Maximilien Degonville

Gwenn Boedec

Marc Leonetti

Oblate to prolate transition of a vesicle under flow

Keywords: . 87.16..Dg Membranes, bilayers, and vesicles 47.15.G-Low-Reynolds-number (creeping) flows

published or not. The documents may come

Oblate to prolate transition of a vesicle under flow

Introduction

Dynamics of vesicles under flow has triggered numerous studies in the past decades, motivated in part by the connection with red blood cells dynamics in blood flow since their interfacial mechanics share similar properties. Indeed, both are phospholipidic bags, separating an inner viscous fluid from the suspending outer fluid. Vesicles are also a member of a broad family of soft objects which may be described loosely as droplets with "complex" interfacial properties, starting from clean drops described by a constant surface tension and extending to capsules (polymerized elastic interface) [1] or polymersomes (vesicles for which the membrane shear viscosity plays a dominant role) [2,3,4]. Studies are generally interested in how the coupling between hydrodynamics and interfacial mechanics selects the dynamics under flow: for instance, when submitted to a simple shear flow, both vesicles [5] and capsules [6] exhibit a tank-treading motion consisting of a fixed ellipsoidal shape and a rotation of the interface along this fixed shape, as well as other type of motions, such as tumbling and trembling when internal viscosity is increased [7].

Another question of particular interest is how the deformability of these objects can lead to cross-streamline migration, in particular in the context of a wall-bounded flow, a simple configuration which allows to determine the leading-order contribution of channel's wall-induced migration. Indeed, for these objects, fluid flows are generally in the creeping regime. In this regime, the equations gov-Send offprint requests to: gwenn.boedec@univ-amu.fr, marc.leonetti@univ-grenoble-alpes.fr erning fluid flows are linear and reversible. Thus, a symmetric rigid body submitted to a shear flow near a wall does not migrate away from the wall. On the contrary, if the body is deformable, the symmetry with respect to the flow may be broken and will induce a lift velocity pushing the particle away from the wall, as first observed experimentally for drops in [8].

Far from the wall, the migration velocity can be related to the particle stresslet and has been shown theoretically to scale as v lif t ∼ γR 3 0 /h 2 [9]. This far-field scaling can be recovered either by approximating the shape of the vesicle by an ellipsoid [10], or by expansion around the spherical shape [11,12]. This far-field scaling was confirmed numerically [13,12] and measured experimentally under microgravity [14]. Close to the wall, the question of how the hydrodynamic lift force can trigger unbinding of an adhering vesicle was tackled numerically and analytically in two [15] and three [16,17] dimensions, and also investigated experimentally [18,19,20]. Gravity effects were also investigated numerically [17,21,13]. While a deflated vesicle close to a wall is expected to have a flattened shape due to gravity or adhesive interaction [22], most of the focus has been devoted to the dynamics of elongated shapes.

Indeed, among soft objects, a notable peculiarity of vesicles is that multiple (locally stable) shapes at thermal equilibrium may exist, among them the prolate (cigar-like) and the oblate (disk-like) shapes [23,24]. Which family is the globally stable equilibrium shape depends only on the reduced volume : for quasi-spherical shapes, prolate shapes are the one with the least bending energy, while for more deflated vesicles, oblate shapes are the global minimum. In this respect they offer a unique system to study the influence of the initial condition on the dynam-ics and how external forcing by hydrodynamics may lead to a transition from one shape to the other. Most of the aforementioned works were devoted to the study of prolate vesicles, especially in the near-spherical regime v ∈ [0.9, 1]. In the context of wall-bounded shear flow, Zhao et al. [13] considered oblate shapes for a reduced volume of v = 0.65 for one value of shear rate, but the transition to prolate was not studied. In the context of free space shear flow, Spann et al. [25] studied the phase diagram of an initially biconcave vesicle as a function of dimensionless shear rate and viscosity contrast λ for two values of reduced volume. They found that for a given capillary number, increasing the viscosity contrast λ stabilizes the biconcave shape, which otherwise may transition towards prolate shape. For a reduced volume of 0.65, and at low capillary numbers, biconcave shape was metastable for all viscosity contrast. Increasing the capillary number while keeping viscosity contrast fixed leads to a transition to prolate shapes, occurring for λ = 1 at a capillary number in the range 0.6-1.0. On the other hand, for a reduced volume of 0.75, a viscosity contrast higher than a value in the range 3.5-4 was necessary to stabilize the biconcave shape. Noguchi and Gompper [26,27] studied a similar phase diagram for a reduced volume of 0.59 but with membrane viscosity instead of viscosity contrast, and found that including membrane viscosity also stabilizes the oblate shape. Note that none of the aforementionned studies characterized the nature of the transition, nor studied its phase diagram in (v, Ca) space.

In this paper, we investigate how the coexistence of oblate and prolate shapes is altered by the hydrodynamic stresses, and characterize the nature of the transition. We study how the critical shear rate depends on the reduced volume, and provide a phase diagram in the (v, Ca) space. As an example where this bistability has physical consequences, we study lift dynamics close to wall. The paper is organized as follows: first, we describe the model used and its transcription into a numerical model. Then, we focus on the hydrodynamic-induced transition from oblate to prolate shapes. We then analyse lift dynamics of oblate vesicles, both below and above critical shear rate. We conclude with some observations of high capillary number lift dynamics.

Model and numerical method

A vesicle is a drop enclosed by a self-organized fluid phospholipid membrane which possesses peculiar properties : lacking a reference configuration, the membrane behaves as a bi-dimensionnal incompressible fluid on a curved surface. There is however an energetic cost associated with the curvature of the membrane, measured by the Helfrich free energy :

E κ = κ 2 S (2H) 2 dS (1)
where κ ≈ 20 k B T is the bending modulus and H is the mean curvature of the surface S. The dynamics of the vesicle under flow thus results from the interplay between the elastic forces f κ related to bending energy such as δE κ = -S f κ • δx dS, and the fluid stresses related to the surrounding (inner and outer) media. An additional specificity of this phospholipidic membrane is that surface flows are locally incompressible to preserve the density of phospholipids. This induces an additional stress in the form of a bidimensionnal pressure (homogeneous to a surface tension) γ, which is a Lagrange multiplier of the surface divergence free constraint

∇ S • v m = 0 (2)
where ∇ S is the surface divergence, and v m is the membrane velocity field. This membrane velocity field is the restriction on the membrane of the surrounding velocity fields v m (x) = v i (x) = v e (x), which are governed by the Stokes equation

η i,e ∆v i,e -∇p i,e = 0 , ∇ • v i,e = 0 (3)
where η i,e are the internal and external viscosities. Far from the wall, the velocity is imposed as a simple shear flow

v e (x) → |x|→∞ v ∞ = γye x (4)
Examples of faccurate codes for three-dimensional simulations of vesicles in flow were published in 2011 [29,30,31]. Here the surface meshing is different. To solve this set of equations, we introduce a discretization of the interface S as a collection of Loop subdivision elements. With this discretization, bending membrane forces can be computed using a finite element method. This loop elements have also been used in the case of capsules and droplets with surface viscosities [32,33]. The Stokes equations for inner and outer fluid are solved by a boundary element method which, in the case of equal inner and outer viscosities, relates the velocity on the interface to the force densities via an integral relation :

v i (x) = v ∞ i (x) + 1 8πη S G w ij (x, x )f m j (x ) dS(x ) (5)
where G w ij are the cartesian components of the Green function taking into account the presence of the wall at y = 0 [28], and f m j are the cartesian components of the membrane forces f m = f κ + f γ . This equation is computed at each node of the mesh describing the interface. The Lagrange multiplier γ is determined by imposing that ∇ s • v = 0 at each node of the mesh. Finally, the interface position is advanced by using a trapezoidal method

x t+dt = x t + dt 2 v(x t ) + v(x t+dt ) (6)
where the unknown position x t+dt is found by an iterative method. More details and validation of the numerical method may be found in [34].

The dynamics of a vesicle under shear flow is described by two dimensionless numbers, the reduced volume v and the capillary number Ca defined by

v = V 4 3 π( S 4π ) 3/2 (7)
where S, V are the area and the volume of the vesicle.

Ca = η γR 3 0 κ (8)
where R 0 = 3V 4π 1/3 is the radius of the sphere having the same volume.

To study the influence of the initial shape on the wall migration, we set deflated vesicles at a distance h = 1.1R 0 of the wall. The initial shape is obtained by letting an oblate/prolate ellipsoid relax (Ca = 0) according to (5).

We consider reduced volumes in the interval [0.59, 0.75].

Since both oblate and prolate equilibrium shapes exist for reduced volumes in this interval, starting with an oblate (resp. prolate) ellipsoid, the shape relaxes towards the oblate (resp. prolate) thermodynamic equilibrium shape. The shear flow is then started abruptly, and the evolution of the vesicle is monitored (see figure 1): we measure h, the vertical position of the center of gravity of the vesicle, the axis L 1 , L 2 , L 3 of the ellipsoid having the same tensor of inertia, and the angle of inclination θ of the shape. If prolate shape is the minimum global, an initial condition of oblate shape can still be obtained experimentally before starting flow if the vesicle settles to the bottom of the chamber with a well chosen Bond number: see the diagram, figure 1 of Kraus et al [22].

Oblate to prolate transition

Before looking into the influence of the initial shape on the dynamics, we characterize the zone for which two possible long term solutions can exist, and the nature of the transition between these two states. To do so, we study the evolution of an initially oblate vesicle submitted to shear flow for various capillary numbers. As will be shown at the end of this section, the presence of the wall affects only slightly the quantitative results for the transition, and thus the results presented in this section is applicable in both cases. We introduce a shape parameter

β = (L 1 /R 0 -1)(L 2 /R 0 - 1)(L 3 /R 0 -1)
where the L i are the axis of the ellipsoid having the same tensor of inertia as the vesicle. Furthermore, L 1 L 2 L 3 = R 3 0 due to the constraint on the volume. Thus, the shape parameter allows to distinguish between prolate (β > 0) and oblate (β < 0) shapes. Figure 2 shows the evolution of this shape parameter as a function of dimensionless time γt, for different capillary numbers. Two regimes clearly emerge: for weak flows (Ca < Ca c ), an initially oblate shape is slightly deformed by hydrodynamic stresses, but remains oblate (β < 0) for all times. By contrast, under sufficient hydrodynamic stresses (Ca > Ca c ), an initially oblate shape will be stretched by the flow and will eventually transition towards a prolate shape (β > 0). This transition is illustrated in figure 3, where a side and a top view of the vesicle are provided. After the flow is started, the shape is stretched in the shear plane. Because volume and surface are invariants of the dynamics, this stretching is accompanied by a thinning of the shape in the Z direction, perpendicular to the flow. The dynamics of the profile in the Z -Y cross section is interesting to understand the transition: just before the transition, the cross profile which had a characteristic "discocyte" shape, passes through a "pancake" shape which is convex (while the discoctye was not). Looking solely at bending forces in this plane, it is clear that a convex shape such as the one shown at t = 6 in figure 3 is not stable and will evolve towards a circular shape. This is because, in this plane which is perpendicular to the flow plane, the flow acts only indirectly, through the constraint of volume and surface preservation. Thus, there is no external flow that can counter the effect of bending forces in this plane, which leads to the evolution towards circular profile.

Clearly, the dimensionless time needed to switch from the oblate state to the prolate state depends on the capillary number, as it increases with decreasing capillary num- ber. Because characterizing the nature of the transition is not a straightforward task, we will later use this dependency to determine the type of bifurcation. We define the transition time t tr as the instant such that β(t tr ) = 0. This transition time and the critical capillary number depend on the reduced volume of the vesicle. In figure 4a, we represent the evolution of the transition time as a function of the capillary number. In the high Ca regime, one could expect that the transition time decays to zero. However, because we always start our simulations with a symmetric shape whose axis are aligned with the wall, the vesicle has to first tilt towards the maximal stretching axis (located at π/4 in the xy-plane), before transiting from oblate to prolate. We note t ∞ the time needed to reorient the shape in flow, and use it as a fitting parameter to adjust our curves. On the other hand, at low capillary number, this transition time diverges at a critical capillary number as t tr ∼ 1 √ Ca-Cac . This divergence is characteristic of a saddle-node transition. Note that the nature of the transition can be understood intuitively: for a given reduced volume v < 0.75, without external shear, two families of solutions exist, corresponding to local minima of the bending energy. Because bending energy is a continuous function of the deformation, these minima are connected by a local maximum, which is a stationary unstable point. Energy can be considered as a double-well curve of a state quantity, named x. The transition occurs when this unstable branch collides with the stable oblate branch. Indeed, increasing the capillary number Ca decreases the oblate well up to vanish for Ca = Ca c (v). Close to this point, dynamics of oblate solution is governed by the dimensionless normal form of saddle-node transition:

ẋ = r + x 2 (9)
where r is proportional to Ca -Ca c (v). In the case r < 0, there are two fixed points, one stable and another unstable: x = ± √ -r. Otherwise, there is no solution: the oblate solution does not exist anymore. Scaling variables such that x = √ rX and t = T / √ r leads to: Ẋ = 1 + X 2 which does not depend on r and is straightforward to integrate between two quantity states. Thus, the necessary time scales as 1/ √ r, a characteristic of saddle-node transition.

Note that the transition as function of reduced volume for a fixed non zero capillary number is also a saddle-node transition, as shown in the figure 4-b : the evolution of the transition time is well-described by t tr ∼ 1 √ v-vc . To further characterize the transition, we also measure the amplitude of the shape parameter as a function of the capillary number, as shown in figure 5-a. In the range of studied reduced volume ([0.592, 0.75]), a prolate branch of solution exists for all capillary numbers, while the oblate branch of solution exists only for weak flows, and ceases to exist discontinuously at Ca = Ca c . Rescaling Ca as Ca c and β as β/β max allows the collapse of the transition curves for three different reduced volumes. We have checked the sensitivity to perturbations in the subcritical case Ca < Ca c where both prolate and oblate are local minima of bending energy. The initial oblate shape is elongated in the direction of flow preserving volume, area and β < 0 (oblate shape). In the case v = 0.63, we obtain a transition oblate to prolate for an initial deformation of 1 % and Ca/Ca c = 0.5/0.515 ≈ 0.97 and for an initial deformation of 4 % and Ca/Ca c = 0.375/0.515 ≈ 0.73: see two green asteriks on prolate branch (upper branch) in figure 5-a. The smaller the capillary number, the larger the initial perturbation.

This oblate to prolate transition is characteristic of shear flow and the value of critical capillary number is moderately affected by the presence of a wall: for a reduced volume of v = 0.59, the critical capillary number in wallbounded shear flow is Ca c ≈ 0.88, while it is Ca F S c ≈ 1.05 in free-space. There is thus a roughly 15% decrease in the critical capillary number due to the presence of a wall at h = 1.1R 0 . The reduction in capillary number is due to the lubrication layer which holds the bottom part of the vesicle while the upper part is lifting: there is thus an additional stretching of the vesicle which triggers the transition sooner.

Finally, we also investigate how this transition is affected by the reduced volume. The oblate shape is the global minimum [23] of the bending energy for reduced volume in the range [0.592, 0.651]. For higher reduced volume, for which the prolate shape is the global minimum, discocyte solutions can still exist for small capillary numbers, indicating that the oblate familly is metastable even under weak flows. For a given reduced volume, the critical capillary number Ca c is determined by the fit presented in figure 4. The evolution of Ca c as a function of reduced volume is presented in figure 5-b, together with a subset of the simulated points, indicating the zone where two solutions may be found, depending on the initial condition, and labelled as "bistable". The parameters where only prolate shapes were found (irrespective of the initial conditions) are labelled "prolate". As can be seen in figure 5-b, there is a good agreement between the zones determined by simulation points alone and the zones determined with the fit of transition time divergence. We also note that the range of possible capillary numbers compatible with oblate shapes decreases with increasing reduced volume: for v = 0, 59, the critical capillary number is around 0.9 while for v = 0.725 it is only 0.07. For a reduced volume higher than 0.74, we observe that a small amount of hydrodynamic stress is sufficient to trigger the oblate to prolate transition. This is in good agreement with the energetic stability analysis which shows that oblate shapes become a local minimum for v < 0.75 [35]. For v = 0.648, we find a critical capillary number of Ca c = 0.44. This capillary number is consistent with the range [0.6; 1.0] given by [25] for similar conditions (v = 0.65,λ = 1) because the definition of capillary number is based on a different length scale: [25] uses the radius of the sphere of equivalent area as the length scale, while we use the radius of the sphere having the same volume as the length scale. With the same definition of capillary number than [25], we find a critical capillary number of 0.68.

Lift dynamics

A typical simulation is shown in figure 6 : a discocyte shaped vesicle is placed in a weak shear flow (Ca = 0.375). Because of the fore-aft symmetry of the initial shape, the initial lift velocity is zero. The shape of the vesicle adapts itself by stretching in the flow direction, while the membrane also develops a tank-treading motion. There is thus a pressure gradient in the lubrication film, the highest pressure being found at the front of the vesicle [15,16]. This induces a torque which is balanced by the torque acting on the vesicle when its longest axis changes its orientation in flow, which leads to a reorientation of the vesicle with a positive tilt, as shown in figure 1. In a second phase, the vesicle moves away from the wall, keeping roughly a constant deformation and tilt angle. This deformation measured by Taylor parameter D xy and orientation angle θ depends on the capillary number and the reduced volume. In the weak capillary regime (where oblate shapes are stable), increasing the capillary number increases the deformation D xy defined in figure 1 while the angle of inclination θ decreases. At larger capillary numbers, the evolution is reversed: D xy is a weakly decreasing function of the capillary number, while θ is a weakly increasing function of Ca. Rescaling the lift velocity by γ, all curves for weak flow collapse on a master curve v lif t = v lif t (h) which depends only on the vesicle height h. Far enough from the wall, the asymptotic regime is v lif t ∼ h -2 with a prefactor depending on the particle stresslet as shown in [13]. In [13], the authors show that, in this asymptotic regime, a prolate vesicle has always a higher lift velocity, because the amplitude of its particle stresslet is higher. However, close to the wall, the situation is different: while oblates still have a lower stresslet, they now have a (roughly 50%) higher lift velocity than prolate ones, as shown in figure 7-a. Note that, in the vicinity of the wall, lift velocity is dominated by lubrication effects. An oblate vesicle, having an higher area facing the wall than a prolate one, leads to a more efficient lubrication layer beneath the vesicle, which in turn leads to an higher lift velocity.

Upon further increase of the capillary number, the initially oblate shape is stretched by the flow into a prolate shape, as discussed previously, and shown in figure 7-b. This shape transition has also a signature in the lift velocity, which shows a local "bump" characteristic of the transition. The closer to the transition, the higher the local "bump" is, as shown in figure 7. After the transition, the lift velocity is the same, irrespective of the initial shape, as shown in figure 7-a for Ca = 2.

Finally, for high (Ca ≥ 10) capillary numbers, the migration from the wall is accompanied by the appearance of transient asymmetric shapes, as shown in figure 8. Far enough from the wall, the asymmetry vanishes as the shape converges towards the free-space one. Increasing the capillary number leads to a more pronounced asymmetry, which might even lead to the formation of a transient tethered shape, as preliminary simulations at higher capillary numbers (≈ 50) indicate: figure 9. These shapes are reminiscent of the PB state obtained by [36] for a deflated vesicle (v ≈ 0.6) in shear flow with a viscosity contrast (λ < 1.0). Note that the appearance of this transient tethering is not related to the oblate-prolate transition, as it appears independently of the initial shape when starting either with an oblate or a prolate. It is due to the asymmetry in the hydrodynamic stresses applied to the vesicle : the presence of the wall leads to an additional stress in the lubrication layer. This tends to 'pin' the vesicle near the wall, while the upper part is lifted and stretched by the flow.

Conclusion

We studied how hydrodynamic stresses can trigger the transition between an oblate shape and a prolate shape. We provide an accurate diagram showing in which region of the (v, Ca) plane bistable solutions may be found. For quasi-spherical vesicle, no bistable region was found. For more deflated vesicles, a bistable region was found for weak flow, with an upper bound increasing when decreasing the reduced volume. Note that for deflated vesicles (v ∼ 0.6) viscosity contrast [25] or membrane viscosity [26,27] were both found to increase the critical capillary number, thus stabilizing the oblate shape. The presence of the wall leads to a slightly decreased critical capillary number, due to the additional stress. We then studied lift dynamics of oblate vesicle in the weak flow regime and found that, contrary to the far-field prediction, oblate vesicle near the wall have an higher lift velocity when compared to prolate vesicle with the same reduced volume. Finally, we showed that, for high capillary numbers, wall-induced hydrodynamic stresses may also lead to asymmetric shapes, independently of the initial shape.

Acknowledgements

This work has benefited from financial support from the ANR 2DVISC (grant no. ANR-18-CE06-0008), from Labex MEC (grant no. ANR-11-LABX-0092), from A*MIDEX (grant no. ANR-11-IDEX-0001-02) and from CNES. This and capillary number. Close to the wall, an oblate vesicle has a higher lift velocity, while a prolate vesicle has an higher lift velocity far from the wall. (b) -Dimensionless lift velocity as a function of dimensionless centroid height, for a v=0.635 vesicle, starting with an oblate shape. Critical capillary number for this reduced volume is ≈ 0.52. The transition from oblate to prolate shape is visible in the lift velocity as "bump", whose relative importance is higher closer to the transition (see e.g. Ca = 1 vs Ca = 10 curves).

Fig. 1 .

 1 Fig. 1. Sketch of the situation considered here : an ellipsoidal vesicle is placed in a wall bounded shear flow. The deformation is characterized by the Taylor parameter D: here, with our choice of axis, Dxy = (L1 -L2)/(L1 + L2).

Fig. 2 .

 2 Fig. 2. Evolution of the shape parameter as a function of dimensionless time, for different capillary numbers, and for a v = 0.635 vesicle.

Fig. 3 .

 3 Fig. 3. Oblate to prolate transition triggered by hydrodynamic stresses (v = 0.635, Ca = 2). Snapshots in the Z -Y plane (upper row), in the X -Z plane (middle row) and in the X -Y plane (bottom row) for different dimensionless times (from left to right γt = 0; 1; 2; 4; 6; 8; 12;).

Fig. 4 .

 4 Fig. 4. Evolution of the transition time as a function of capillary number, for different reduced volume. Symbols correspond to simulation points, solid lines corresponds to fit by 1/ √ Ca -Cac + t∞. This behavior is characteristic of a saddle-node bifurcation. t∞ corresponds to the time needed to reorient the vesicle in the flow. (b) Evolution of the transition time as a function of reduced volume for a fixed capillary number (Ca = 0.5). Symbols correspond to simulation points, solid lines corresponds to fit by 1/ √ v -vc.

Fig. 5 .

 5 Fig. 5. (a) Evolution of the shape parameter (rescaled by maximal value) as a function of capillary number rescaled by critical capillary number, for different reduced volume. All points collapse to two branches of solutions, one prolate (β/βmax ≈ 1), one oblate (β/βmax < 0). (b) Critical capillary number separating the bistable region from the prolate region as a function of reduced volume. The symbols represent a subset of simulation points in the weak capillary regime. The curve is obtained by fitting transition time divergence for various reduced volume, see figure 4. Solid line and symbols represent the data for a vesicle released close to a wall h/R0 = 1.1, while dashed line represent data for a vesicle in unbounded shear flow.

Fig. 6 .

 6 Fig. 6. Dynamics of lift of a biconcave vesicle (v = 0.635; Ca = 0.375): Snapshots in the Z -Y plane (top row), X -Z plane (middle row) and X -Y plane (bottom row), for different dimensionless times (from left to right γt = 0; 2; 5; 10.

Fig. 7 .

 7 Fig.7. (a) -Migration dynamics of a v=0.635 vesicle, as a function of initial shape (oblate -dashed lines, prolate -solid lines) and capillary number. Close to the wall, an oblate vesicle has a higher lift velocity, while a prolate vesicle has an higher lift velocity far from the wall. (b) -Dimensionless lift velocity as a function of dimensionless centroid height, for a v=0.635 vesicle, starting with an oblate shape. Critical capillary number for this reduced volume is ≈ 0.52. The transition from oblate to prolate shape is visible in the lift velocity as "bump", whose relative importance is higher closer to the transition (see e.g. Ca = 1 vs Ca = 10 curves).

  work was granted access to the HPC resources of Aix-Marseille Université financed by the project Equip@Meso (ANR-10-EQPX-29-01) of the program "Investissements d'Avenir" supervised by the Agence Nationale de la Recherche. M.D. was supported financially by a fellowship cofunded by the region "Provence-Alpes-Côte-d'Azur" and the CNRS -INSIS. LRP is part of the LabEx Tec 21 (Investissements d'Avenir -grant agreement ANR-11-LABX-0030) and of the PolyNat Carnot Institute(Investissements d'Avenirgrant agreement ANR-11-CARN-007-01).

7 Fig. 8 . 5 Fig. 9 .

 7859 Fig. 8. Asymmetric shape of a prolate vesicle (Ca = 10). Snapshots in the X -Z plane (top row) and X -Y plane (bottom row), corresponding to dimensionless heights of h R 0 = 1.6; 2.3; 3.5; 5


Meritxell Genovart
email: m.genovart@csic.es

Roger Pradel

Transience effect in capture-recapture studies: the importance of its biological meaning

Keywords: Capture-recapture, Capture effect, Cost of first reproduction, Demography, Multievent, State-Space models, TEST 3.SR, Transients

Capture-recapture (CR) models are an essential tool for estimating demographic parameters in most animal and some plant populations. To avoid drawing incorrect conclusions in any statistical inference, a crucial prerequisite is to assess the goodness of fit of a general model to the data. In CR models, a frequent cause of lack of fit, is the so-called transience effect, which is due to a lower expectation of re-observation of individuals marked for the first time as compared to other individuals. The transience effect may result either from different biological causes or from the sampling procedure.

A transience effect is usually treated by distinguishing at least two age-classes in the survival probability, but other approaches may be more suitable. Here we develop a conceptual and analytical framework for including a transience effect in capturerecapture models. We show the implementation of two additional parametrizations that incorporate a transience effect. With these parametrizations, we can directly estimate the "transience probability" defined as the probability that a newly caught individual disappear from the population beyond what is expected based on the behavior of the previously caught individuals in the same sample. Additionally, these parametrizations allow testing biological hypotheses concerning drivers affecting this probability. Results from our case study show differences between parametrizations, with the parametrization most currently used giving different estimates, especially when including covariates. We advocate for a unifying framework for treating a transience effect, that helps clarifying the ideas and terminology, and where the biological reasons should be the rule for choosing the appropriate analytical procedure. This framework will also open new and powerful ways for the detection and exploration of ecological processes such as the costs of the first reproduction or the deleterious effects of some types of marking.

Introduction

Capture-recapture (CR) models are an essential tool for estimating and analyzing factors driving some demographic parameters (e.g. survival, recruitment, dispersal) in most animal populations and some plant populations [START_REF] Lebreton | Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies[END_REF][START_REF] Kéry | Extinction rate estimates for plant populations in revisitation studies: importance of detectability[END_REF]. As in any statistical inference, in CR models, assessing the goodness of fit (GOF) of a general model to the data is a crucial prerequisite to avoid drawing incorrect conclusions [START_REF] Agostino | Goodness-of-fit techniques[END_REF]. Significant lack of fit of the general model should force to investigate the reasons for this lack of fit and adapt the model accordingly. CR models are based on four key assumptions [START_REF] Lebreton | Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies[END_REF][START_REF] Cooch | Goodness of fit testing at Program MARK: a gentle introduction[END_REF], sometimes known as the 'Cormack-Jolly-Seber assumptions': 1) individual marks are not lost or missed; 2) all samples are instantaneous relative to the interval separating occasions, and each release is made immediately after the sample; 3) every marked animal present in the population at time (i) has the same probability of recapture; 4) every marked animal in the population immediately after time (i) has the same probability of surviving to time (i +1) [START_REF] Pollock | Capture-recapture models: A review of current methods, assumptions and experimental design[END_REF][START_REF] Pollock | Goodness-of-Fit Tests for Open Capture-Recapture Models[END_REF]. For most capture-recapture studies we know or confidently accept that assumptions 1 and 2 are met, and assumptions 3 and 4 are typically the most important in terms of GOF testing [START_REF] Pollock | Goodness-of-Fit Tests for Open Capture-Recapture Models[END_REF]. GOF tests are present as part of the available software programs for capture-recapture data goodness-of-fit testing or in specialized R packages [START_REF] Choquet | U_Care2.2 : User's Manual[END_REF][START_REF] Gimenez | R2ucare: An r package to perform goodness-of-fit tests for capture-recapture models[END_REF]. A violation of one of these tests, TEST 3.SR, is the so called transience effect, meaning that the individuals captured for the first time ('new individuals') have a lower expectation of being re-observed in the future, as compared to individuals of the same sample that had been captured previously ('old individuals') [START_REF] Pradel | Capture-recapture survival models taking account of transients[END_REF]. As an example, in a review of CR studies analyzing data from breeding sites, Oro and Doak [START_REF] Oro | Estimating costs of first reproduction and consequences for local population dynamics in a long-lived species[END_REF] found that a transience effect was detected 37% of the times. In most cases, this heterogeneity in local survival is treated by considering at least two age classes in the survival probability. However, this procedure may not always reflect adequately the underlying biological phenomenon. Even if very rarely used (but see [START_REF] Genovart | Exploiting uncertain ecological fieldwork data with multi-event capture-recapture modelling: an example with bird sex assignment[END_REF][START_REF] Weinbach | Resilience to climate variation in a spatially structured amphibian population[END_REF][START_REF] Cayuela | Population dynamics and age-dependent mortality processes in tropical reptiles[END_REF]), there are other ways to account for a transience effect, which may be more appropriate depending on the biological phenomenon that caused transience, and we may want to approach the issue from different perspectives. With these other parametrizations, we could directly estimate the "transience probability" i.e. the probability that a newly caught individual disappear from the population (die or permanently emigrate) beyond what is expected based on the behavior of the previously caught individuals from the same sample. With these parametrizations we could even test biological hypotheses concerning the external drivers (e.g. climate) and internal factors (e.g. age, sex) affecting this probability. Additionally, terminology around transience is actually ambiguous (Box 1). We think that there is a need to distinguish the statistical transience effect from its potential biological causes, of which the presence of transients as individuals transiting through the area, i.e. non-resident individuals, is only one possibility. Here we expose different possible ways to incorporate a transience effect in capture-recapture models and different biological reasons that may be behind it, and suggest the best analytical way to include this effect in our models in each case. Our main goal is to provide an updated conceptual and analytical framework to deal with transience effects in CR studies.

Conceptual and analytical framework

Biological meanings of a transience effect

A deviation on the TEST 3.SR may result from different biological causes or even relate to the marking procedure. We enumerate below the main biological explanations for the presence of a transience effect in the data.

Age. In many species, survival or permanent dispersal may be different for individuals of different ages, thus if for example individuals are marked as young or juveniles, the lower probability of reencounter of newly captured individuals may be due to an age effect (i.e. juveniles have lower survival or higher permanent dispersal) (i.e. [START_REF] Brown | Survival of Timber Rattlesnakes (Crotalus Horridus) Estimated by Capture-recapture Models in Relation to Age, Sex, Color Morph, Time, and Birthplace[END_REF]). In this case, the terminology "residents and transients" is not adequate. In cases when we are not interested in estimating parameters of younger individuals, we may want to remove the first capture in all capture histories. This simplifies the model (no age effect), at the expense of some loss of information.

True transients. In some cases, some individuals included in our data set may not strictly belong to the study population; they are transiting through the study area. In this case, the terminology "residents and transients" is adequate, as some individuals only seen once are transients, whereas those seen on subsequent occasions are all residents [START_REF] Nott | Demographic monitoring and the identification of transients in mark-recapture models[END_REF]. This may occur at breeding sites (i.e. colonies for birds and mammals or ponds for amphibians) with some captured individuals being breeders (residents) and others just individuals caught transiting the area (transients) [START_REF] Weinbach | Resilience to climate variation in a spatially structured amphibian population[END_REF][START_REF] Schmidt | Apparent survival of the salamander Salamandra salamandra is low because of high migratory activity[END_REF][START_REF] Perret | Transience, dispersal and survival rates in newt patchy populations[END_REF]; another example of the analysis of true transients would be some studies at stopover sites or wintering areas, where residents (i.e. those making a stopover or hibernating) and transients are often present [START_REF] Schaub | Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain[END_REF]. Additionally, for example, when studying a population at its breeding place but not strictly marking individuals when found breeding (e.g. at the nest or burrow), some sampling methods (e.g. birdcalls or decoys) may increase the transience effect by enlarging the number of individuals attracted and not belonging to the study population (e.g. [START_REF] Sanz-Aguilar | Recapture processes and biological inference in monitoring burrow-nesting seabirds[END_REF]). When true transients occur, we would like to estimate our demographic parameters of interest (e.g. survival) without taking into account information from these individuals. We may also be interested in estimating the proportion of real transients in the population, and in determining the factors affecting it.

Marking effect. Also marking an individual may affect it in such a way that it decreases survival or induces permanent emigration. This would be a researcher effect and can only occur when manipulation takes place for marking. For instance, it has been demonstrated that, following the helicopter roundup of moulting adults of lesser snow geese accompanied by non-flying goslings, 25 to 30% of the birds undergoing this likely stressful procedure for the first time emigrate permanently [START_REF] Pradel | Transient animals in a resident population of snow geese: Local emigration or heterogeneity?[END_REF]. Apart from including this capture effect in our models to get reliable demographic parameter estimates, we may also be interested in estimating this transience probability, in this case the probability of emigration or additional mortality due to the marking effect, and in determining the factors affecting it, external and internal.

Cost of first reproduction. Reproduction is costly, especially first time reproduction, and the presence of a link between survival and reproduction is a concept underpinning the theory of life-history evolution [START_REF] Roff | The evolution of Life Histories. Theory and Analysis[END_REF][START_REF] Stearns | The Evolution of Life Histories[END_REF]. When many animals enter the marked population as first-time breeders, a transience effect can indicate a cost of first reproduction. This is because individuals breeding for the first time may have a higher mortality or permanent dispersal probability than experienced breeders (e.g. [START_REF] Tavecchia | Sex-and Age-Related Variation in Survival and Cost of First Reproduction in Greater Flamingos[END_REF]). We may be interested in estimating this transience probability in first-time breeders and exploring the factors affecting the costs of the first reproduction.

Multievent parametrizations

Models dealing with transience can be implemented as Multievent models [START_REF] Pradel | Multievent: an extension of multistate capture-recapture models to uncertain states[END_REF] or equivalently as state-space models [START_REF] Gimenez | State-space modelling of data on marked individuals[END_REF][START_REF] Royle | Modeling individual effects in the Cormack-Jolly-Seber model: a statespace formulation[END_REF]. In the following, we build our models in the Multievent framework, but the same conceptual ideas can be implemented as state space models. Multievent models hold two levels, 1) the field observations called "events" encoded in the capture histories, and 2) the "states" defined to match the biological questions; those can only be inferred.

We describe three general parametrizations that include a transience effect in our models. The first one is the most vastly used approach, and involves including at least two age classes when analyzing survival probabilities ("Transience effect as an age effect"; parametrization A). The second one treats transience as a different individual state, thus, there would be individuals "transients" and others "residents" ("Transience effect as a state"; parametrization B). In this parametrization, individuals are intrinsically transients or residents before and independently of the first observation.

The third analytical procedure analyzes transience as a possible transition for those individuals captured for the first time ("Transience effect as a transition"; parametrization C). In this parametrization, the change of state occurs after this first sampling occasion and may result from the sampling procedure itself (i.e. effect of marking) or from other experiences undergone simultaneously to this first sampling occasion (e.g. effect of first breeding attempt). We describe below the implementation of the three mentioned Multievent parametrizations to account for a transience effect. For all models, the symbols for parameters are: ϕ: Survival probability, Tr: Transience probability, which is context dependent, p: Recapture probability. Several kinds of dependency may be considered on these parameters (e.g. constancy, time dependency, or dependence on individual or environmental covariates). Models are described by means of row-stochastic matrices, i.e. each row contains the parameters of a multinomial distribution. Consequently, the total of cell probabilities per row is 1.

Because of this constraint, one and only one cell probability in each row will be calculated as the complement to 1 of the others. For the sake of clarity, all models are kept as simple as possible; we only correct for a transience effect, we do not consider groups, and, except for parametrization A, we do not consider age effects. But these models can be modified to correct for a trap effect if the GOF testing points to such an effect [START_REF] Pradel | Modeling Trap-Awareness and Related Phenomena in Capture-Recapture Studies[END_REF], and include as much complexity as needed, in terms of states and observations, to answer our research question.

Transience as an age effect (A)

The individual states considered are: Individual alive (A) and dead (D), this last state not being observable. The possible events are: not seen (0) and seen [START_REF] Lebreton | Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies[END_REF]. Even if not essential from a mathematical point of view, from a conceptual point of view, to include a transience effect as an age effect, individuals captured for the first time should be younger that those seen on later occasions (i.e. individuals marked as juveniles). In this parametrization, the initial state, i.e. the state at first encounter of an individual, may only be A.

Initial state:

We have one transition matrix, which models survival probabilities from the state at t (in row) to the state at t+1 (in column):

And the matrix of event probabilities (E) relating states in row and events in columns:

In this parametrization, the transience effect is rendered by considering at least two age classes.

Transience as a state (B)

In this parametrization, transience is seen as a preexisting individual state. The individual states considered are: Individual transient (AT), individual resident (AR) and dead (D), this last state not being observable. In all parametrizations, but especially in this case, the dead state (D) does not necessarily imply the death of the individual but more generally represents the permanent departure of the individual from the study population. The possible events are: not seen (0) and seen [START_REF] Lebreton | Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies[END_REF]. In this parametrization, the initial state, i.e. the state at first encounter of an individual, may be AT or AR.

Initial State:

Transition matrix:

Event probabilities matrix:

As transients, by definition, cannot be re-observed after their initial encounter, detection of transients is intrinsically zero after the first encounter and this must be enforced in the model. In this parametrization, we may test for environmental or individual factors or variables potentially affecting the transience probability by modeling the initial state probabilities. This may serve for instance to detect whether there are more transients at some occasions or under some environmental or individual conditions. However, one may be cautious to note that the proportion of transients among the newly marked, τ, which is what we estimate as Tr, differs in general from the proportion in the whole population, T, the interesting parameter (Fig 1). They differ because of two unrelated features: the relative detectability of transients and residents, and the proportion of the population that is already marked. That τ differs from T is not by itself a problem when one wants to assess the influence of a variety of covariates as long as the relationship is monotonous. Thus, if one may assume that the relative detectability of transients and residents and that the proportion of the population that is already marked do not change, τ is a valid proxy of T. The second point is yet probably untrue during the early years of any study when the proportion marked tends to increase. Pradel et al. [START_REF] Pradel | Capture-recapture survival models taking account of transients[END_REF] have derived a formula relating τ and T, when T, ϕ, and p are constant. This may serve as a basis to decide for how many of the early occasions τ is not a valid proxy of T. [START_REF] Pradel | Capture-recapture survival models taking account of transients[END_REF] also provide the exact formula relating τ and T at each occasion when detectability can be assumed the same for residents and transients; the correction factor is then simply the expected proportion of unmarked in the sample. If samples are large enough, the observed proportions can be used instead and an estimation of T can be implemented.

Alternatively, an independent scan of the population can be used to estimate this proportion.

Transience as a transition (C)

In this parametrization, the individual states considered are: Individual alive (A) and dead (D), this last state not being observable. The possible events are: not seen (0) and seen [START_REF] Lebreton | Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies[END_REF]. In this case, the transition part of the model involves two steps. The first one accounts for the transience probability, i.e. the probability that an individual following its first encounter dies or departs definitively from the study area as a direct or indirect consequence of the initial observation (see 'capture effect' and 'cost of first reproduction' above), and the second one accounting for the survival process of those that remain.

Initial State:

Transition matrices: In this parametrization, we estimate the transience probability at the first step of transitions. In this step, we may test for variables driving transience probability. As the transience effect is specific to the first encounter, the probability of transience at later occasions is equal to zero.

Which parametrization should I use?

To choose the best parametrization to analyze our data we should consider: a) are we interested in these individuals that generate this transience effect? b) do we need to assess factors affecting the transience probability? and when possible c) which is the biological origin of our detected transience effect. Based on that, we are going to choose the most appropriate capture-recapture parametrization (Fig 2). If all individuals are marked at the same age, and there are consequential differences due to age between individuals marked and individuals captured simultaneously but previously observed, parametrization A will be useful. If we rather suspect that some individuals that have been sampled are transiting through our study area and do not belong to our study population (e.g. studies at stopover sites or at breeding sites but not sampling sure breeders), we suggest to choose parametrization B. In case we suspect that some individuals may be affected by the first sampling procedure (e.g. effect of marking) or by other events occurring during their first sampling (e.g. effect of first breeding attempt), we suggest to use parametrization C. If we do not know a priori the biological causes of the transience effect, we suggest choosing parametrization C, because it is the one that would involve the fewest biological assumptions and the most flexible for hypotheses testing.

Decision tree for selecting the best parametrization to render a transience effect in capture-recapture models. Parametrization types are A: Transience effect as an age effect, B: Transience effect as an individual state and C: Transience effect as a transition.

Worked example

As an example, we analyze a long-term data set (1999-2018) of adult Scopoli's shearwaters Calonectris diomedea diomedea breeding at the Aire islet (Menorca, western Mediterranean) (see S1 File on supporting information for details on the study species and study area). We detected a transience effect with the GOF test ( 2 17 χ = 66.42, p<0.001). Given that our database only include adults, first release does not correspond to a specific age, thus parametrization A is not the best option to model the transience effect. Additionally, as we sample individuals breeding at the nest, all our individuals are surely residents. According to Fig. 2, parametrization C is the best analytical procedure here. For the sake of illustration and comparison, we are nonetheless going to present the other two parametrizations.

Under each analytical procedure and based on previous results [START_REF] Genovart | Contrasting effects of climatic variability on the demography of a transequatorial migratory seabird[END_REF], we build different models to test for constancy, time dependency, and to test for an environmental covariate potentially affecting transience and survival probabilities, namely the Southern Oscillation Index (SOI) (http://www.cru.uea.ac.uk/cru/data/soi/soi.dat) (see S1 File on supporting information for the practical implementation of the different multievent modeling approaches). Model selection relies on QAICc (i.e., the Akaike Information Criterion duly corrected for small sample size) [START_REF] Burnham | Model selection and multimodel inference: a practical information-theoretic approach[END_REF].

Results from our case study evidence some differences between parametrizations (Table 1 and2). Parametrizations B and C are identical from a statistical point of view and give us exactly the same estimates (Table 1 and2); they only differ from a conceptual point of view. However, parametrization A estimates a different parameter and, when including covariates, the models differ also from a statistical point of view (Table 1 and2 Previous studies have rejected an effect of marking in this Scopoli's shearwater study with an even more invasive type of marking [START_REF] Igual | Short-term effects of data-loggers on Cory's shearwater (Calonectris diomedea)[END_REF]. Hence, we interpret the transience effect in this dataset as a non-negligible cost of first reproduction. Estimates of mean transience and adult survival probabilities (and 95% Confidence Intervals in parentheses) for Scopoli's shearwaters on the colony of Aire (from models 9, 10 and 11, Table 1). Based on a good knowledge of natural history features of the study system and the biological model used, the biological reasons of the transience effect should be the guide for choosing the appropriate analytical procedure. In our case example we had enough previous information to determine the most plausible biological meaning of the transience effect, however, which is the best choice when the demography of the focal species is poorly known or when multiple causes (e.g., natal dispersal, cost of first reproduction, age-dependent survival) may lead to a transience effect? As suggested, the age of marking should guide us to the first decision; if we do not know the age at marking, or if individuals are marked both as juveniles and adults, we should avoid parametrization A. Then between parametrizations B and C, if we do not have enough information to guess the most plausible biological meaning of the transience effect, we suggest to choose parametrization C, as previously mentioned, and we could attempt to discriminate between biological meanings of the transience effect. For example, we could use a measure of the intensity of manipulation at each occasion or a change in the marking technique at some point for assessing a marking effect; or the comparison of luring or sampling methods to detect the presence of true transients. This framework will help clarifying the ideas and terminology when dealing with a transience effect. Additionally, these parametrizations directly estimate the "transience probability" and allow testing biological hypotheses about factors driving this

probability; this open new and powerful ways for the detection and exploration of factors affecting ecological processes such as the costs of the first reproduction or the deleterious effects of some types of marking in natural populations.

  ). If adult survival estimates obtained from each parametrization are identical, parametrization A gives survival of individuals captured for the first time and parametrization B and C give us the transience probability, i.e. the probability that a newly caught individual disappear from the population (die or permanently emigrate) beyond what is expected based on the behavior of the previously caught individuals from the same sample. Thus caution should be made when choosing between each analytical procedures, especially between parametrization A versus B or C.

Fig 1 .

 1 Fig 1. Transients proportion in a sample and a population under a capture-

Fig 2 .

 2 Fig 2. Decision tree for selecting the best parametrization to include a transience

Table 1 . Model selection and hypothesis tested Model Param. Transience Survival Recapture np Deviance QAICc QAICc

 1 Model selection and hypothesis tested with Multievent modelling. Param.= Parametrization type: A:Transience as an age effect, B: Transience as an individual state and C: Transience as a transition; np = number of parameters; w i = weight of model i. a1=individuals seen for the first time; SOI=Southern Oscillation index as a covariate; t=time varying; ctant=constant over time. QAICc: Akaike information criterion corrected for small sample size; DeltaQAICc: the QAICc difference between the current model and the one with the lowest QAIC value; w i : Akaike's weight of the model. The models best fitting our data are shown in bold.

	w i

Table 2 . Mean transience and adult survival probabilities.

 2

1

  Transience in parametrization A is not the transience probability, but the survival probability of all individuals captured for the first time.We provide here an updated framework for including a transience effect in capturerecapture models. Even if the implementation is detailed for working in the Multievent framework, the same conceptual ideas can be implemented as state space models.We advocate for treating transience as an age effect (parametrization A) only in those cases where there is evidence that the transient effect is due to a difference in age of individuals captured for the first time. Otherwise, and especially in those cases where we want to infer factors driving the transience probability, we strongly encourage the use of parametrizations B and C, treating transience as an individual state or as a transition. Parametrizations B and C are identical from a statistical point of view and will give us exactly the same estimates. However, we suggest the use of parametrization

		A	B	C
		transience as an	transience as an	transience as a
		age effect	individual state	transition
	Transience 1	0.66 (0.57-0.73)	effect 0.28 (0.19-0.38)	0.28 (0.19-0.38)
	Adult survival	0.90 (0.88-0.92)	0.90 (0.88-0.92)	0.90 (0.88-0.92)
	Discussion

B if we suspect the existence of "transients and residents" in our study sample, and parametrization C on the other scenarios.

Acknowledgments

We are very grateful to Raül Escandell for his fieldwork in Aire islet and to all people helping with fieldwork over the years. We thank the Regional Balearic Government for permits to access the study colony. We would also thank the Editor Benedikt R.

Schmidt, Hugo Cayuela and another anonymous referee for providing helpful and constructive comments to improve the manuscript. We dedicate this work to the memory of our close friend and colleague Lluís Jover.


Anna Bon
email: a.bon@vu.nl

Jaap Gordijn

Hans Akkermans

Victor De Boer

André Baart

Cheah Wai Shiang

Sze San

Community-centered, Project-based ICT4D Education in the Field

Keywords: ICT4D education, Community Service Education, Information System development life cycle, Context, User-centered approach, Ecosystem awareness, Sustainability

The growing demand for user-centered, sustainable, collaborative and ecosystemaware ICT4D programs and projects brings requirements for education of a new generation IC-T4D professionals. This paper presents a post-graduate field-based course in ICT4D that teaches how to co-create and deploy community-centered ICT services. The course has been jointly organized and deployed by Vrije Universiteit (VU) Amsterdam and Universiti Malaysia Sarawak (UNIMAS) for a mixed group of computer science, information science and artificial intelligence students from VU and UNIMAS. The course design, experiences, outcomes and evaluation are presented in this paper.

1

The need for skilled, hands-on ICT4D professionals "Design with the User", "Collaborate", "Understand the Existing Eco-system", and "Build for Sustainability" are the new imperatives, set by the international donor community for the ICT4D practitioners community. Formulated as the new "Principles for Digital Development" [START_REF] Waugaman | From Principle to Practice: Implementing the Principles for Digital Develop-ment. The Principles for Digital Development Working Group[END_REF] these requirements mark a trend towards more agile, usercentered, and innovation-oriented approaches in ICT4D projects and practice (e.g. [START_REF] Hippel | Democratizing Innovation: The Evolving Phenomenon of User Innovation[END_REF][START_REF] Dingsøyr | A decade of agile methodologies: Towards explaining agile software development[END_REF][START_REF] Gottesdiener | Rational Software White Paper[END_REF]). The trend results from the concerns about high failure rates in donorfunded ICT4D projects, especially where projects target the poorest populations and regions [11]. However, bringing agile, collaborative, user-centered approaches into practice in poor low-resource development environments is a challenging endeavor. Low resource, development regions may have a lack of (physical, digital, energy) infrastructures. There may be high illiteracy rates, low purchasing power or a variety of other complex social, economic, cultural and environmental factors [START_REF] Gyan | The Web, Speech Technologies and Rural Development in West Africa, An ICT4D Approach[END_REF][START_REF] De Boer | Radiomarché: Distributed Voice-and Webinterfaced Market Information Systems under Rural Conditions[END_REF]. In recent years, new, user-centered approaches to ICT4D and Digital Development have emerged that use state-of-the-art concepts, theories and methods from e.g. agile Information Systems (IS) engineering [START_REF] Doerflinger | A software development methodology for sustainable ictd solutions[END_REF][START_REF] Ferrario | Software engineering for 'social good': integrating action research, participatory design, and agile development[END_REF][START_REF] Haikin | A Framework to Assess Participation and Empowerment Impacts of ICT4D Projects[END_REF][START_REF] Bon | Developing ICT Services in a Low-Resource Development Context[END_REF]. These approaches can guide operational, socio-technical ICT4D developments in challenging and demanding development contexts. However, bringing with these frameworks into practice requires field experience and social, technical and organizational skills. Where do ICT4D professionals obtain these skills and knowledge?

Currently, ICT4D education at graduate or post-graduate levels is oriented at social, economic, development or policy studies (see e.g. a recent textbook on ICT4D [START_REF] Heeks | Information and Communication Technology for Development (ICT4D). Routledge[END_REF]). However, these ICT4D curricula do not train ICT4D professionals for fieldbased information systems/requirements engineering under complex, real world conditions. Technical studies such as computer science, artificial intelligence and information science have traditionally focused on technology development for the "wealthy", connected world, and have not yet included ICT4D in their curricula. Recent trends in ICT4D policy and technological innovation are demanding a new type of ICT4D education, that caters for social, technical, practical and organizational knowledge and delivers professionals with hands-on skills and a reflective attitude.

In this paper we present design and implementation of a new type of communitycentered, project-and field-based ICT4D education. The paper is structured as follows: in section 2 we discuss the theoretical framework to be used for course design. In section 3 we outline the course objectives, structure and assessment criteria; in section 4 we illustrate this with a case of a field-based post-graduate ICT4D course which was deployed in June 2018 at the Universiti Malaysia Sarawak; in section 5 we discuss the evaluation and the outcomes and give recommendations and perspectives for further adjustments and deployments.

Theoretical frameworks for practical and societal ICT4D education

In terms of course design, an appropriate theoretical framework for designing and rolling out community-oriented, field-based ICT4D education is (Community) Service Learning (CSL) [3]. CSL is designed to connect theory and practice and meet challenging social problems while educating students for a life as responsible citizens [3,[START_REF] Duffy | Service-learning in engineering: What why and how? age[END_REF]. CSL uses an experimental learning methodology that integrates meaningful community service with instruction and reflection to enrich the learning experience, learn civic responsibility, encourage civic engagement and strengthen communities for the common good [START_REF] Lopez | Participatory learning process through ict4d projects[END_REF]. Moreover, CSL is useful for field-based operational IC-T4D, as it combines two different (but related) goals: (i) an educational goal based on learning-by-doing and reflection (ii) a societal goal to serve the community, by cocreating meaningful solution.

For the course content, we used ICT4D 3.0 as theoretical framework. This framework matches the Community Service Learning philosophy, fits a learning-by-doing approach and offers a methodology how to "do" socio-technical development in the field [START_REF] Bon | Developing ICT Services in a Low-Resource Development Context[END_REF]. The framework is displayed in Figure 1 as an intention-strategy map [START_REF] Rolland | Capturing System Intentionality with Maps[END_REF]. It consists of five components that cover the complete information system (IS) engineering life-cycle : (i) context analysis, (ii) needs assessment, (iii) use case and requirements analysis, (iv) sustainability assessment, (v) engineering, deploying, evaluating. ICT4D 3.0 differs from policy-oriented frameworks, as for example ICT4D 2.0 (e.g. [START_REF] Heeks | ICT4D 2.0: The next phase of applying ICT for international development[END_REF]), in its practical, hands-on, grassroots approach.

Designing a post-graduate course: ICT4D in the Field

Based on the theoretical frameworks (i) CSL for education, and (ii) ICT4D 3.0 for course content, we designed a post-graduate course in ICT4D. This consisted of setting course objectives, defining content, methodology, admission criteria and assessment method. The course design is evaluated by its implementation.

Course objectives

Based on the above considerations of section 1, we formulated the course objectives as:

─ to make the next generation of information and computing scientists aware of the potential role of ICTs for the developing and emerging world, with a strong appreciation for the highly diverse and complex contexts (in contrast to a one-sizefits all-approach), social-cultural factors and human needs that must be addressed; ─ to equip students with relevant field research and development methods and skills to develop technologies in a (poor) rural/suburban community/developing region; ─ to acquire and reflect on the experience of carrying out a full life-cycle of a realworld software development project in the field, learning to be able to deal with unfamiliar and complex contexts, and engage with communities with their specific contextual constraints, needs and goals.

Course structure

Based on the time available (a 6 ECTS course equals one month of full time education), the course takes 4 weeks for field research and designing, modeling, engineering, testing and deploying an information system, that must serve local community's needs.

The course comprises lectures, field visits to (rural or sub-urban) communities where the envisaged users live and work. Interviews and focus group discussions, user test and feedback sessions are part of the course assignment(s). The course starts with lectures in which important topics are reviewed: (i) use case, context and requirements elicitation and analysis; (ii) conceptual modelling of information systems; (iii) selected technical aspects of ICT4D projects; (iv) value modelling and economic sustainability analysis; (v) guidelines and protocols for interviewing and focus group discussions with local communities. Based on meetings with user groups and context analysis in the field, students form teams (of 4-6 persons). Each team selects a relevant use case to elaborate, build a prototype, test with the users, evaluate, improve and deploy. Sustainability analysis and value modeling are part of the assignment. Students work in self-organizing teams, dividing tasks and working closely together. Users must be involved to make sure their needs and (business) requirements are met. The course involves reflection and open dialogue. The constraints and opportunities of the real world context are taken into consideration. The students interact and receive daily feedback from the lecturers. At the end of the course the results of the project are presented by the student teams to the users, local experts and other stakeholders, during an official symposium.

Assessment

Assessment of the student work is based on four deliverables: (i) a personal (individual) reflection about the course and the student's role in the process, and what he/she learned from it. Each team delivers a: (ii) a working information system/app, tested and validated by key users, documented and available as Open Source (group work);

(iii) a group presentation/pitch during the end conference; (iv) a technical report containing the following items: ─ Context description; ─ High level system design and user scenario; ─ A justification of the project (short); ─ Interviews typed out (not necessarily literal), containing relevant info (e.g. all user and business requirements, key points, important details etc.); ─ A use case and analysis report according to the structured narrative format including a stakeholder analysis, a system architecture, information concepts (activity diagram, class diagram, user interaction diagram, deployment diagram), summary of requirements in MoSCoW [START_REF] Clegg | Case method fast-track: a RAD approach[END_REF] method terms; fidelity. ─ A sustainable value model/multiple scenarios using the e 3 value method [START_REF] Gordijn | Value based requirements engineering: Exploring innovative e-commerce ideas[END_REF] for quantitative and qualitative assessment of sustainability; ─ Report of user tests (preferably two cycles); ─ A reflection of the (iterative) process, and user interactions with the group (how were the interviews); ─ A discussion section on the outcomes and what aspects of the project need further research.

To be admitted to this course students must dispose of technical (programming, modeling, requirements engineering) skills and social/communication skills. Moreover, a specific attitude is expected as well: openness to other cultures, willingness to collaborate in an interdisciplinary team, a hands-on mentality, a social orientation and a reflective nature.

4

The case of "ICT4D in the Field" in Sarawak, Malaysia

The post-graduate course "ICT4D in the Field" was designed, implemented and evaluated as a joint educational program by Vrije Universiteit Amsterdam (VU) and Universiti Malaysia Sarawak (UNIMAS). The initiative was built on previous experiences. VU was already offering a (6 ECTS) classroom-based ICT4D course in the master tracks of computer science, information science and artificial intelligence, without field work, and running an interdisciplinary research program on ICT4D since 2009. UNIMAS was engaged since 1999 in ICT4D research projects to connect remote communities in Sarawak: eBario and Long Lamai [START_REF] Songan | Implementing information and communication technologies for rural development: Lessons learnt from the ebario project[END_REF]. Both universities were committed to implementation of Community Service Learning in their educational curricula.

In June 2018 eleven master students computer science, information science and artificial intelligence from VU joined a group of ten computer science students from UNIMAS in Sarawak, Malaysia for a one month ICT4D project-based field course, by a joint VU/UNIMAS lecturers team. The course was hosted at the UNIMAS campus. The students followed classes, did group assignments and participated in community visits. The course was organized according to the framework ICT4D 3.0, starting with a general context analysis. This led to the definition of three student projects in which each project proceeded with the next steps of the framework (see Figure 1).

Context analysis

The project started with a joint context analysis by all students and lecturers, to become familiar with local environment and eco-systems. The visited a banana plantation and a small banana factory, talked to inhabitants of a sub-urban community or kampung PJ and visited a community primary school. Focus groups discussions and interviews with users led to a list of project ideas. Three mini-ICT4D student projects were selected, based on (i) relevance for the community and (ii) technical feasibility for an ICT4D student project. The projects, dubbed BannaTree, Appong and EDUCOMX are briefly described in the following paragraphs. The full reports are available at https://w4ra.org/student-papers/.

BannaTree student project

During the context analysis field visits the students learned that the government of Sarawak wants to increase income and standards of living of banana contract farmers by providing support to banana chips factory and improving work conditions of the farmers.

Elicitation of needs:

According to the interviews with farmers and other experts, banana crop yields are poorly planned. The farmers plant and harvest bananas called pisang sekaki. The bananas are sold to small local factories where they are processed as chips and packed for (international) export. Lack of information on timing and amount of expected crop yields leads to planning problems for the factory, waste of bananas for the farmers and supply inconsistencies for the banana chips export. To improve co-ordination and logistics between farmers and chips factory, the students proposed an information system with a mobile users interface: the BannaTree application.

Analysis of use cases and requirements:

The information system and information processes were modeled by the students according to the users and business requirements they collected. To understand the complete use case and elicit the exact requirements for the system they modeled the complete work process and information flows. Many discussions were held with key experts and users to check the validity of the requirements. Much attention was given by the students to stakeholder analysis, and to understanding their operational goals. The proposed model was discussed with the banana farmers, representatives of the banana factory, fertilizer experts, the agricultural department and a key expert from UNIMAS. The discussions led various times to changing requirements and two major redesigns of the system in one week.

Engineering, deploying, evaluating:. The first working prototype of the BannaTree application was delivered in the third week of the course to two local experts. See https://github.com/aoelen/banna/blob/master/README.md for source codes and documentation.

Sustainability analysis and reflection:

The students became aware of local ecosystems and the complexity of (sometimes opposite) interests from various stakeholders on this use case. The requirements changed several times, as users changed their ideas. This made time to finalize and fine-tune the application a stress-full activity for the days before deadline. The team learned about the complexity of the use case and discussed their own roles as reflective practitioners. The project was ready in time, but the final delivery test to the banana farmers was unfortunately cancelled due to time constraints. Long-term impact and sustainability could not be evaluated in a real world setting due to the time constraint. An important lesson learned here is that more time than the available 4 weeks is needed for a project with a high level of complexity. The learning experience for the students was positively evaluated.

The Appong student project

The second use case, dubbed Appong, consisted of a need of (independent smallholder) sugar palm farmers, to increase the sales of their product: Gula Apong.

Elicitation of needs:

From interviews with the kampung inhabitants, in the first course week, the students learned about Gula Apong, a traditional Malaysian sweet/sugar, produced from the Nipa palm by family farms in the mangrove forests along the estuaries of the Sarawak river. The Nipa juice is collected manually and produced (boiled) locally, in the forest. This work is challenging: there are ants and snakes in the forest. When the tide is high, the mangroves are a dangerous place for the farmers, risking crocodile attacks. The government wants to support the small producers, because Gula Apong producers protects the mangrove forests, which are environmentally protected areas of important biodiversity. Moreover, there is a growing consumer market for Gula Apong which needs further exploitation.

Analysis of use cases and requirements:

The Appong use case and project was tackled by a mixed group of students from VU and UNIMAS. The group held interviews at the Ministry of Entrepreneurial Development (MIED) in Sarawak, and visited Gula Apong farmers in the forest. They saw how the farmers collected Nipa juice and observed the production and packaging of Gula Apong at home. They visited the farmers' homes, to get to know the local livelihoods. A mobile app named Appong was proposed. It will serve three types of users: (i) potential customers to help increase sales; (ii) producers, to stimulate cooperation between producers; (iii) the local government. The government can support the farmers, in return for their production data, by providing them rights to harvest in the public mangrove forests.

Engineering, deploying, evaluating: After requirements elicitation a prototype was built. It allows Gula Apong producers to enter information about offerings and sales locations. A web interface was designed for the government, to monitor the data on overall sales activities of Gula Apong in the region. The prototype was presented by the students for evaluation at the Ministry of Industrial and Entrepreneurial Development in Sarawak.

Sustainability analysisand reflection:

The economic feasibility of the Appong application in an ecosystem is assessed using the e 3 value methodology [START_REF] Gordijn | Value based requirements engineering: Exploring innovative e-commerce ideas[END_REF]. The e 3 value method maps the actors (or market segments) in a value network, the value they exchange with each other, and calculates for each actor the income this generates. This model can evaluate quantitatively the feasibility of a complete value network.

4.4

The EDUCOMX student project

During interviews with parents, school teachers and children in Kampung PJ, the ICT4D students were told that, although English is widely spoken in urban areas of Malaysia, in poor rural areas English education at primary schools lags behind the ur -ban (private) schools. Since school subjects such as science and math are taught in English, poor knowledge of English limits education of rural children. Elicitation of needs: Learning English is of key importance for children in the kampung, according to parents and school teachers who were interviewed. Since educational resources at public schools are not sufficient, alternative learning methods are sought. The students propose a mobile Smartphone app, (as most of the families here in the kampung own a Smartphone, despite a lack of Internet connectivity). The app, built by a mixed group of VU and UNIMAS students is dubbed EDUCOMX. The app can be run on Smartphone or in any browser. UNIMAS has deployed a number of XO laptops (see Figure 3) for user tests of this application in the kampung.

Use case and requirements analysis:

The students decide to build EDUCOMX as a pilot mobile app to teach children English. They try to make the app engaging, while staying in the scope of the English learning methods used at school. For the use case and requirements analysis, the students meet a group of children in the kampung, and ask them to draw their favorite superheroes and write down their hobbies and aspirations, as inspiration for the artwork that will be used for the reading material of the app. This process has great significance in context analysis -as taught by the ICT4D 3.0 framework -user interviews and stories are often deciding factors in the success of an application deployed in the field.

Engineering, deploying and evaluating: For EDUCOMX, gamification methods are used, as a game will unconsciously help improve your English. The design looks playful and includes animations, sounds, fun colors and images. A scoring system is implemented. Users (children) can continue through chapters that look and feel like game levels, which they have to complete in order continue to the next level. Status bars show user progress and scores. User testing with children was a central activity of this project. After one week of building and lab testing, the first user tests for the prototype were carried out in Kampung PJ with a group of 21 children in the age range of 10 -12. The children were able to freely play with the application, in the style of a living lab see Figure 3. The children liked the concept of the comic books and started to read the sentences aloud so it was good to hear their reading skills. The questions at the end were a bit difficult for them. We noticed that there were too many pages before the quiz started so after some pages they were having a hard time concentrating on reading. The children seemed to enjoy unlocking new chapters and were motivated to get all questions correct. They started to share answers with other children to help them unlock new chapters. They liked the sound effects of answering the questions. We noticed that the return button and the logout button were at a wrong place on the screen and should be moved.

Sustainability and reflection:

There are two main contextual issues here, that influence design decisions: (i) connectivity and (ii) specificity. First, in rural areas there is no (consistent) internet access, making online content not a design option. Second, the now existing digital learning platforms are not tailored to the local culture or specific wishes of the end users (the children). The user tests were the central activity of this project and provided much new insights. The students learned how to make design decisions based on context and user requirements. The sustainability analysis revealed the need for further exploration of the use case. This will be followed up by UNI-MAS.

Project results

The three student groups delivered the reports and software in time and presented the projects at a community ceremony at Kampung PJ and at a final ICT4D conference at UNIMAS. Personal reflection reports were submitted. All students passed the assessment. A short video clip on the making-of the student projects can be seen at https://w4ra.org/2018/06/28/a-living-lab-in-kampung-pinggan-jaya-sarawak-shortclip/.

Evaluation and conclusion

In line with CSL principles, evaluation of the ICT4D course is done along two axes: (i) did the course achieve its educational goals? (ii) are the student projects carried out in a community-centered way and do the results meet local needs? To evaluate if (i) the educational goals were met, we evaluate if this course makes the next generation of information and computing scientists aware of the potential role of ICTs for the developing and emerging world, with a strong appreciation for the highly diverse and complex contexts, social-cultural factors and human needs. Based on the student assessments, the reflection reports and the group deliverables, the course "ICT4D in the Field" has achieved its educational and societal goals. The students have collaborated sufficiently with the users, learned from the context, and worked iteratively, testing and improving the technological solution according to users' requirements.

To evaluate if (ii) the project is community-oriented and context-sensitive, we assess the student projects and the number of interactions and design decisions, based on local requirements. Concerning effectiveness and results, it is clear, in terms of serving the community, that one month is far too short to do a full context analysis, engage with the users, build a long term relationship and deploy and test a working information system. In terms of ICT4D 3.0, this was only a first cycle deployment including a number of sub-iterations. It is important to foster long-term partnerships and work with local partners. The long relationship between UNIMAS and the local communities is key in the successful collaboration. The collaboration between VU and UNIMAS is characterized by commitment and trust.

A great challenge for long-term sustainability of the course is funding. Organization and planning of a course between two continents is an interesting experience, which takes efforts and time. For the lecturers the lecturing effort is bigger than in regular courses. In terms of community service orientation, the project is a first step towards deployment of sustainable ICT4D solutions. Continuation of the project will be needed to build up long lasting relationships with the communities and achieve long-term sustainability. The student reflection reports show that the whole experience of designing, building, deploying

Fig. 1 .

 1 Fig. 1. ICT4D 3.0 Framework how to build ICT services in low resource environments using a collaborative, adaptive, iterative approach [2].

Fig. 1 .

 1 Fig. 1. Use case analysis for the Appong student project

Fig. 1 .

 1 Fig. 1. Testing the EDUCOMX app with children in the kampong. EDUCOMX is platform independent. Here it is deployed on XO laptop from OLPC.

Acknowledgements

The authors are grateful to Associate Professor Dr. Johari Abdullah, Dean of the Faculty of Computer Science and Information Technology from UNIMAS and Frans Snijders, Director of the International Office of VU Amsterdam for their great support to the project. We thank the community leader and inhabitants of Kampung Pinggan Jaya for their hospitality and collaboration. We thank the students Amir AziziMusa, Aron van Groningen, Ludwig Hoon, Kuan Huiggy, Giorgi Kikolashvili, Allard Oelen, Hameedat Omoine, Judith Schermer, Anastasios Sidiropoulos, Deva Ramakrishnan, Linh Tran, Chris Valladares, Nip van Wees for their work. We thank Leeuw van Moerkerken for filming and producing the video clips of the project. This project is part of the W4RA research program.