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Shear-Induced Detachment of Micrometer-Scale Soft Droplets Embedded onto a Rigid Substrate. Relation to Biological Systems Karine Mougin, Hamidou Haidara,* and Jacques Schultz Institut de Chimie des Surfaces et Interfaces (ICSI), CNRS, 15 Rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France Received February 14, 2001. In Final Form: May 16, 2001 Under isothermal conditions (∼22 °C here), a liquid crystalline (LC) drop at rest onto a structured surface may retract upon an early stage of spreading, leading to the formation of a uniform array of micrometer-scale droplets, which exhibits a high environmental stability in water. We took advantage of this specific dewetting pattern and softness of the LC material to investigate the shear-induced detachment of these adhering micrometer-scale soft objects, induced by the retraction of an evaporating water drop. Two resisting factors were shown to critically control the detachment of the LC droplets: (i) the intrinsic adhesion to the surface via molecular bonds and (ii) the cohesion of a transition layer arising from the homeotropic ordering of the LC molecules near the surface. Both resisting barriers were reduced by setting a slight thermal disorder to the system (T ∼ 40 °C), leading to the complete detachment of the LC material within the drying drop. These competitive processes of particle sticking vs detachment are relevant to many biological systems. For instance, they may control the drying of biodispersions on tissues or cellcovered substrates. These results may therefore provide some insights into the understanding of these phenomena.
Introduction A renewed growing interest for wetting and related phenomena at heterogeneous and rough surfaces has recently emerged among the surface and interface science community. This renewed interest, in contrast with that of earlier works1-3 in the domain, is driven by the tremendous opportunity that easy-to-handle imaging techniques and structured surfaces (elaboration of sized heterogeneous domains and roughness) offer today,4-7 for the model investigation5,8-10 of wetting and interface phenomena. This unique situation is allowing real-time interactions between theoretical predictions and model experiments on fundamental issues, which still remain to be understood. This involves, for instance, length-scale and topological effects of micrometer- and nanometersize domains or roughness on interface forces and phenomena at the underlying surface (wetting, line tension, film stability, pattern formation, etc.).8-10 Aside from these fundamental issues, these investigations also have a strong technological relevance due to the large spectrum of wetting phenomena, which, in addition to many natural processes, covers most basic and growing technological applications (coatings and biotechnology to cite a few). In * Corresponding author. E-mail:
[email protected]. (1) Good, R. J. Am. Chem. Soc. 1952, 74, 504. (2) Shepard J. W.; Bartell, F. E. J. Phys. Chem. 1953, 57, 458. (3) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (4) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Science 1997, 273, 1425. (5) Drelich, J.; Miller, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf. A 1994, 93, 1. (6) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger H. J.; Sita, L. R. Langmuir 1998, 14, 241. (7) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610. (8) Lipowsky, R.; Lenz, P.; Swain, P. S. Colloids Surf. A 2000, 161, 3. (9) Konnur, R.; Kargupta, K.; Sharma, A. Phys. Rev. Lett. 2000, 84, 931. (10) Haidara, H.; Mougin, K.; Schultz, J. Langmuir 2000, 16, 7773.
all these applications, especially those involving a solid substrate, the ultimate state and property of the system for a given fluid (simple liquid, polymer solution or melt, biofluid) may drastically depend on the chemistry, domain size, topology, or roughness at the substrate surface. These effects may even become more complex when the surface roughness and heterogeneity are on the micrometer or nanometer scale and composed of soft objects. In this case, length scale effects as well as viscous deformation, lowtime-scale molecular reorientation and possible detachment, and collection of the soft objects may simultaneously intervene, depending on which wetting regime (dynamic or static) is experienced by the system. An example of such a situation is found in biology where both fundamental investigations and clinical essays often imply the wetting and/or drying11,12 of biofluids (biodispersions), either on “homogeneous” substrates, tissues (heterogeneous surfaces), or substrates embedding attached soft membranous objects (cells, bacterial colonies, vesicles, etc.). We here present a model experiment toward the fundamental understanding of the behavior of such systems. The study first involves the formation of a stable pattern (under experimental conditions) of micrometerscale soft droplets (µdroplets) at rigid substrates. This first step to which a letter was recently dedicated13 will only be briefly presented. The dewetting dynamics of evaporating aqueous drops are then investigated at these soft microdroplet arrays as a function of both the underlying surface and receding velocity of the shrinking drop. These two parameters, respectively, determine the competition between two processes, (i) the adhesion of the soft particles to the wall and (ii) the magnitude of the shear stress they experience. On the basis of these two competitive driving processes, a simple phenomenological approach is proposed, which satisfactorily allowed us to interpret the experimental results. (11) Alexandrova, L.; Tsekov, R. Colloids Surf. A 1998, 131, 295. (12) Ra¨dler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 4539. (13) Haidara, H.; Mougin, K.; Schultz, J. Langmuir 2000, 16, 9121.
10.1021/la0102399 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/24/2001
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Figure 1. AFM phase contrast mode pictures of the complete nanoheterogeneous molecular surfaces: (a) hts/nh2 samples showing hts nanoislands (clear domains) within the nh2 terminated amine continuum (dark background); (b) nh2/hts sample with nh2 amine domains (clear) within the hts continuum.
Experimental Section The nanoheterogeneous surfaces are binary molecular films, composed of two organosilane compounds, the methyl terminated hexadecyltrichlorosilane Cl3Si(CH2)15CH3 and the amine terminated n-(6-aminohexyl)aminopropyltrimethoxysilane (OCH3)3Si(CH2)3NH(CH2)6NH2, respectively referred to as hts and nh2. Both molecules were supplied by ABCR, Karlsruhe, Germany. The detailed description of the self-assembling technique, regarding the formation of homogeneous molecular films of organosilanes onto silicon plates, can be found elsewhere.14 More specifically, the heterogeneous surfaces were obtained through a sequential (two-step) process, based on the time control of the early stage nucleation and domain growth of solvent-coated organosilane films, self-assembled onto silicon wafers (SiO2). The discrete nanodomains were first realized by immersing the virgin silicon wafers in millimolar solutions (1 mM), of either hts in carbon tetrachloride (1 min for hts nanodomains) or nh2 in ethanol (6 min for nh2 nanodomains). The complete nanoheterogeneous binary surfaces were obtained by assembling a continuous monolayer of the second molecular compound around the nanodomains, in the remaining virgin room of the SiO2 substrate. For this second (and final) step, we used the vaporphase adsorption technique15 under reduced pressure, in a dynamically evacuated chamber (1 h at ∼10-2 Torr). Before this final step, the nanodomain-coated SiO2 substrates were thoroughly rinsed with the pure solvent to remove free and loosely adsorbed nanodomain phase molecules. The nanodomain-coated substrates are then exposed to the vapor of the continuum phase molecules. In the following, heterogeneous surfaces composed of hts nanodomains in the amine (nh2) continuum are referred to as hts/nh2, whereas those composed of nh2 nanodomains in hts continuum will be referred to as nh2/hts. The typical roughness at these nanoheterogeneous surfaces, which mainly arises from (14) Ulmann, A. An introduction to Ultrathin Organic Films, from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991; pp 245-253. (15) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230.
the difference in the thickness of the two molecular phases (respectively 1 and 2 nm for nh2 and hts domains), lies in the sub-nanometer range, as estimated both by ellipsometry and by AFM. In addition to the AFM phase contrast mode (Figure 1a,b), the compositional heterogeneity at the binary structured surfaces was confirmed, using both X-ray photoelectron and infrared spectroscopies.10,13 The advancing and receding contact angles (θa, θr) of water on these bare heterogeneous substrates were found to be quite similar: (87, 56°) for hts/nh2 and (86, 51°) for nh2/hts. It is noteworthy mentioning here that this similarity in the macroscopic wetting properties also corresponds to surface fractions in the composing molecular phases, which are identical at both heterogeneous surfaces (∼70% nh2, 30% hts), as determined from AFM pictures in Figure 1a,b. It therefore seems that so far as nanoscale structures are involved, the macroscopic wetting properties at heterogeneous surfaces are essentially dominated by the surface fraction of the constitutive domains, irrespective of their topological distribution (hts in nh2 continuum and vice versa). A more complete description regarding the details of the elaboration and characterization of the nanoheterogeneous surfaces, including both height and phase mode AFM images, can be found in ref 10. The formation of the µdroplet arrays has consisted of depositing 2 µL of a liquid crystal (LC) solution onto the nanoscale heterogeneous surfaces.13 The LC material, the 4-cyano-4-nhexylbiphenyl (6-CB) from Merck, was used at a concentration of 2.5 mM in cyclohexane. Upon the early spreading and solvent evaporation steps, the remaining LC film shrinks from both structured surfaces, leading to an instability of the receding front (Figure 2a), which generates the formation of the pure LC µdroplet patterns (Figure 2b,c). It is noteworthy mentioning here that no such dewetting patterns were obtained on the corresponding homogeneous surfaces, respectively composed of hts and nh2 molecular films.13 The average diameter (2Rd) and height (hd) of these µdroplets, as measured by a laser profiler, were, respectively, 23 ( 5 and 1.3 ( 0.2 µm. These soft LC microdroplets were found to exhibit, under ambient conditions, a high environmental stability in water. This high structural and morphological
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Mougin et al. Table 1. Receding Contact Angle θr and Velocity U of the Evaporating Water Drop at the Heterogeneous Substrates Embedding the Μicrometer-Scale LC Droplet Arraysa av over six measures θr ( 1° 22 °C
40 °C
U (µm/s) 22 °C
40 °C
hts/nh2 + µdroplets array 52 ( 3 70 ( 4 2.4 ( 1 5.6 ( 1.5 nh2/hts + µdroplets array 55 ( 2.5 72 ( 3 2.6 ( 1 6.5 ( 2 a The average velocities are determined in the middle range of the receding drop size, around R(t) ∼ R0/2, under ambient relative humidity, ∼33%.
performed under an optical microscope equipped with a videocamera system for the observation and recording of both evaporation and detachment dynamics. The average receding velocity U of the evaporating water drop was determined in the middle range of the initial drop radius, around R(t) ∼ (R0/2). The initial diameter, 2R0, of the water drop was ∼2.8 ( 0.2 mm, at 22 and 40 °C, and independent of the underlying heterogeneous surface patterns (hts/nh2 and nh2/hts). This clearly indicates that the wetting behavior of the water drop is dominated, at both substrates, by the LC µdroplets array, as are the receding contact angles shown in Table 1. These average initial diameters of the water drops, which are determined by the deposited volume (1 µL), the temperature, and the wettability of the µdroplets-covered substrates, were kept fixed during all of our experiments. Therefore, no systematic investigation of the influence of this drop size was done during this work. Nevertheless, we verified that a 2-fold increase in the drop coverage (diameter) had no influence, at both 22 and 40 °C, on the results hereafter presented.
Results and Discussion
Figure 2. Optical micrographs showing (a) the instability of the receding front of the LC drop and the formation of initial µdroplets along the triple phase line at nh2/hts surfaces and (b) the µdroplet arrays in the medium range of the receding drop radius, R(t) ∼ R(t)0)/2, at the nanoheterogeneous hts/ nh2 surface and (c) at the nanohetrogeneous nh2/hts surface, as they appear in Figure 1a,b. stability in water was used to investigate the shear flow and detachment induced by the retraction of an evaporating water drop at these embedded micrometer-scale soft droplets. To verify whether the detachment of the droplets at the underlying nanoscale heterogeneous surfaces was affected by the topological organization of the molecular domains (hts and nh2), at identical surface fraction in the domain phases, the experiments were conducted on both µdroplet-covered hts/nh2 and nh2/hts substrates. The detachment experiment properly has consisted of depositing ∼1 µL of a water drop onto the micrometer-scale droplets array, from which retraction upon evaporation provides the shear stress and subsequent de-adhesion of the embedded micrometerscale soft LC droplets. Two sets of experiments were performed: one at 22 °C within the bulk nematic temperature range (15-29 °C) of the pure LC material and the second at 40 °C in the bulk isotropic state. Before each experiment, the samples were thermally equilibrated in a thermostated cell, under a constant ambient relative humidity RH ∼ 33%. The experiments were
The characteristic parameters of the experiments, including both the receding velocities U and contact angles θr of the evaporating water drop onto the µdroplet arrays are summarized in Table 1. It is worth noting about these data (Table 1) that θr is higher at 40 °C, as compared to 22 °C. This result arises from both the lowering of the µdroplet adhesion and its softening at 40 °C, which all favor the detachment and early retraction of the evaporating water drop, as demonstrated hereafter in the discussion. In Figure 3a,b are shown the representative snapshots of the evaporation dynamics of the water drops at 22 °C, respectively, at hts/nh2 and nh2/hts surfaces. In comparison, the results of the evaporation and detachment experiments at 40 °C are given in Figure 3c,d, respectively, at hts/nh2 and nh2/hts substrates. The final states of the detachment commonly observed, irrespective of the nature of the underlying substrates (hts/nh2 or nh2/hts), are given in Figure 4a,b, respectively, at 22 and 40 °C. These results, at 22 and 40 °C, were systematically observed over six independent experiments, at both hts/nh2 and nh2/hts substrates, demonstrating the quite good reproducibility of the experiments and observed results. Basically, the behavior of the LC µ droplets during the dewetting of the evaporating water drop can be accounted for by two competing forces: (i) a resisting force arising from the adhesion of the µdroplets to the underlying substrate and (ii) the shear force arising from the viscous friction they experience within the receding wedge. The balance between these two forces then essentially determines the detachment (existence or not), whereas the extent of that detachment (partial or complete) may strongly depend on the relative magnitude of these forces, as compared to the intrinsic cohesion of the LC droplet. The simplest expression of this force balance is given by the original Coulomb’s law,16 which relates the frictional (16) Kendall, K. Nature 1986, 319, 203.
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Figure 3. Optical micrographs showing the detachment and peripheral aggregation of the confined soft LC µdroplets at the drying drop/heterogeneous substrate interface: (a, b) at hts/nh2 and nh2/hts surfaces, respectively, at T ) 22 °C; (c, d) at hts/nh2 and nh2/hts surfaces, respectively, at T ) 40 °C.
Figure 4. Optical micrographs showing the collected LC material in the final state of the detachment at both heterogeneous surfaces at (a) 22 and (b) 40 °C.
force fµ experienced by an attached particle to the total normal applied load (fadh+ fextern) as fµ ) µ(fadh+ fextern) ) k + µfextern, where µ is the friction coefficient and k ) µfadh, the intrinsic friction under zero external load, fextern ) 0, due to adhesion forces (van der Waals, hydrogen, and
electrostatic bonds). For a liquid drop of viscosity ηD flowing past a wall with a velocity U and adhering to the surface (U ) 0 at the wall), the shear stress within the liquid in the receding wedge of the drop is given by the velocity gradient along the fluid thickness h, as σ ∼ ηD(∂U/∂h). For
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small h, as in the drop wedge, the gradient (∂U/∂h) is averaged by (U/hd), leading, at first order to σ ∼ (ηDU/hd), where U is the mean fluid velocity and hd, the local thickness of the receding wedge, taken at the apex of the LC microdroplets. If π(Rd)2 is the average contact area of the LC µdroplets of radius Rd and WA their adhesion energy per unit area, the frictional force balance, fµ ) µfadh + fσ ) 0, which finally drives the detachment, can be expressed as Rd(σRd-µWA) ≈ Rd[(ηDU/hd)Rd-µWA], where fσ ) π(Rd)2σ ∼ π(Rd)2(ηDU/hd) and fadh ) π(Rd)2WA. The main statement of this phenomenological expression is that the frictional force developed within the retracting wedge and experienced by the µdroplets should overcome their adhesion, for the detachment and collection to occur. In deriving the above expression, we mainly assumed the no-slip condition at the µdroplet/substrate boundary, which is here reasonable, at least for the experiments performed at 22 °C as discussed later on. The second assumption is that both forces (adhesion and shear) are supposed to work, for each droplet, over a characteristic distance, which is on the order of the droplet radius Rd. In the following, we used the above phenomenological approach to interpret our results, highlighting the specific contribution of the LC transition layer at the substrate, in the overall force balance and detachment process. At 22 °C, the behavior of the system is largely dominated by the adhesion of the µdroplets to the underlying surfaces, as shown by the LC spots left behind the receding water drop (Figure 3a,b). In addition to the pure intermolecular adhesion forces, one may expect the LC to form at this temperature (nematic state), a transition layer17 of higher order, and cohesion at these substrates (θe of LC/substrates ∼ 25°), leading to an overall increased detachment barrier. What these results clearly indicate is that the limiting factor in these shear-induced detachment processes actually arises from all the wall effects, including both pure intermolecular adhesive bonds and intrinsic rigidity of a transition layer, which may result, for instance, from the homeotropic ordering induced by a high anchoring energy.17 We tried to bring an experimental proof toward the above interpretation of the partial detachment of the LC µdroplets observed at 22 °C, at both hts/nh2 and nh2/ hts surfaces. This was done by taking the LC droplets, a few degrees (10 °C) above their isotropization temperature (29 °C), supplying this way the thermal disorder necessary for the reduction of the adhesive and structural interface barrier to the detachment. The results of these experiments at T ) 40 °C are given in Figure 3c,d, respectively, at hts/nh2 and nh2/hts substrates. As compared to the experiment at 22 °C, the average velocity of the receding water drop is ∼2.5 times higher here (see Table 1), for a viscosity18 ηw(40 °C) ∼ 0.65 mPa s (vs 0.95 mPa s at 22 °C). As shown in Figure 3c,d, the most striking feature of these experiments at 40 °C is the complete de-adhesion and collection of the LC droplets within the receding drop, for both underlying surfaces. Furthermore, the detachment almost develops up to minimum drop radius (Figure 4), despite the increasing peripheral accumulation of the LC, which strongly reduces the retraction velocity, as it favors the thinning (by evaporation) and eventually the rupture of the water drop. The qualitative difference observed in the behavior of LC droplets at the two temperatures may arise from both competing terms (WA and σ) in the driving equation. If one first considers the variation in the magnitude of the shear term, based on (17) Hubert, P.; Galerne, Y. Appl. Phys. Lett. 1997, 71, 1050. (18) CRC Handbook of Chemistry and Physics, 69th ed.; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press Inc.: Boca Raton, FL, 1998-1989.
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those of U and ηw at 22 and 40 °C (see above), the maximum difference one can expect in the driving force, (σ40/σ22) ∼ (η40U40/η22U22), is on the order of 1.7. Such a magnitude was already obtained at 22 °C, in the late stage of the evaporation, where the receding velocity increases up to 10 µm/s. Still, no detachment was observed in this range, as shown in Figure 3a,b, demonstrating that the detachment at 40 °C could not be explained by the 2-fold increase in the dewetting speed of the water drop (Table 1) and related shear-induced stress. Direct evidence that the above 2-fold increase in the flow velocity was irrelevant in accounting for the observed detachment was brought by a complementary experiment, where the µdropletcovered substrate was vertically lifted at a much higher retracting speed (900 µm/s), from a water bath, at ambient temperature. At this retracting velocity of the (air-watersubstrate) contact line, the induced shear stress at T ) 22 °C, σ ) ηwU/hd ∼ 0.7 Pa, is 2 orders higher than the maximum value developed in the evaporating water drop experiments (σmax) 4.5 × 10-3 Pa, for U ∼ 6 µm/s). Nevertheless, the LC µdroplets essentially remain attached at the substrates, showing only at a few locations a slight distortion in the µdroplet profile and local distribution. On the other hand, the high-retracting-speed experiment from the water bath, at T ) 40 °C, resulted in the complete detachment and collection of the LC µdroplets, the same as for the low-retracting-speed experiment with the evaporating water drop.19 One therefore should seek a more relevant mechanism, capable of bringing a significant lowering into the adhesion term, to explain the drastic change observed in the detachment of the µdroplets at 40 °C. As already mentioned above, the resisting term to the detachment here involves two contributions: an adhesion force arising from pure intermolecular bonds and a structural term due to the intrinsic cohesion of a highly ordered interphase layer, which may arise from the strong anchoring of the LC molecules at the wall. Both contributions are temperature dependent. The adhesion (anchoring) mainly involves here hydrogen bonds (H-bonds) between the terminal surface amines (-NH2) and the cyano groups (-CtN) of the LC molecules, of which the energy is lowered at 40 °C (586 K) by ∼2 kT/molecule (or equivalently, 2 × 2.478 kJ/mol). In the same way, the intrinsic cohesion of the highly ordered interphase transition layer built up on the anchoring between the LC molecules and the wall is significantly reduced by the thermal disorder above the isotropization temperature (29 °C). Since no remaining spot was left behind during this detachment, these results suggest that the modification in the driving force balance at 40 °C is such that the resisting barrier is brought to about zero (WA ∼ 0), allowing the complete detachment of the micrometer-scale droplets. The thermal energy (2 kT) then constitutes an estimate of the LC adhesion, which can be determined from the number N of molecular bonds (mole/area) at the droplet/substrate interface. If one assumes the LC molecules to adsorb in the homeotropic alignment at the H-bonding NH2 domains (surface fraction ∼75%) of the heterogeneous surfaces, N is given by the LC molecular area (0.28 nm2) and the droplet’s contact area π(Rd)2. The magnitude of the LC adhesion energy WA ≈ 2 × 2.478N obtained from these data is ∼30 mJ m-2, which is 40% lower than what is estimated from the LC (19) We did not think it necessary to incorporate into this report the video pictures of these high-retracting-velocity experiments, which are quite identical to those obtained at 22 and 40 °C in the evaporating water drop experiments, and herein shown in Figure 3. These results as well as the details concerning these complementary experiments are available to readers, by request from the corresponding author.
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surface tension γLC (∼26 mJ m-2) and contact angle θ (25°) at these substrates, using the Young-Dupre´ relation γLC(1 + cos θ). Since we here assumed the adhesion to mainly arise from interface H-bonds between the cyano end-groups of the 6-CB and the terminal amines NH2 of the surface, this energy actually constitutes a lower limit, which only accounts for the contribution of H-bonds to the LC droplet adhesion. It therefore does not involve the background contribution due to London-van der Waals dispersion forces. Finally, the combination of the detachment experiment with the thermal activation provides a method for probing on such attached soft particle objects the magnitude of the specific interactions (H-bonds and electron acceptor-donor bonds) which may be involved at the interface. To that unique regard, it may constitute an interesting tool for the investigation of the attachment of vesicles and cells to various substrates. These experiments and related results proved to be an interesting model for the investigation of wetting dynamics and shear-induced detachment of micrometer-scale soft objects, embedded onto a rigid substrate. We here studied, through a rather simple phenomenological approach, one aspect of these dynamics, the competition between frictional and adhesion forces, which controls the shearinduced detachment of the adhering particles. In this specific case of soft LC material, the dragged µdroplets are found to coalesce and aggregate in a peripheral ring around the receding water drop (Figure 3), just in a way similar to rigid colloidal particles in a drying dispersion. What these results clearly demonstrate is that the shear flow induced under normal conditions by a drying drop is by far of a too low magnitude to overcome the intermolecular adhesion energy and interphase ordering, which arise from the contact of these soft and H-bonding µdroplets to H-bonding substrates. Compared to these soft
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materials, the detachment of rigid spherical-like particles exhibiting identical intermolecular forces would require a much lower energy, both the contact area and bulk dissipation being minimum in this case. Finally, whenever shear forces are dominant in the system, this interestingly allows the entire detachment and collection of the randomly distributed soft µobjects, leading to an estimate of their adhesion energy to the underlying substrates. Conclusion In this paper, we used the shear flow induced by the retraction of an evaporating water drop to investigate the detachment of soft LC µdroplets adhering to a rigid and H-bonding substrate. Under normal conditions (20 °C, ambient relative humidity), only a partial detachment due to the bulk cohesive rupture of the µdroplets was observed, the adhesion and interfacial ordering of the soft and highly polar LC material being much higher than the shear stress developed by the retracting water drop. By adjusting the substrate temperature slightly above the LC isotropization, the intermolecular adhesion bonds at the particle/substrate interface could be overcome, leading to the entire de-adhesion of the attached particles, and an estimate of the LC µdroplet adhesion energy. This investigation has direct relevance to the wetting and drying of biofluids over attached cells or similar soft particles (vesicles) at various substrates, where both the detachment and peripheral aggregation processes, similar to those observed in this study, often develop. To that regard, these results may constitute a step forward in the understanding of the underlying mechanisms that control such biological processes. LA0102399
