), 500 A. B) Piezoelectric droplet injection of aluminum nitrate precursor solution, 50µs exposure time, 3.2 m.s -1 injection speed, N2 plasma (2 slm), 25 A (dc pulsed arc). B1: early stages of thermal fragmentation, B2: transport of materials, B3: plasma jet streamlines close to the substrate. We is the Weber number.

Figure

  Figure 5B also highlights significant amount of vapors that are pointed out by Figure6. It depicts the molecular emission spectra of AlO (𝐵 2 Σ + -𝑋 2 Σ + ) between 430 and 550 nm for different distances from the nozzle exit (9, 18 and 30 mm where the coatings are deposited). It has to be underlined that the spectra are recorded when the arc voltage reaches its mean value (Figure4). The spectrum obtained for the time-shift condition (at 30 mm) corresponds to a droplet emission by the printing head when the arc voltage is at its minimum value. Each spectrum integrates the radiative emission from all the height of sight (Y vertical direction, Y=0 being the torch axis). The progressive decomposition of the precursor is observed along with the formation of the AlO molecule which is supposed to form alumina (2𝐴𝑙𝑂 (𝑔) + 𝑂 → 𝐴𝑙 2 𝑂 3 (𝑙𝑖𝑞) ). The emission profiles following the height of sight reveal inhomogeneities in the AlO species number distribution and a higher emission in the plasma fringes in contact with the oxygen of the surrounding air. The time-shifted spectrum has significant lower emission assuming a lower plasma temperature and stressing the influence of the synchronization of liquid injection on in-flight heat transfers.

Figure 6 .

 6 Figure 6. a) Emission spectra of AlO (𝐵 2 Σ + -𝑋 2 Σ + ) b) Emission profile following the height of sight for different distances from the nozzle exit. The acquisition of spectra is performed at the mean arc voltage for an exposure time of 60µs. The time-shifted condition corresponds to an arc voltage at its minimum value. The Y-direction is perpendicular to the torch axis referenced by zero.

  Figure 7.

Figure 7 .

 7 Figure 7. ZnO (a) nanorods (b) cauliflower like structure (62) (Reproduced with kind permission of Elsevier)

Figure 8 .

 8 Figure 8. Supersaturation ratio for Y2O3 at (a) different initial concentration with fixed cooling rate and (b) different cooling with fixed monomer concentration (63) (Reproduced with kind permission of IOP)

  0 = 𝜂𝑉 ̅ 𝐼 𝑚⁄ where 𝑚̇ is the mass flow rate of plasma forming gases. It is worth noting that it also includes the specific kinetic energy of the plasma jet so that ℎ ̅ 0 = ℎ ̅ 𝐿 + 𝑢 ̅2 2 ⁄ where ℎ ̅ 𝐿 is the mean specific enthalpy at nozzle exit. By considering that 𝑢 ̅ is close to 1000 m.s -1 at atmospheric pressure, the kinetic energy only represents less than 10% of the total energy and is usually neglected.

	The measured enthalpy at nozzle exit is a quantity averaged in time but also in the cross section
	at nozzle exit, i.e. ℎ ̅ 0 =	8 𝑑 2 ∫ ℎ ̅ (𝑟)𝑟𝑑𝑟 𝑑 2 0

1 -𝑄 𝑙𝑜𝑠𝑠 𝑃 𝑒 ⁄ . The mean specific enthalpy produced by the torch and available at nozzle exit is Cathode Cathode Cathode basically evaluated by ℎ ̅