Hubert R Klein
email: hubert.klein@univ-amu.fr

Matthieu Rocheron

Christian Curtil

FM-AFM with hanging ber probe for the study of liquid / liquid interfaces

    Copyright FM-AFM with hanging fiber probe for the study of liquid / liquid interfaces

Introduction

Understanding and engineering of liquid interfaces has interested both fundamental science and industry for decades. [START_REF] De Gennes | Capillary and Wetting Phenomena[END_REF] Interfaces between two immiscible liquids are exploited to control nucleation [START_REF] Grossier | Predictive Nucleation of Crystals in Small Volumes and Its Consequences[END_REF] and crystallization, [START_REF] Bonnett | Solution crystallisation via a submerged liquid liquid phase boundary: oiling out[END_REF][START_REF] Galkin | Control of protein crystal nucleation around the metastable liquidliquid phase boundary[END_REF] for droplet generation in microuidic devices, [START_REF] Doufène | Microuidic Systems for Droplet Generation in Aqueous Continuous Phases: A Focus Review[END_REF] and 1 for asymmetric chemical synthesis. [START_REF] Hassan | Droplet Liquid/Liquid Interfaces Generated in a Microuidic Device for Assembling Janus Inorganic Nanohybrids[END_REF] They also serve as templates for the self-assembly of nanoparticles. [START_REF] Turek | Plasmonic Ruler at the Liquid Liquid Interface[END_REF] Many of these applications manipulate small volumes, which means that liquid interfaces need to be probed at a local scale. Atomic Force Microscopes (AFM) are undoubtedly good candidates for the job.

Since their invention, AFMs have been widely used in liquid media. For example, they have opened the way to the exploration of wetting and capillarity on a local scale, [START_REF] Xiao | Investigation of Humidity-Dependent Capillary Force[END_REF][START_REF] Dupré De Baubigny | Shape and Eective Spring Constant of Liquid Interfaces Probed at the Nanometer Scale: Finite Size Eects[END_REF] of surface forces between a colloid and a liquid interface, [START_REF] Hartley | Surface Forces and Deformation at the Oil Water Interface Probed Using AFM Force Measurement[END_REF] of the structure of liquids close to a surface, [START_REF] Zhmud | Evaluation of Surface Ionization Parameters from AFM Data[END_REF] or even the imaging of living cells. [START_REF] Haberle | Force microscopy on living cells[END_REF] During the last decade, they have been applied to the imaging of bubbles, drops, [START_REF] Uddin | Novel Characterization of Microdrops and Microbubbles in Emulsions and Foams Using Atomic Force Microscopy[END_REF][START_REF] Munz | Size Dependence of Shape and Stiness of Single Sessile Oil Nanodroplets As Measured by Atomic Force Microscopy[END_REF] nanoparticles or self-assemblies at the interface of two liquids, [START_REF] Costa | Real Space Imaging of Nanoparticle Assembly at Liquid -Liquid Interfaces with Nanoscale Resolution[END_REF] but also to the study of the wetting dynamics of coated air / water interfaces. [START_REF] Guo | Wetting Dynamics of a Lipid Monolayer[END_REF] At the same time, several teams have successfully studied the possibility of using an AFM as a microrheometer. 1719 Working in a liquid environment greatly reduces the sensitivity of standard oscillating AFM probes. The probes have to be completely immersed, which results in over-damping of the oscillator. [START_REF] Uddin | Novel Characterization of Microdrops and Microbubbles in Emulsions and Foams Using Atomic Force Microscopy[END_REF][START_REF] Munz | Size Dependence of Shape and Stiness of Single Sessile Oil Nanodroplets As Measured by Atomic Force Microscopy[END_REF] Moreover, their tapered shapes hamper the interpretation of rheological measurements. This led to the development of near constant diameter dedicated probes for AFM rheological measurements, in the form of a whisker, [START_REF] Mechler | Nanoscale velocity-drag force relationship in thin liquid layers measured by atomic force microscopy[END_REF] or hanging cylindrical ber. [START_REF] Xiong | Development of an atomic-forcemicroscope-based hanging-ber rheometer for interfacial microrheology[END_REF] These oscillating probes require only the whisker or the ber to be immersed in the liquid, which solves the problem of over-damping. Sensitivity can be further improved by using quartz tuning forks (QTF) as oscillators instead of standard AFM cantilevers.

The use of quartz tuning forks as force probes was envisaged very early on by the pioneers of AFM. 21 QTFs are high quality factor (Q 10 4 ) and sti (k 10 4 N.m -1 ) oscillators. The quality factors of these oscillators are much higher than for standard AFM probes, which gives them unrivaled force sensitivity as low as a few tenths of pN.Hz -1/2 . Coupled with their high static stiness in the range of 10 4 N.m -1 , 22 this makes them very suitable for work in liquids and/or at soft interfaces. [START_REF] Tamayo | High-Q Dynamic Force Microscopy in Liquid and Its Application to Living Cells[END_REF] Moreover, operated in Frequency Modulation mode (FM-AFM ), they allow independent measurement of the conservative and dissipative parts of the interaction. [START_REF] Albrecht | Frequency modulation detection using high-Q cantilevers for enhanced force microscope sensitivity[END_REF] We report here how a quartz tuning fork (QTF) in a hanging ber geometry (i.e. a cylindrical ber glued to a prong of the tuning fork, and oscillating along its major axis) was used to perform AFM measurements at the interface between two liquids. We describe the dedicated AFM and probes developed for this purpose, and the principle of FM-AFM measurements in liquids. Validation of the technical feasability of this setup is obtained through quantitative rheological measurements in simple uids. Finally, we present the rst results of measurements at the interface between two immiscible liquids, poly-dimethylsiloxane (PDMS) and water.

Experimental Experimental setup

Measurements were performed with a glass ber glued to a quartz tuning fork in a hanging ber geometry, where the ber oscillates along its long axis and is partially dipped in the uid (see Figure 1). The advantage of this method is that the oscillator is not damped by the liquid, allowing precise measurements of the ber's interaction with the liquid. [START_REF] Devailly | Mode coupling in a hanging-ber AFM used as a rheological probe[END_REF][START_REF] Xiong | Development of an atomic-forcemicroscope-based hanging-ber rheometer for interfacial microrheology[END_REF][START_REF] Jai | Wetting an Oscillating Nanoneedle to Image an Air Liquid Interface at the Nanometer Scale : Dynamical Behavior of a Nanomeniscus[END_REF] The QTF probe was used in a home made AFM mounted on an inverted optical microscope (TE2000-U, Nikon). The uid is contained in a liquid cell with a diameter of 30 mm and a depth of 3 mm, the bottom consisting of a glass cover slip changed for each new experiment. The uid sample can be scanned in the three directions using a piezoelectric actuator (P527-3CL, Physik Instrumente) positioned under the liquid cell and operated in closed-loop mode . Scanning is performed by an open source digital signal processing controller (MK2-A810, Soft dB), driven by the GXSM software. [START_REF] Zahl | Open source scanning probe microscopy control software package GXSM[END_REF] The QTF holder (containing a piezo dither), positioned above the liquid cell is mounted on a vertical 22mm piezoelectric slider operated in closed-loop mode (Q521, Physik Instrumente). It oers a bidirectional repeatability of 80nm for controlled long range vertical movements in uids. The QTF is excited mechanically by a piezo dither, in mechanical contact with its base, and its oscillation amplitude is measured using a dierential voltage amplier (DLPVA-100, 40dB gain, 100kHz bandwidth, Femto) to ensure an optimal signal to noise ratio. [START_REF] Jahncke | Choosing a preamplier for tuning fork signal detection in scanning force microscopy[END_REF] Furthermore, the 1TΩ input impedance of this amplier eliminates the need to compensate for the stray capacitance of the tuning fork, [START_REF] Grober | Fundamental limits to force detection using quartz tuning forks[END_REF] as virtually no current ows through it. The AFM is operated in the FM-AFM mode using a phase-locked-loop device (PLL, HF2LI, Zurich Instruments) driven by the LabOne software. In this mode, the QTF oscillates at its resonance frequency, and the frequency shift ∆f with respect to the natural resonance frequency in air is monitored. A proportional-integral-dierential (PID) closed loop is used to modulate the excitation voltage A ex sent to the piezoelectric dither, in order to maintain the amplitude of oscillation constant. Monitoring the A ex signal gives access to the dissipation of the oscillator (see FM-AFM measurements section for details).

The whole setup is operated on an optical table in a basement, to ensure minimum mechanical vibrations and temperature variation. It enables the probe to be positioned above a liquid interface, and allows control of the dipping depth relative to the air-liquid interface level. The setup also allows independent measurement of the conservative and dissipative parts of the interaction between the ber and the uid.

Building and calibration of the FM-AFM probe Our hanging ber probe was produced from a quartz tuning fork. The quality factor is dened as Q = f 0 /∆f where f 0 is the resonance frequency of the QTF and ∆f its full frequency width at half maximum (which we will note FWHM in the following) of the tuning fork amplitude < z > at resonance.

The aluminium can containing the QTF, sealed under residual vacuum, was rst cut suciently to make the prongs of the tuning fork accessible, using a watchmaker's lathe.

The ber, a pure silica optical ber (initial diameter 125µm) was chemically thinned down in 30% hydrouoric acid to the desired radius, [START_REF] Kbashi | Fabrication of Submicron-Diameter and Taper Fibers Using Chemical Etching[END_REF] typically r = 5µm for this study. The thinned ber was glued to the top of one prong of the QTF and perpendicular to it (see the micrography of gure 1) using a UV curing glue (Nordland optical adhesive 81). It should be noted that the glue is applied at a temperature of 5°C, this gives it the texture of a gel during application and prevents the glue from wetting the entire ber and thus modify its surface.

The ber was then cut to the desired length (typically 5mm). To avoid a strong decrease in the quality factor of the oscillator, the masses of the two prongs need to be be as similar as possible; a glue droplet was therefore deposited at the top of the other prong of the QTF. All these operations were performed using micromanipulators under a binocular microscope. This protocol typically led to QTF probes with a resonance frequency in air of around 32 kHz, and a quality factor in the range of 10 4 .

Using a QTF probe as a force probe require determining not only its spring constant, but also its electrical sensitivity, so as to control the oscillation amplitude. This was because we were measuring the piezo-electrical response of the quartz to the oscillation amplitude of the QTF prongs. The QTF was modeled here as two coupled oscillators to avoid underestimation of the spring constant. The eective spring constant k ef f , and eective mass m ef f were estimated by a geometrical analysis of the prong's dimensions and by measuring of the frequency of the in-phase (f i ) and anti-phase (f a = f 0 , the resonance frequency) oscillation modes of the QTF, as described by Castellanos-Gomez et al. [START_REF] Castellanos-Gomez | Dynamics of quartz tuning fork force sensors used in scanning probe microscopy[END_REF] The electrical sensitivity α was then determined by measuring the thermal noise spectrum of the bare QTFs. These procedures are described in the Supplementary information, and the results are summarized in Table 1 for the two kinds of QTFs used in this study. We used QTFs in the form of radial cylinders measuring 8 x 3mm (QTFA), and 6 x 3mm (QTFB) respectively. 2.26(±0.11).10 -6 1.12(±0.06).10 -6 α (m.V -1 ) 4.9(±0.7).10 -8 6.9(±1.1).10 -8

Materials

Liquid poly-dimethylsiloxane (PDMS) trimethylsiloxy terminated,

(CH 3 ) 3 SiO(Si(CH 3 ) 2 O) n Si(CH 3 ) 3
was purchased from Alfa Aesar. It was used as obtained: molar mass 1250 g.mol -1 , density 0.935 and kinematic viscosity 9-11cSt at 25 • C.

The water used for this study was deionized with a resistivity of 18MΩ.cm.

The QTFs were watch quartz crystals, of nominal resonant frequency 32768 Hz. We used radial cylinders measuring 8 x 3mm (QTFA) and 6 x 3mm (QTFB) with a 12.5 pF load capacitance , purchased from AEL CRYSTALS ( references X32K768L104 and X32K768L009 respectively ).

The QTF probes were built using single mode bers with a cladding diameter of 125µm and a pure silica core of diameter 3.5µm, dual acrylate coated, purchased from Thorlabs.

The coating was removed by dissolution in dichloromethane.

FM-AFM measurement principle

Quantitative rheological measurements necessarily involve measuring the conservative and dissipative parts of the ber's interaction with the uid. These measurements are obtained by comparing the resonance spectrum of the oscillator, compared to the spectrum in air. [START_REF] Devailly | Mode coupling in a hanging-ber AFM used as a rheological probe[END_REF] Figure 2 shows three resonance spectra recorded in air and at two dierent depths in a 10cst PDMS bath. Compared to the spectrum recorded in air, the resonance frequency undergoes a negative shift when dipped in liquid, and this shift increases with the dipping depth. The ber, while oscillating in the liquid, puts a layer of uid in motion at its periphery.

The thickness of this moving uid layer depends on the viscosity and density of the uid, [START_REF] Tirado | Translational friction coecients of rigid, symmetric top macromolecules. Application to circular cylinders[END_REF] through the skin thickness of a viscous ow induced by an oscillating plane in a liquid: 32

δ = 2η ρω (1)
with η the dynamic viscosity (Pa.s) and ρ the density of the uid (kg.m -3 ), ω the angular frequency (rad.s -1 ).

Since the resonance of the oscillator is given by

f 0 = 1 2π k ef f m ef f (2)
with k ef f and m ef f the eective stiness and mass, the drag of this layer of uid increases the eective mass of the oscillator, and therefore causing a decrease in the resonance frequency.

This decrease in resonance frequency is expected to vary linearly with the length of the ber immersed.

Meanwhile, the FWHM of the spectra increases, indicating increased dissipation, also related to the density and viscosity of the uid.

Both the dragged mass and dissipation are expected to vary linearly with the length of the ber immersed in the uid. Because of the quality factor of the oscillator, acquiring such spectra at appropriate resolution takes a few minutes using a lock-in amplier. We use an alternative method to obtain the same information faster: servo control of the frequency and amplitude of the oscillations. We use a PLL to maintain the oscillator at its resonance frequency, and monitor the frequency shift ∆f with respect to the natural resonance frequency in air f 0 . Under these conditions, the elastic and inelastic responses of the oscillator are in quadrature, [START_REF] Giessibl | Advances in atomic force microscopy[END_REF] and the normalized frequency shift reads

∆f f 0 1 2 ∆k k - 1 2 ∆m m (3)
and gives access to variations in the oscillator's eective stiness and mass. When the ber penetrates a uid, the formation of a meniscus is accompanied by an increase in the stiness of the oscillator. Then, when the ber is moved in the uid, since the stiness of the oscillator is expected to be constant, an added mass per unit length can be calculated using

∆m * = -2m ∆f f 0 * (4)
where ∆f

f 0 *
is the relative frequency shift per unit length. At an interface between two uids, wetting phenomena 1 can result in an additional stiness [START_REF] Jai | Wetting an Oscillating Nanoneedle to Image an Air Liquid Interface at the Nanometer Scale : Dynamical Behavior of a Nanomeniscus[END_REF] that will also modify the frequency shift. A fuller discussion of this point is contained in the section on investigating a PDMS / water interface.

A PID closed loop maintains the oscillation amplitude at a given setpoint by varying the amplitude of the mechanical excitation A ex with respect to the amplitude of excitation in air A 0 . From this observable, we can extract a friction coecient β in the form of [START_REF] Giessibl | Advances in atomic force microscopy[END_REF] :

∆β = β 0 A ex A 0 -1 (5)
with

β 0 = k/ω 0 Q 0 (N.m.s -1
), the intrinsic friction coecient of the tuning fork in air, Q 0 being the quality factor of the oscillator in air, and ∆β = β -β 0 . A friction coecient per unit length β * can be measured as the slope of the friction coecient β's evolution with depth.

Results and discussion

Dragged mass and friction coecient of simple uids

Although the focus of this study was liquid-liquid interfaces rather than high resolution viscosity or density measurements, an important rst step was quantitatively measuring the ∆m * and β * coecients. We thus performed measurements in liquids of various densities and viscosities whose properties are reported in Table 2. ∆m * and β * were obtained following the protocol described in the Experimental section, using two dierent probes : a bered QTFA (f 0 = 32116 Hz, Q = 12950, ber radius r = 4.5µm) and a bered QTFB (f 0 = 32110 Hz, Q = 9600, ber radius r = 5.5µm).

Immersion depth was varied over 250 µm and the vertical speed during measurements was v = 1µm.s -1 . The oscillation amplitude of the ber was set at z = 1 nm. All measurements were performed at 22°C.

To compare the coecients measured, which depend both on ber radius and on the viscosity and density of the uids, we express them as dimensionless parameters, using the model developed by T. Ondarçuhu et al. [START_REF] Dupré De Baubigny | AFM study of hydrodynamics in boundary layers around micro-and nanobers[END_REF] This model revisits the one established by G.K.

Batchelor for a cylinder moving steadily in a viscous liquid. [START_REF] Batchelor | The skin friction of innite cylinder moving parallel to their length[END_REF] Liquid behavior is described by the dimensionless r/δ parameter, r being the radius of the moving cylinder, and δ the skin thickness of the viscous ow dened in the previous section. Dimensionless drag mass C m and friction coecients C β are expressed as

C m = r δ (g 1 (r/δ) -g 2 (r/δ)) (6)
C β = r δ (g 1 (r/δ) + g 2 (r/δ)) (7)
with the function g

g(u) = K 1 [(1 -i)u] K 0 [(1 -i)u] = g 1 (u) + ig 2 (u) (8)
where K n denotes the modied Bessel function of the second kind of order n, g 1 and g 2 being the real and imaginary parts of g.

The drag mass ∆m * and friction coecient β * per unit length introduced in the previous section are then expressed as :

∆m * = 2π η ω C m (9)
β * = 2πηC β (10)
Figure 4 presents the measurements performed on the pure liquids of Table 2 in the form The viscosity of the uids varies over 3 orders of magnitude, and the measurements reasonably reproduce the orders of magnitude of the model. Moreover, the measured values are in good agreement with those reported in the study by T. Ondarçuhu et al, [START_REF] Dupré De Baubigny | AFM study of hydrodynamics in boundary layers around micro-and nanobers[END_REF] recorded using dierent setups at dierent amplitudes and frequencies of oscillation. This conrms the reproducibility of the measurement protocol, allowing us to apply it to the study of the interface between two immiscible liquids. We test our setup on the widely studied PDMS / water interface, using a liquid PDMS with a kinematic viscosity of 10cSt (10 -5 m 2 .s -1 )

Investigating the 10cst PDMS / water interface

In terms of uid properties, an interface between two immiscible liquids such as siloxane and water, can be seen as a discontinuity in viscosity and density. The schematics of Figure 5 depict the expected general shape of the approach / retract curves of frequency shift and dissipation signal.

As the ber approaches the PDMS / water interface (step 1), both the length of the ber immersed in PDMS and the mass of the uid dragged by the ber increase. Accordingly the frequency shift decreases linearly with a slope related to PDMS viscosity.

At the interface crossing (step 2) the water forms a meniscus around the ber resulting in an additional stiness of the system, [START_REF] Yazdanpanah | Micro-Wilhelmy and Related Liquid Property Measurements Using Constant-Diameter Nanoneedle-Tipped Atomic Force Microscope Probes[END_REF][START_REF] Jai | Dynamical behavior of an evaporating nanomeniscus: A boundary condition problem at the local scale[END_REF] and thus a positive frequency shift jump according to equation 3.

When the ber moves in water (step 3), the frequency shift decreases linearly with a slope that depends on water viscosity. During retraction, when the ber moves in water (step 4), the water dragged mass decreases. We thus expect to nd the same absolute value for the slope of the frequency shift as in step 3. Frequency shift approach / retraction curves are slightly separated. This is to account for a possible hysteresis of the contact angle and/or a transient anchoring of the triple line on reversal of the movement.

At some point (step5) the contact line is pinned at the apex of the ber, and the upward motion of the ber causes the contact angle to decrease from its equilibrium value down to zero. When the contact angle has a zero value, the meniscus breaks.

Following this (step6 ), the frequency shift is expected to vary with the decreasing dragged mass of PDMS, up to the surface of the PDMS bath.

The same reasoning can a priori be applied to the dissipation signals, with variations opposite to those of the frequency shift, and obviously dierent values for slopes in the linear portions of the curves. The only expected dierences concern meniscus formation and breaking: the changes in conservative interactions (stiness variations) are not expected to inuence dissipation. However a specic dissipation excess can occur during the meniscus formation / breaking, as a result of discontinuities around the contact line, [START_REF] Huh | Hydrodynamic model of steady movement of a solid/liquid/uid contact line[END_REF][START_REF] Guo | Direct Measurement of Friction of a Fluctuating Contact Line[END_REF] represented on the diagram as a sharp peak.

Here, the measurements were performed on a millimetric drop of water placed at the bottom of the liquid cell lled with PDMS. As a rule, the interface was initially located at a depth of 150 to 200 microns below the PDMS surface. Measurements were made as soon as possible after the drop was deposited, typically within 15 minutes. We used a QTFB sensor, with a ber of radius r = 6.5µm (f 0 = 32132.4 Hz, Q = 7970, β 0 = 2.95.10 -5 N.m -1 .s). The vertical speed of the ber was set to 0.5µm.s -1 . The bandwidth was set to 10Hz and 100Hz respectively for the PLL and the PID loops. The vertical oscillation amplitude of the ber was set at z = 2nm.

Figure 6: Simultaneous measurements of frequency shift and dissipation at the interface between 10cst PDMS oil and water using a QTFB with a ber of radius r = 6.5µm, f 0 = 32132.4 Hz and Q = 7970. Oscillation amplitude is A RM S = 2nm, and displacement occurs at a constant speed of 0.5µm.s -1 . The decrease in frequency shift with the immersion depth is due to the increase in mass of the uid dragged by the ber, while the positive contribution reects the increased stiness of the system arising from the water interfacial tension. The vertical dotted lines mark the depth at which the wetting of the ber occurs during downward motion.

Figure 6 shows typical conservative and dissipative signals recorded during a forward and backward vertical motion of the oscillating ber in the vicinity of the interface. The initial depth of the PDMS / water interface is 180µm for the data presented here. Data are presented in the form of a normalized frequency shift ∆f /f 0 and a friction coecient, β in N.m -1 .s. The zero position on the graph does not correspond to the PDMS / air interface, but rather to the beginning of the signal recording. The curves exhibit the general behavior described by the diagrams in Figure 5.

During downward motion, frequency shift and dissipation signals vary linearly with depth, and the ratio of their slopes in PDMS and water corresponds to the ratio of the square root of the kinematic viscosities of PDMS (10cSt) and water (1cSt) as expected, with 21% and 24% error respectively for frequency shift and dissipation. Fiber wetting is observed at the immersion depth of 64µm, leading to a positive frequency shift. Frequency shifts and dissipation evolve linearly below this point, their slope depending on the viscosity of the water.

As the interface is approached, at an apparent separation of about 10 microns, a positive contribution to the frequency shift is measured, indicating an increase in the eective stiness of the oscillator. As no change in the dissipation signal is observed, it can safely be assumed that this is an elastic eect. While the sudden increase is due to water wetting the ber, we attribute the monotonic variation observed over several microns to the deformation of the oil/water interface. This deformation arises from the hydrodynamic force exerted by the probe on the interface. [START_REF] Davis | The lubrication force between two viscous drops[END_REF] It is accompanied by a variation in the eective stiness of the interface, [START_REF] Horn | The eect of surface and hydrodynamic forces on the shape of a uid drop approaching a solid surface[END_REF] and was described in earlier AFM studies. [START_REF] Hartley | Surface Forces and Deformation at the Oil Water Interface Probed Using AFM Force Measurement[END_REF][START_REF] Aston | Quantitative Analysis of Fluid Interface -Atomic Force Microscopy[END_REF] The deformation caused by a drainage lm between the probe and the interface is interpreted using the augmented Young-Laplace equation, [START_REF] Manica | Hydrodynamic forces involving deformable interfaces at nanometer separations[END_REF][START_REF] Bai | Water Film Drainage between a Very Viscous Oil Drop and a Mica Surface[END_REF] with wetting occurring at the breakdown of the drainage lm. In this regime the amplitude of the displacement of the ber reects the amplitude of the vertical deformation of the interface. Determining the distance between the ber and the interface, i.e. the thickness of the drainage lm, requires a modeling described in ref. [START_REF] Manica | Hydrodynamic forces involving deformable interfaces at nanometer separations[END_REF] It is beyond the scope of this study, but can be estimated to a few hundred of nm.

During the upward motion of the ber, while the ber apex is in the water, the height of the ber immersed in PDMS remains constant. Thus, the slope of frequency shift and dissipation measured is comparable to the slope measured during the downward motion, and again reects the viscosity of the water. The breaking of the water meniscus and the dewetting of the ber result in a negative frequency shift. It occurs at a depth of 36µm, above which, frequency shifts and dissipation continue to evolve linearly, with a slope that reects PDMS viscosity.

We estimate the height of the water meniscus by measuring the distance between the wetting and dewetting events, nding h = 27(±1)µm. Using the expression of James [START_REF] James | The meniscus on the outside of a small circular cylinder[END_REF] for the height of a meniscus on a cylinder, and taking a contact angle value and a capillary length for water in PDMS of respectively 45 • and 2.7mm, [START_REF] Svitova | Wetting Behavior of Silicone Oils on Solid Substrates Immersed in Aqueous Electrolyte Solutions[END_REF] we obtain a meniscus height of 26µm in excellent agreement with our estimation.

Both frequency shift and dissipation approach / retraction curves are slightly separated. This is likely due to a transient anchoring (between 120 and 105 microns) of the triple line on reversal of the movement (and thus a reduction of the contact angle) on defects at the surface of the ber probably resulting from the chemical thinning of the ber. This is accompanied by an excess of dissipation and slightly decreased eective stiness. In addition, the receding contact angle is expected to be lower than the advancing one, [START_REF] Yazdanpanah | Micro-Wilhelmy and Related Liquid Property Measurements Using Constant-Diameter Nanoneedle-Tipped Atomic Force Microscope Probes[END_REF] which also contributes to the decrease in eective stiness observed during the rst part of upward motion.

It should be noted that the dissipation curves do not clearly exhibit excess of dissipation at the interface crossings. This suggests that any specic dissipation processes occuring in the meniscus are below the sensitivity threshold of our probe.

Monitoring the displacement of the PDMS / water interface

We have so far considered that water is not soluble in PDMS for the sake of simplicity. However, while very low, water solubility in PDMS is not zero; it can be estimated to a few ppm by weight. [START_REF] Vogel | Mutual Solubilities in Water-Permethylsiloxane Systems[END_REF] Actually, Grossier et al [START_REF] Rodríguez-Ruiz | Monitoring Picoliter Sessile Microdroplet Dynamics Shows That Size Does Not Matter[END_REF][START_REF] Grossier | A parameter to probe microdroplet dynamics and crystal nucleation[END_REF] showed unambiguously that microdroplets of water immersed in liquid PDMS do evaporate. Attempting to observe this phenomenon, we record numerous approach / retract curves like those in Figure 6. The results of measurements over four hours on the same water droplet are presented in the left part of Figure 7, following the protocol detailed in the previous section, and using the same probe. For greater clarity, only a few representative frequency shift approach curves are plotted, and the curves have been surimposed along the frequency shift axis at the depth of 20 µm to better highlight the displacement of the interface. A vertical piezo slider operated in closed loop, is used to re-position of the ber at the same initial depth with a resolution better than 100nm before each new recording. This depth corresponds to the zero shown for the depth axis in Figures 7 and8. The rst frequency shift curve labeled t 0 is that of Figure 6 , and the others are comparable, with an oset in position and in frequency shift values. Not only do these observations prove the repeatability of the measurements, but the recession of the interface position is clearly visible, and shows a continuous shift to greater depths.

In order to plot the evolution of this position with time, the depth at which the ber reaches the water below the PDMS (as dened in the previous section) was recorded on each curve, with an uncertainty of 1 µm. On the right, Figure 7 plots of the evolution of the interface position with time during the experiment. It varies linearly, so that a linear t yields a recession speed of 1.1(±0.1).10 -7 m.min -1 .

We now check for any changes other than the position of the interface, as both position and ∆f values vary during the experiment. Shifting the approach ∆f curves on the immersion depth axis to superpose the point where the ber enters the water, reveals that the positive frequency shift attributable to water wetting the ber decreases over time (see graph in Supplementary information). This suggests a temporal evolution of the interface properties, likely a decrease in interfacial tension. Interfacial tension is very sensitive to the presence of contaminants at the interface, and such contamination over a duration of several hours in an open environment cannot be ruled out.

Interfacial tension estimation

We measured the interfacial tensions of the PDMS / water system using the pending drop method. The results are summarized in Table 3. The values of the interfacial tensions of water and PDMS are in agreemnt with the published values, and the value of the PDMS/water interfacial tension is median to those of the air/PDMS and air/water interfaces. The interfacial tension of the PDMS / water interface can also be estimated from the frequency shift curves during the upward motion of the ber. During retraction, when the meniscus gets pinned at the apex of the ber, the receding contact angle will decrease down to zero upon breaking of the meniscus, thereby altering the eective stiness of the oscillator.

This evolution is visible on the retraction frequency shift curves shown in the left graph of Figure 8 (at t 0 and t 0 + 240min). It appears as a sharp decrease in frequency shift from the point marked by an arrow, as the ber moves upward upon the water meniscus breaking.

The frequency shift then varies linearly again, with a slope corresponding to a dragged mass of PDMS. Prior to the breakup, the eective stiness variation ∆k of the oscillator can be extracted using equation 3, by removing the contribution of the dragged mass of the PDMS from the point marked by an arrow. The evolution of ∆k is plotted on the right graph of Figure 8, the zero depth and stiness corresponding to the breakup of the water meniscus. At t 0 and t 0 + 240min, we clearly observe that the stiness reaches a maximum value, which we associate with the meniscus stiness.

Figure 8: Plot of the frequency shift (left) and eective stiness (right) at the PDMS water interface at t 0 and t 0 + 240min during upward motion of the oscillating ber. The point where the water meniscus is pinned at the apex of the ber is marked by arrows. The eective stiness is proportional to interfacial tension. To calculate it, the dragged mass contribution of PDMS (linear t plotted as dashed lines at t 0 ) to the frequency shift is rst subtracted from the data, above the depth marked by arrows. We associate its maximum value with the meniscus stiness. From this value, with a liquid tank radius L = 8.7mm, and a ber radius r = 6.5µm, we found an interfacial tension γ = 60(±7)mN.m -1 at t 0 , and γ = 29(±3)mN.m -1 at t 0 + 240min. See text for details.

Using the work of T. Ondarçuhu group on the eective spring constant of liquid interfaces, 9 we can extract from it an estimate of the interfacial tension γ int ∼ = ∆k ln(L/r)/2π, L being the radius of the tank containing the liquid, and r the radius of the ber.

This analysis began with frequency shifts recorded at the PDMS / air interface (see Supplementary information), to obtain an estimated PDMS surface tension of γ = 18(±2)mN.m -1 in agreement with the measured values of the surface tension of PDMS.

At the PDMS / water interface, we estimate the interfacial tension at γ int = 60(±7)N.m -1 at time t 0 . Interestingly, it decreases to γ int = 29(±3)N.m -1 at time t 0 +240min. This reduction, larger than the experimental errors, 51 is consistent with a contamination of the interface accompanied by decreased interfacial tension. [START_REF] Azizian | Surface Tension of Binary Mixtures of Ethanol + Ethylene Glycol from 20 to 50[END_REF] It should be noted that a measurable decrease in interfacial tension at the oil / water interface 10 was reported in an earlier AFM study, in the presence of trace surfactants at the interface. The estimated interfacial tension is in agreement with the results obtained by the pending drop method and the previously published values. [START_REF] Svitova | Wetting Behavior of Silicone Oils on Solid Substrates Immersed in Aqueous Electrolyte Solutions[END_REF][START_REF] Ismail | Interfacial Structure and Dynamics of Siloxane Systems: PDMS Vapor and PDMS Water[END_REF] The amplitude of the interface deformations apparent on the approach frequency shift curves (shown in Supplementary information), and discussed above, depends on the interfacial tension. A decrease in interfacial tension must be accompanied by a decrease in the amplitude of the deformation before wetting. We indeed observe a decrease in the distance traveled by the ber from the point where an elasto-hydrodynamic contribution is measured to the point where wetting occurs. The value of 9.5 ± 0.5µm measured at time t 0 decreases to 6.5 ± 0.5µm at t 0 + 270 min, consistent with the evolution of interfacial tension.

Taken together, these preliminary results clearly show that we are able to probe an interface between two uids with our hanging ber AFM. In addition to being quantitatively sensitive to viscosity change, this setup enables us to observe the wetting of the ber and thus to estimate the interfacial tension. It also enables us to locate the interface and monitor its displacement while water 'evaporates' into the oil. Thanks to the sensitivity of the probe, the interface's elasto-hydrodynamic response is measured as the ber approaches it, before wetting occurs. This non-contact interaction with the interface, combined with the FM-AFM operating mode, opens the way to locally measuring the elastic and dissipative properties of the interface at a constant distance over time.

Although the sensitivity of the force measurements is signicantly improved compared to conventional AFM probes, the technique has some limitations. The deeper the interface, the more the sensitivity of the measurements is reduced due to the damping of the oscillator, leading to a strong decrease of the quality factor of the probe. The same factor also limits the viscosity of the uids that can be studied with these probes. Moreover, during the measurements, the localization of the interface is linked to the existence of a density and/or viscosity gradient, as well as to the wetting of the ber when crossing the interface. It appears dicult to apply this technique to interfaces that do not meet these specications.

Regarding the lateral resolution of this type of measurements, the limitation comes from the skin thickness of the moving uid layer. In the case of the results presented here, it is in the range 3-10 microns, but is completely dependent on the kinematic viscosity of the uids.

It should be noted that the probe as presented here, is not the most suitable to study the deformation of an interface, because the shape of the ber apex is not well dened. We plan to use a spherical end ber for future studies, for example by attaching a coloidal silica sphere at the end of the ber, in order to model the contact geometry correctly.

Conclusion

This paper presents a method of measuring the interaction forces in a simple uid, or at an interface between two liquids using an original FM-AFM in a hanging ber geometry.

After detailing the manufacture and calibration of this very high sensitivity force probe based on a cheap watch quartz tuning fork, we demonstrate its ability to make quantitative and reproducible measurements of simple uid properties.

Furthermore, we provide evidence that this AFM is suitable for measurements at the interface between two liquids. In particular, the repeatability of the measurements allows a moving interface to be monitored over time. The evaporation of a millimetric drop of water immersed in PDMS is observed, yielding a reasonable estimate of the interfacial tension and highlighting its decrease over time due to contamination.

These capabilities, and preliminary results on the interface between two immiscible liquids, pave the way for interface manipulation, such as triggering crystallization in liquid micro-reactors. [START_REF] Grossier | Predictive Nucleation of Crystals in Small Volumes and Its Consequences[END_REF] Moreover, we also show that non-contact interactions can be envisaged, in the spirit of the work of E. Charlaix's on using elasto-hydrodynamic interaction for the noncontact measurement of elastic properties of thin lms. [START_REF] Steinberger | Nanoscale Flow on a Bubble Mattress: Eect of Surface Elasticity[END_REF][START_REF] Leroy | Hydrodynamic interactions for the measurement of thin lm elastic properties[END_REF] This technique could be fruitfully applied to the study of complex uid-uid interfaces. [START_REF] Fuller | Complex Fluid-Fluid Interfaces: Rheology and Structure[END_REF][START_REF] Jaensson | Tensiometry and rheology of complex interfaces[END_REF] But also in the eld of biology, to the mechanical properties of membranes, as well as the the phenomena of locomotion exploiting the deformation of interfaces. [START_REF] Trouilloud | Soft Swimming: Exploiting Deformable Interfaces for Low Reynolds Number Locomotion[END_REF] Abreviations

Figure 1 :

 1 Figure 1: Left : Schematic diagram of the experiment. One end of a ber of radius r glued to a prong of a quartz tuning fork is dipped from a height h above the the air -liquid interface. It oscillates along its axis with an amplitude z. The frequency and oscillation amplitude of the QTF prong are monitored. Right : SEM image of a bered QTF. The prongs of the QTF protrude from the cut packaging. A glass ber thinned to a radius r is glued perpendicularly to the end of one prong, and the same amount of glue is applied to the other prong to balance the mass. The ber radius is r = 4.5µm.

Figure 2 :

 2 Figure 2: Resonance spectra of a bered QTFA (ber radius r = 4.5µm) in air and with the ber immersed at dierent depths in 10cst PDMS oil. The dashed lines are the best ts using a Lorentzian function. Modeling yields a resonant frequency f 0 = 31795.8Hz, and a quality factor Q = 12950 in air. As the length of the immersed ber increases, resonance frequency decreases due to increased drag mass, and FWHM increases due to increasing dissipation within the uid.

Figure 3 ,

 3 Figure3, shows the evolution of the frequency shift and friction coecient with respect to the immersion depth of a glass ber of radius r = 4.5µm in water and 10 cSt PDMS, under downward ber motion. The recording time for a whole curve is about 5 minutes. As these two values depend on the contact surface between the ber and the uids, they vary linearly with the length of the ber immersed. Their slopes ∆m * and β * obviously vary signicantly with the viscosity of the uids.

Figure 3 :

 3 Figure 3: Evolution of the frequency shift (left) and dissipation (right) of a QTFA with respect to the immersion depth of a ber of radius r = 4.5µm in water or 10 cSt PDMS oil. Frequency shift and friction coecient vary linearly with the immersion depth, and slopes ∆m * and β * signicantly depend on the viscosity of the uids.

Figure 4 :

 4 Figure 4: Dimensionless drag mass and friction coecients determined as C m = ∆m * ω 2πη and C β = β * 2πη as a function of r/δ for liquids of dierent viscosities and densities (listed in Table 2). The solid lines correspond to the theoretical values of C m and C β from the model. See text for details.

Figure 5 :

 5 Figure 5: Schematic representation of signals of frequency shift ∆f /f 0 (left) and friction β (in N.m.s -1 , right) with changes in the geometry of the ber and interface when the force probe crosses the interface between two liquids of dierent viscosities.

Figure 7 :

 7 Figure 7: Monitoring of the interface depth with time over four hours following the protocol used to record data shown in Figure 6. Left : As water evaporates into PDMS, the volume of the droplet decreases, causing the interface to shifts downward. To better visualize this displacement, the frequency shift curves have been surimposed along the frequency shift axis at the depth of 20 µm. Right : Plot of the interface position over time. The dashed line is the best linear t giving an interface recession speed of 1.1(±0.1).10 -7 m.min -1

Table 1 :

 1 Summary of stiness and electrical sensitivity calibration of bare QTFs. Calibration measurements were performed at T=22°C.

		QTFA	QTFB
	L (µm)	4000 ± 120	3200 ± 96
	T (µm)	590 ± 12	400 ± 7
	W (µm)	350 ± 7	330 ± 7
	f i (Hz)	16000	17300
	f 0 (Hz)	32764	32757
	Q	13930	9200
	k ef f (N.m -1 ) 8.8(±0.3).10 4	4.7(±0.16).10 4
	m ef f (kg)

Table 2 :

 2

	calculated

Summary of properties of uids used for mass and friction coecient measurements at 22°C. Fluids including PDMS oils of various viscosities, a uorinated oil (GPL103), water and glycerol / water mixtures. Glycerol / water mix viscosities and densities were

Table 3 :

 3 Summary of interfacial tension measurements using the pending drop method. Measurements were performed at T=21.5°C.

	Interface	Interfacial tension (mN.m -1 )
	Air / water	72.1 ± 0.6
	Air / PDMS	20.4 ± 0.2
	PDMS / water	37.1 ± 1.2

Table 4 :

 4 Abbreviations used in the text

	AFM	Atomic Force Microscope
	FM-AFM Frequency-Modulation Atomic Force Microscope
	FWHM	Full Width at Half Maximum
	L	Length
	PDMS	Poly-DiMethylSiloxane
	PID	Proportional-Integral-Dierential
	PLL	Phase-Locked-Loop
	Q	Quality factor
	QTF	Quartz Tuning Fork
	T	Thickness
	W	Width

Acknowledgement

The authors thank Dr T. Ondarçuhu for helpful discussion concerning the quantitative rheological measurements, M. Lagaize for his crucial technical support (design and production of mechanical parts, 3D printing), and D. Chaudanson for the SEM imaging of the QTF

probe.

Supporting Information Available


René Chalon
email: rene.chalon@ec-lyon.fr

Bertrand David
email: bertrand.david@ec-lyon.fr

Assistive systems for special mobility needs in the Coastal Smart City

Keywords: Smart City, mobility of people and goods, assistive systems for mobility, special needs, tourism, on the beach, on the sea, boat mobilities

The Smart City is an important evolution for cities of different sizes and contexts. Multifaceted proposals are made all around the world. Mobility support and its evolution is an important part of this issue. In this paper we propose to study a particular context, which is devoted to showing how a Coastal city can become a Coastal Smart City. We present our study and proposals based mainly on special mobility configurations, the role and functionalities of corresponding assistive systems, and the HCIs implemented in practice. Finally, we integrate all these aspects in the architecture of the assistive system managing them.

Introduction

The Smart city originated in the media sector, which refers to integration of urban systems and services by using various information technologies or innovative concepts to improve the efficiency of resource utilization, optimize urban management, and improve the quality of life of citizens. Today, countries world-wide realize in varying degrees the idea of smart cities and the different implementation efforts. However, there is no denying that in the 21st century, the age of smart is here.

The Smart City is not only the application of new information technologies, but also concerns the participation of citizens in the various activities of the city with the intelligence of humans, combined with Artificial Intelligence in different forms. A variety of classifications try to clarify this huge domain. One of these proposes six dimensions: environment, economy, living, mobility, people, and government with the following orientations: wireless city, smart home, smart transportation, smart public service and social management, smart urban management, smart medical treatment, green city and smart tourism [START_REF] Caragliu | Smart Cities in Europe[END_REF].

It is also possible to examine whom these evolutions concern. By avoiding the generic "to everybody", we classify the answers by age: kids, teenagers, adults, seniors; by implication in the city: citizen, neighbor, student, worker, administrator, governor; without avoiding important orientation to all persons with deficiencies.

We can also try to draw up a list of reasons for: better common welfare and neighborhood, energy, transportation of goods and passengers, information dissemination on culture, sports, social services, tourism, etc.

The Smart City is an area where all ICT possibilities can be used: data accessibility, data processing, information access exchange and manipulation in static and mobility situations, using wired or wireless networks, mainly in contextual situations. Multiple ICT technologies are used to solve these problems such as Internet, Internet of Things (IoT), Location-Based Services (LBS), Big Data and Open Data, as well as Artificial Intelligence with Machine and Deep Learning.

The presentation at this conference is steered towards mobility in the city, as well as towards HCI devices allowing propagation and collection of appropriate information to the users concerned.

The next sections of this paper are organized as follows. After a short state-of-the art in section 2, we identify in section 3 multiple aspects of our field of study which is a coastal city. We try to identify characteristic situations to consider, as well as corresponding activities and/or applications in order to manage them. Data collection, management, and evolution are studied in section 4. The architecture of an assistive system and its role in the field are studied in section 5 based on data exchanges with the ground and with users. Appropriate HCI supports and devices are presented and discussed at appropriate places in the paper.

2

State-of-the-art

To carry out a complete state-of-the-art for the Smart City would appear to be totally impossible as the amount and diversity of work is large and diversified. It would be hard enough to establish a common definition and classification, and indeed this has not yet been achieved, while listing all approaches and contributions seems totally impossible. We thus propose that the reader consults our paper [START_REF] Yin | A literature survey on smart cities[END_REF]. In relation to transportation aspects, a relevant reference is [START_REF] Kolski | Human-Computer Interactions in Transport[END_REF]. The Japanese view seems interesting [START_REF] Hosaka | Japan creating 'smart city' of the future[END_REF], as is also Singapore's perception of emerging technologies [START_REF] Ng | Embracing Emerging Technologies: The Case of the Singapore Intelligent Nation 2015 Vision[END_REF]. The questions on the effectiveness of smart cities make for instructive reading [START_REF] Giffinger | Smart cities ranking: an effective instrument for the positioning of the cities?[END_REF], as does also the view presented to CIOs [START_REF] Washburn | Helping CIOs understand "smart city" initiatives[END_REF]. [START_REF] Nam | Conceptualizing smart city with dimensions of technology, people, and institutions[END_REF] directly relates to our approach. From the coastal city point of view, we shall now suggest two references [START_REF] Isla | From touristic villages to coastal cities: The costs of the big step in Buenos Aires[END_REF][START_REF] Chuai | Land use and ecosystems services value changes and ecological land management in coastal Jiangsu, China[END_REF].

Coastal city characterization and how it becomes smart

To show how a city evolves into a smart city, specifically in the case of a coastal city, we need first to characterize what a Coastal city is. Then, we identify progressively different dimensions of its evolution into a Smart City. We limit our work to the dimension of management of mobility and the assistive system, the role of which is to manage corresponding behaviors. We start with the definition of a Coastal City: A Coastal City is a city that is located by the sea or the ocean. Its activities are related to the sea, i.e. fishing, boat transportation, boat and ship repairs, as well as possibly boat and ship construction. A large majority of coastal cities in France also have important tourist activities related to the proximity of more or less popular beaches. Swimming, diving, snorkeling, yachting, kitesurfing, and fishing are common practices.

This aspect of welcoming tourists is dominant in the organization of City life. It introduces the notion of the high and low seasons, mainly by the number of inhabitants that can vary considerably between these seasons (their number can be multiplied by 10 or more in summer). As such, the requirements of life organization are totally different. In the low season, a few inhabitants can live quietly without stress, noise or traffic jams. However, economically speaking, this period is a "waiting" period for a large majority of inhabitants who rely on their tourist businesses to live (retired people excluded), whether in the form of restaurants, shops, accommodation or recreation activities.

It seems natural to study the evolution of this kind of city into a smart city in the context of high season city life, while only later showing that in the low season it is possible to propose appropriate limited behaviors and city services adapted to this season and its inhabitants.

We begin our study with the general problem of circulation in a city with a large population density. We can inherit classical smart city solutions for different kinds of mobility: pedestrian, bike, scooter, motorbike, car, truck, and public transportation (bus and, for large cities, tram and subway). The solutions implemented are traffic reorganization with pedestrian zones, one-way streets and roads, prioritized lanes for public transportation, emergency services, and taxis, pedestrian and bike lanes. In coastal cities two evolutions can be considered: (1) organization of circulation between specific zones, i.e. park-and-ride and downtown, downtown and recreation areas and beaches, etc. and (2) new transportation modes such as boat taxis and individual boats, pedaloboats. New protected lanes can also be created for automated shuttles between these zones.

It is also important to consider the dynamicity of these reorganizations, which can be permanent (available all the time) or evolve according to the time scale (each month, week, day, weekdays, weekend, morning, afternoon, evening, night, per hour or in unpredictable exceptions). As we will see later, signaling must be appropriate (static or with appropriate dynamicity), as must also be propagation of applicable rules to different users. The goal is evolution of the circulation map during the high season in order to optimize fluidity and preferential transportation. Creation of park-and-ride can be associated with public automated transport to the city, the harbor (port) and the beaches. Management of priority for several kinds of vehicles such as buses, delivery trucks, emergency vehicles, and vehicles for disabled people can also be considered.

Management of circulation in the city, in the harbor/port, and on the sea

Let us now recall an approach to mobility devoted to dynamic lane management on which we worked [START_REF] Wang | Dynamic Road Lane Management Study: A Smart City Application[END_REF]. Its objective was to create these lanes dynamically only when buses are present and to leave all lanes open to general traffic when buses are not present. This allows management of general traffic speed. The main technologies used are: a Location-Based Service integrating bus detection sensors; an intermediation platform collecting sensor information and determining dynamic bus section activation and deactivation; in-the-field infrastructure and/or embedded vehicle interface receiving instantaneous information on selected situations (Fig. 0). From the HCI point of view, it is important to indicate the present situation on in-the-field indicators, as well as on the screen in the vehicle. Another non-coastal specific service is devoted to better and easier parcel delivery to inhabitants or stores. In this case, we proposed to create delivery areas which can be reserved as priority by delivery vehicles, in order to shorten delivery times [START_REF] Patier | A new concept for urban logistics: Delivery area Booking[END_REF]. In this context, the main data are: delivery addresses of parcels to be distributed and the geographical location of delivery areas to be reserved. An algorithm or interactive tool is used to create a delivery trip integrating the schedule of use delivery areas allowing transported parcels to be delivered to destination addresses. A mobile interaction tabletbased tool allows the driver to modify the trip if a traffic jam problem or other problems occur (Fig. 0). [START_REF] Patier | A new concept for urban logistics: Delivery area Booking[END_REF].

Fig. 3. Parcel distribution based on delivery area reservation

Management of circulation in the commercial port/harbor is totally specific and has no relation to the touristic part of the city, in order to preserve appropriate working conditions in this part of the city and on the sea (Fig. 4).

In the context of the coastal city, a specific service is devoted to management of circulation in the port, taking into account weather conditions and the state of the sea, as well as access to different parts of the port (industrial, professional, sailing, and yachting). In this context, not only local mobility can be managed, but also all the activities of fishing boats and cruising boats (as we will see later). From the HCI point of view, integrated screens and mobile tablets will be used by the crew. This service can be generalized to the overall management and supervision of boats in the harbor and on the sea. The ecosystems for fishing and cruising boats (Fig. 5) are schematized in Figures 6 &7. The principle is to identify all concerned actors and situations and to indicate their behaviors, their interrelations and contributions to appropriate trip and work conditions. In the past we studied this kind of ecosystem for buses (e-bus) and trucks (e-truck) [START_REF] David | User-oriented System for Smart City approaches[END_REF] and adapted them to these situations.

Beaches and sea

Another important aspect for coastal cities is management of mobility on the beaches and on the sea. On the beach several activities are possible, more or less dynamic: sunbathing, walking along the seashore, carrying out various sports or family activities. In all cases, it is important to look out for other beach users, to avoid accidents and ensure you do not disturb them. In the recent COVID period, on many French beaches, city mayors at local level or, more globally, government authorities have determined several categories of rules: prohibit access to beaches, authorize only "dynamic use of the beach" (no sitting or sunbathing, only walking and running), respect of distances between groups of users, either natural or physically materialized zones, with the possibility of reserving them (Fig. 8 &9). On the sea (Fig. 10), the creation of activity zones is a common approach resulting in the creation of specific zones with supervised swimming, diving, and snorkeling activities, as well as sailing, yachting, surfing, kitesurfing, with either visual supervision, or more instrumented electronic monitoring. In this case, each actor (swimmer, sailor, kite surfer has at his/her disposal a connected watch, or more sophisticated devices able to communicate his/her position and related situation (OK, emergency call, etc.). On the beach, life guards are equipped with a receiver allowing them to monitor the situation and trigger the appropriate rescue actions. Another service that can be used in downtown, near the parking area, on the harbor and near the beaches, is an information shelter. Its objective is to provide potential passengers and visitors with appropriate information on transportation lines and their schedules, using appropriate screens, as well as providing local, cultural and sports programs, commercial advertising, etc. (Fig. 11). Last but not least, a service we would like to mention is the Pedestrian Drive, the aim of which is to provide fresh food distribution based on box-lockers [START_REF] Chalon | Hotspot based mobile web communication and cooperation: ABRI+ Bus Shelter as a hotspot for mobile contextual transportation and social collaboration[END_REF]. The idea is to convey fresh food to the beach instead of asking clients to go to a supermarket. The pedestrian drive is a variant of the concept of the supermarket drive, specially designed for pedestrians. Located as near as possible to client locations (beaches, sports arenas, etc.) and accessible 24 hours a day, it allows ordered goods to be collected at any time. It is able to store not only ambient temperature products but also fresh and frozen goods. This fresh product box-locker can be either owned by a supermarket firm and totally integrated into the ordering, management and logistics process of the firm or can be managed independently. This case offers an interesting support system for the shared economy, as the role of the pedestrian drive manager is to manage such use for multiple providers. His/her role is to ensure Internet access for all provider offers and to organize the global supply chain with consumer information on availability of ordered goods in the box-locker. This makes it possible to support the circular economy as a short circuit of agricultural goods (Fig. 12). Fig. 12. Box/Lockers for fresh and frozen food [START_REF] David | Box/Lockers' contribution to Collaborative Economy in the Smart City[END_REF].

For all these identified activities, it is fundamental to define how to collect the data on which they are based and to determine their management and evolution. The following section 4 is devoted to this issue.

Of course, this list is not complete and has been voluntarily limited to mobility aspects. We could continue to add other aspects and, in particular, reexamine our propositions from the view of special adjustment to disabled people, as we have already done in our previous papers [START_REF] Yin | Assistive systems for special needs in mobility in the Smart City[END_REF][START_REF] Li | Assistive Systems for Mobility in Smart City: Humans and Goods[END_REF].

Data identification and their management -dynamicity

For all identified activities and applications, it is important to identify the data needed and to determine their evolution, i.e. dynamicity. We identified several levels and providers of data dynamicity.

Data dynamicity

The first, basic, level, is totally static, which means not smart, but useful for a large majority of city users (on the whole or by category): physical panels, boards and signs, as well as circulation signs. A contextual example can be seen in Fig. 13.

The second level, with very low dynamicity, is based on boards with QR codes. In this case, in the field (city), the information indicator is static, and dynamicity is provided by the information sent to the user's smartphone from the website address encoded in the QR code. This approach is appropriate for optional information, which is accessed only by interested users and known only to them, in the form of historical information for example, but also for commercial and advertising purposes. The third level of dynamicity is characterized by the need to propagate information that is of general interest and needs to be commonly known. The solution is to propagate it directly on users' smartphones or on GPS vehicle screens. Another solution is to use public active screens and information boards. This is the case for circulation information for dynamically managed circulation lanes such as we described earlier. This kind of dynamicity is usually determined by external decision centers working on collected or predefined data.

The fourth level of dynamicity is a contextual one, i.e. it is related to particular cases or situations that can be produced by an action on a local device, manually or automatically. A classic example is a sender located on a bus, which activates the opening of a barrier accessing a restricted portion of the road. This kind of sender (or remote control) can be either integrated on the vehicle (bike, car, bus, boat, etc.) or used manually by the user, often in emergency situations. These devices are attributed by the relevant authority. The actuator associated with the remote control can be either mono-located and mono-answer acting on a single door, barrier, etc. or integrated in a system assisting more sophisticated behaviors. Management of this dynamicity requires appropriate association between sender(s) and receiver(s) in order to organize the corresponding behaviors.

Emergency and rescue management

The highest level of dynamicity concerns emergency and rescue situations, mainly related to weather and environmental situations, such as floods, landslides, tsunamis, ecological and industrial problems, and fires. In these situations, the objective is to collect appropriate information and transmit it to the relevant organizations in charge of determining appropriate actions and informing those concerned (both those having to take action and those impacted).

From the data collection point of view, it is important to dispose of appropriately distributed sensors in water, air, and elsewhere for fire detection, which are able to transmit the appropriate data to the rescue center. The synthesis of these data is then elaborated, and an emergency signal is propagated to all information media, both individual and collective, in the city with vocal and/or visual announcements indicating the actions to take (leave the sea, put on a mask, move to an appropriate location, etc.).

Data for beach mobility

As we explained earlier, concerning management of mobility on beaches and on the sea in recreation areas, static information is available, mainly to indicate authorized and banned zones and expected users' behaviors. In this case, security observations and rescue actions are based on visual observations. In a more dynamic and secure approach, swimmers and kite surfers are equipped with appropriate transceivers indicating their geographical positions (Fig. 14). These devices are able to activate emergency demands. Life guards are fitted with a receiver designed to observe the movements of everyone in the beach zones and beyond in order to observe users' behaviors and contact each one in order to clarify their situation. They can indicate inappropriate behaviors, such as use of an inappropriate zone or ask the swimmer for the reason for his/her emergency signal and trigger appropriate action.

Data for traffic management

Organization of traffic is a very important aspect. Of course, in the low season, we can consider that static allocation of the circulation map is appropriate. However, in the high season, dynamicity aims at ensuring better traffic speed by introducing several restrictions during different periods of the day with authorized or prohibited vehicles.

Circulation restrictions can vary frequently between buses, trucks, tourist vehicles, and so on. It is also important to take into account the main objective of each zone in the city. In tourist zones, such variation is natural. In industrial zones, mainly dedicated to boat construction and maintenance, priority must be given to the fluidity of this activity. Such dynamicity is either based on collected and preprocessed data (behavioral patterns) or on real-time data. This is then propagated to the assistive system and drivers in an appropriate manner, either by electronically driven signs and/or by propagation of corresponding information to the in-vehicle screens, smartphones, and GPS applications.

Data collection and elaboration

As analyzed in the previous section, data availability is the main problem in the provision of appropriate solutions. For the first levels of dynamicity, work can be elaborated progressively and be used relatively long-term. For greater levels of dynamicity, three different approaches can be used.

The first is totally related to the local context. The aim of local senders addressing local receivers is to obtain local and appropriate answers, such as opening of a read section that is managed.

A more general solution is to be able to share available data that are collected by communicating devices, namely senders and receivers, on site in the field, the city, and its neighborhood. This approach, which is called Open Data, constitutes major progress in general utilization of available data.

Another interesting approach is based on evolution of collected data by the acquisition of new, more recent data and their integration in the elaborated behavior model. This approach is based on a very active research field called Machine and Deep Learning. We decided to devote the next two sections to these last two approaches.

Open data

One of the possible sources of information is access to open data published by different operators, such as municipalities, road operators, etc. These data can be either old data that are synthetized and give general trends or real-time data, indicating what is happening now or a few minutes ago.

Open Data [START_REF] Auer | DBpedia: A Nucleus for a Web of Open Data. The Semantic Web[END_REF] is the idea that some data should be freely available to everyone to use and republish as they wish, without restrictions from copyright, patents or other mechanisms of control [START_REF] Kitchin | The Data Revolution[END_REF] ) is an EU initiative to establish an infrastructure for spatial information in Europe that is geared to help to make spatial or geographical information more accessible and interoperable for a wide range of purposes supporting sustainable development.

One of the most important forms of open data is open government data (OGD), which is a form of open data created by ruling government institutions. The importance of open government data is that it is a part of citizens' everyday lives, down to the most routine/mundane tasks that are seemingly far removed from government. We give the following example [24]: Since January 1, 2015, the Metropolis of Lyon has grouped the missions of the Lyon Urban Community and the Rhône department into the 59 municipalities that make up the territory of Greater Lyon. The Metropolis of Lyon builds and preserves a pleasant living environment for its 1.3 million inhabitants by acting in broad areas: travel, mobility, knowledge, culture, environment, energy, solidarity, public health, childhood, family, city politics, habitat, housing, cleanliness, water, sanitation. It is in charge of major urban facilities and infrastructure, economic development, the attractiveness and influence of the territory. The Metropolis of Lyon is committed to opening up data to strengthen the innovation capacities of stakeholders on its territory and to invent the services and uses of the city of tomorrow.

Machine learning and deep learning

Deep learning is hugely popular today. The past few decades have witnessed its tremendous success in many applications. Academia and industry alike have competed to apply deep learning to a wider range of applications due to its capability to solve many complex tasks while providing state-of-the-art results [START_REF] He | Neural collaborative filtering[END_REF].

In some situations, it seems that deep learning can be used to analyze existing data and lead to interesting solutions. We can take intelligent navigation and travel planning as examples. For intelligent navigation, the intermediation platform considers the starting position and the destination position, combines the time information, meteorological information and other interfering factors, and finally provides the most appropriate one or more recommended paths. As for travel planning, the focus is to intelligently analyze the user's intention his/her behavior and finally make travel recommendations based on the tags that he/she is interested in and his/her browsing history.

Concerning the automated buses between park-and-ride and the main destinations (downtown, several beaches), it would be interesting to study the evolution of the trip trajectories, day-time periods, and the type of passengers. The objective is to determine the appropriate trip trajectory and timetable.

For goods transportation, it is important to start with a case-by-case study of the different destinations. We can then work progressively on these data that we capitalize in order to find main transportation trajectories and propose their evolution if necessary.

In practice, we can use the RNN (Recurrent Neural Network) structure in path planning, including intelligent navigation, travel for the disabled, parcel post and so on. The CNN (Convolutional Neural Network) model can be used for vehicle recognition to assist smart parking. For some scenarios with recommendation tasks, such as goods transportation and travel planning, we can use multilayer perceptron, LSTM (Long Short Term Memory networks), and other models [START_REF] Dziugaite | Neural network matrix factorization[END_REF].

Deep learning-based intermediation has proved very effective. However, the key problem for deep learning in different application scenarios is always data. Once data have been sufficiently collected and tagged, some deep learning-based solutions could be studied and applied.

Mobility assistive system and its relationship with the field

As we explained in the previous sections, the process consisting in transforming a city into a smart city usually starts by identifying the actors concerned, as well as the necessary services and associated data.

In practice, in order to organize and manage these aspects, we need a system that groups the different services and their applications, the data on which they work, and the relationship, if needed, between the field and the system. We call this system an Assistive System, the aim of which is to manage contextually all smart city applications. Its architecture is organized in 4 layers (Fig. 15) where:

Layer number 1 is devoted to applications, while layer number 2 is geared towards management of common data and proposing general services. The aim of the third layer is to work on what is called "Data Vitalization". The idea is to work on data and elaborate new constructions on them that can be used in new applications. An example of this need is to be able to collect and associate data in order to manage situations that were not predictable, such as several cases of rescue for not necessarily easily predictable situations.

The last (fourth) layer aims at acquiring and distributing the data needed for appropriate working of all applications concerning the coastal smart city. In order to take into account the evolution of this assistive system, an appropriate user interface, called the Supportive User Interface, is provided, allowing maintenance staff to provide the modifications requested easily. User Interface [START_REF] Atrouche | A Mashup-based application for the smart city problematic[END_REF].

Appropriate user interfaces, called Supportive User Interfaces, are provided to facilitate these evolutions. The "mashup" technique can be used for this purpose [START_REF] Grammel | An end user perspective on mashup makers[END_REF].

Conclusion

In this paper we studied how a coastal city evolved to become a Smart Coastal City.

The process was mainly based on the study of the actors concerned (inhabitants, tourists, workers, government authorities, etc.) and the corresponding services for each of them and globally. The specificity of a coastal city with activities related to the sea, industry, and tourism were the main instigators of this study. Our reasoning was based on the existence of two seasons (the high season -tourism-oriented -and the low season -inhabitant-oriented).

We proposed what we consider to be major transformations, either specific to the coastal city or adapted from the smart city in general. We identified expected services, associated data, corresponding user interfaces, individual or more common, and we described the links (passive or active) with the field (part of the city concerned) and the need for in-the-field indicators, senders and receivers. The dynamicity of the functional periods was also studied. We only briefly presented technological aspects (assistive system as an intermediation platform, open data use, and Machine and Deep Learning techniques), which we consider out of scope of this conference.

We also avoided the problem of assistance to disabled people, which we studied in our previous publications [START_REF] Yin | Assistive systems for special needs in mobility in the Smart City[END_REF][START_REF] Li | Assistive Systems for Mobility in Smart City: Humans and Goods[END_REF], in which we presented several solutions applicable in Smart Cities and that can be transposed easily to this context. Of course, new solutions, specific to the coastal context (beaches, sea, harbor, boats) can be proposed. This is our objective in the near future, together with the discovery of other Coastal Smart City services and applications.

Fig. 1 .

 1 Fig. 1. Coastal city with downtown, harbor, port, beaches, and boating industrial zone.

Fig. 2 .

 2 Fig. 2. Dynamic lane management [11].

Fig. 4 .

 4 Fig. 4. Industrial coast and navigation zones.

Fig. 5 .

 5 Fig. 5. Cruising and Fishing boats.

Fig. 6 .

 6 Fig. 6. Fishing boat information ecosystem.

Fig. 7 .

 7 Fig. 7. Cruising Boat information ecosystem.

Fig. 8 .

 8 Fig. 8. COVID on the beach [14].

Fig. 9 .

 9 Fig. 9. COVID location management on the beach

Fig. 10 .

 10 Fig. 10. Beach and sea zone management, supervision and rescue organization.

Fig. 11 .

 11 Fig. 11. Bus or Boat shelter for inhabitants' information.

Fig. 13 .

 13 Fig. 13. Surf safety physical panel [19].

Fig. 14 .

 14 Fig. 14. The GPS wristband saving lives [20].

  . The goals of the open-source data movement are similar to those of other "open(-source)" movements such as open-source software, hardware, open content, open specifications, open education, open educational resources, open government, open knowledge, open access, open science, and the open web. Paradoxically, the growth of the open data movement is paralleled by a rise in intellectual property rights [23]. The philosophy behind open data has been long established (for example in the Mertonian tradition of science), but the term "open data" itself is recent, gaining popularity with the rise of the Internet and World Wide Web and, especially, with the launch of open data government initiatives such as Data.gov, Data.gov.uk and Data.gov.in. INSPIRE (Infrastructure for Spatial Information in the European Community

Fig. 15 .

 15 Fig. 15. a/ Architecture of an Intermediation platform with Data Vitalization [27], b/ SupportiveUser Interface[START_REF] Atrouche | A Mashup-based application for the smart city problematic[END_REF].

Acknowledgements. This study is partially supported as regards China by the National Key R&D Program of China (No. 2019YFB2102200) and the National Natural Science Foundation of China (No. 61977003). As regards France, the study was mainly supported by the French Ministry of Ecology (No. 09MTCV37 and PREDIT-G02 CHORUS 2100527197).