Session X45: Focus Session: Soft Matter Physics of Drops, Bubbles, Foams, and Emulsions - Droplet spreading, colliding, wetting

Ultrafast interference of splashing dynamics: where is the air?

A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. While liquid and substrate properties obviously are important for determining the splashing threshold, it has also been shown that the surrounding gas is a crucial control parameter [1]. The mechanism underlying how the surrounding gas affects splashing remains unknown. One suggestion [2,3] has been that upon impact the liquid spreads outwards over a thin layer of gas that has been trapped beneath it during impact. In a sufficiently viscous liquid, splashing occurs at the edge of the drop several tenths of a millisecond after impact [4]. This large separation between impact and splashing, in both time and space, creates an ideal system in which to test whether the initial air pocket remains to influence splashing dynamics. We develop high-speed interference imaging to measure the air beneath all regions of a spreading viscous drop [5]. Although an initial air bubble is created on impact, no significant air layer persists up to the time a splash is created. This suggests that splashing in our accessible range of viscosities is not caused by the presence of a layer of air beneath the liquid, but rather it is initiated at the edge of the drop as it encroaches into the surrounding gas.\\[4pt] [1] L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. \textbf{94}, 184505 (2005).\\[0pt] [2] S. Mandre, M. Mani, and M. P. Brenner, Phys. Rev. Lett. \textbf{102}, 134502 (2009).\\[0pt] [3] L. Duchemin and C. Josserand, Phys. Fluids \textbf{23}, 091701 (2011).\\[0pt] [4] M. M. Driscoll, C. S. Stevens, and S. R. Nagel, Phys. Rev. E \textbf{82}, 036302 (2010).\\[0pt] [5] M. M. Driscoll and S. R. Nagel, Phys. Rev. Lett. \textbf{107}, 154502 (2011).   

The drop impact dynamics of complex fluids on textured surfaces

The deposition of aqueous drops on non-wetting surfaces is an important canonical problem for many applications, including suppressing splash or rebound of sprayed herbicides on intrinsically hydrophobic plant leaves. The addition of a small amount of high molecular weight polymer has been demonstrated to suppress drop rebound on smooth hydrophobic surfaces. The high extensional viscosity of polymer solutions and the increased viscous dissipation near the receding contact line are cited as two distinct anti-rebound mechanisms. Using drop impact experiments on both micro- and nano-textured surfaces with controlled wetting characteristics we examine the roles of viscosity, elasticity and inertia on expansion, retraction, and rebound of well-characterized viscoelastic fluids. By adopting a stick-slip flow model on textured surfaces with various topographic length scales and solid area fractions, we rationalize the dynamics leading to complete rebound following drop impact on nanotextured surfaces even for highly viscoelastic fluids.   

Influence of nanoscale surface roughness on mechanism of dropwise water condensation

Adversely to most potential applications of superhydrophobic coatings, only a few natural and artificial surfaces retain their superhydrophobic characteristics during water condensation. This work addresses the key question of why condensation on such surfaces leads to self-propelled dropwise condensation but causes wetting of other surfaces with water contact angles above 150 degrees. The effects of gradually varying nanoscale roughness of a hydrophobic surface on the mechanism of drop growth and coalescence are observed using electron and light microscopy. It is demonstrated that increasing the nanoscale surface roughness confines the base diameter of the nucleating droplets causing them to grow by increasing their contact angle. The increase in the nanoscale surface roughness also decreases triple line pinning during coalescence, thus allowing formation of nearly spherical drops after merging of two high contact angle drops. The role of the nanoscale roughness in the diameter confinement effect is explained through thermodynamic calculations. Lastly, confined base diameter growth model is derived and compared with experimental results.   

Effect of Nano-Scale Roughness on Particle Wetting and on Particle-Mediated Emulsion Stability

Colloidal particles can strongly adsorb to liquid interfaces and stabilize emulsions against droplet coalescence, the effectiveness of which depends crucially on the particle wettability. From the study of macroscopic solids, surface wetting is known to be influenced strongly by nano-scale roughness (as seen $e.g.$ in the ``Lotus effect'' or in anti-fog coatings); similarly, strong effects of particle roughness on particle-stabilized emulsions should be expected. Here we report the first experimental study of particle wetting and particle-mediated emulsion stability in which particle roughness could be varied continuously without varying the surface chemistry. We demonstrate an enabling method for preparing particles and macroscopic substrates with tunable nano-roughness and correlate the extent of roughness quantitatively with surface wetting (measured \textit{via} the three-phase contact angle) and with emulsion stability (quantifiable \textit{via} the maximum capillary pressure). Our results confirm a dramatic influence of roughness on wetting, emulsion stability, and even the type of emulsion formed (o/w \textit{vs.} w/o) upon mixing oil with an aqueous particle dispersion. Whether particle roughness benefits emulsion stability or not is seen to depend on both the size and shape of the surface features.   

Life Underneath a Leidenfrost Drop

Liquid drops deposited on a hot surface undergo a dramatic transition from boiling to levitating, a phenomena known as the Leidenfrost effect. The drops float on an insulating layer of evaporated vapor, which forms a pocket of high pressure underneath the drop and distorts the liquid-vapor interface. Experiments [1] have examined the lifetime and maximum size of such levitated drops. However, the interface beneath the drop has not been visualized or characterized. Using high-speed laser-light interference, we measure the geometry and fluctuations of the liquid-vapor interface. The interference fringes produced between the bottom surface of the liquid and the hot substrate provide information about the curvature of the vapor pocket beneath the drop as well as the azimuthal undulations along the rim that resides closest to the surface. We measure the speed, wavelength, and amplitude of the fluctuations as a function of the temperature of the substrate, as well as compare our results to recent theoretical predictions concerning the size of the vapor pocket for large drops [2].\\[4pt] [1] A. Biance, C. Clanet, D. Qu\'{e}r\'{e}, Phys. Fluids \textbf{15}, 1632 (2003).\\[0pt] [2] J. H. Snoeijer, P. Brunet, J. Eggers, Phys. Rev. E \textbf{79}, 036307 (2009).   

Voltage Bursting Drops in Solids

Droplets in air or liquids under electrical voltages appear in diverse processes from thunderstorm cloud formation, ink-jet printing, electrospinning nanofibers to electrospray ionization. In these processes, the electrostatic energy competes with surface energy of the drops and causes sharp tips to form on the ends of the drops. Here, we report a physically distinct scenario for droplets in solid matrices under voltages. We show that water drops in elastic polymers can form sharp tips and surprisingly burst into long tubes under applied voltages. The new phenomenon is governed by the elasticity and fracture of the solids, instead of the drops' surface energy as in previous cases. A new scaling is derived for the critical electrical field of the voltage-induced instability of drops in solids. The observations and analyses have significant practical impacts, as they illustrate the mechanism of a major failure mode, defect-induced breakdown, of dielectric polymers, which are widely used as insulating cables and polymer capacitors and transducers.   

Spreading dynamics of a water droplet on a soluble polymer: glass transition effects

We study the wetting dynamics of a droplet of solvent spreading on a soluble polymer coating. The complexity arises from the transfers of solvent and of soluble material through 3 processes: liquid evaporation and recondensation on the substrate, diffusion of the liquid in the substrate from the droplet, and substrate dissolution within the droplet. Indeed, when completely dry, the substrate, although soluble, is initially poorly wetting. Hydration enhances the wettability of the coating and the contact angle is found to decrease at higher humidity or at lower spreading velocity. In this paper, we explore experimentally the situation where hydration itself induces a sharp change in the diffusion coefficient of water in the polymer: this is what happens when the polymer undergoes a glass transition in water content. Diffusion coefficient changes by orders of magnitude upon glass transition, and we show that it results in a sharp effect on the course of the spreading as the hydration will be affected by the change in diffusion in the coating. We validate the previously derived model describing the various spreading regimes observed, and we expand it to account for the glass transition effects. It also successfully describes the results we obtain with other solvents and substrates.   

Water droplet spreading on a soluble polymer: what happens close to the contact line?

We have studied the spreading of a water droplet on a water soluble substrate. Numerous coupled transfer processes are involved in such a situation, leading to complex wetting dynamics. In particular, previous studies have shown the major role of water evaporation from the droplet associated with water uptake by the substrate. However, the processes at stake close to the contact line, where the substrate properties set the wetting angle, have not been understood. We present an experimental study of the phenomena occurring within distances ranging from 10 to 1000 $\mu$m from the contact line of a water droplet spreading on a food polymer layer. We have evidenced a wrinkling pattern inside the droplet close to the contact line, and suggest it results from the swelling of the constrained polymer layer before it dissolves. In addition, using an optical method based on the analysis of Newton's rings, we have measured the hydration profile of the substrate ahead the contact line. We show that the profiles can be understood as a result of the evaporation/water uptake process through air combined with direct water diffusion in the substrate from the liquid wedge.   

Contact angle dependence of the resonant properties of sessile drops

A simple optical deflection technique was used to monitor the vibrations of microlitre sessile drops of glycerol/water mixtures with glycerol compositions ranging from 0$\%$ to $75\%$. A photodiode was used to detect time dependent variations in the intensity of laser light reflected from the droplets. The intensity variations were Fourier transformed to obtain information about the resonant properties of the drops (frequency and width of the resonance). These experiments were performed on a range of different substrates where the contact angle formed by the droplets varied between $38\pm 2^{\circ}$ and $160\pm 4^{\circ}$. The measured resonant frequency values were found to be in agreement with a recently developed theory of vibrations which considers standing wave states along the profile length of the droplet. The widths of the resonances were also compared with theories which predict the influence of substrate effects, surface contamination effects and bulk viscous effects on the damping of capillary waves at the free surface of the droplets. These experiments indicate that the dominant source of damping in sessile liquid droplet is due to bulk viscous effects but that for small contact angles damping due to the droplet/substrate interaction becomes more important.   

Interfacial Effects on Droplet Dynamics in Poiseuille Flow

Many properties of emulsions arise from interfacial rheology. Here we advance theoretical understanding and experimental observation of the dynamics of isolated drops suspended in Poiseuille flow. Stokes flow is assumed in the bulk phases, and a jump in hydrodynamic stress at the interface is balanced by Marangoni forces (linearized with respect to local deviations of interfacial surfactant concentration) and surface viscous forces according to the Boussinesq--Scriven constitutive law. Interfacial diffusion is also included. Our analysis predicts slip, cross-stream migration and droplet-circulation velocities. These results and the corresponding interfacial parameters are separable, enabling a new droplet-based interfacial rheology method to determine interfacial viscosities and Marangoni elasticity. We illustrate such measurements by particle tracking velocimetry of surfactant-stabilized droplets in the Poiseuille flow of a microfluidic device. Small molecule and block copolymer surfactants are examined. This droplet-based interfacial rheology method is attractive since it mimics the natural geometry and length scale of practical emulsions and suspensions.   

Impact of liquid droplets on granular media

The crater formation due to the impact of a water droplet onto a granular bed has been experimentally investigated. Three parameters have been tuned: the impact velocity, the size of the droplet and the size of the grains. The shape of the crater depends on the Weber number at the moment the droplet starts to impact the bed. From the dynamical point of view, the spreading and the receding of the liquid during the impact have been carefully analyzed using image analysis of high speed video recordings. The different observed regimes are characterized by the balance between the impregnation time of the granular bed by the water contained in the droplet and the capillary time responsible for the receding of the drop.   

Droplet Rearrangement in a Sheared Dense Emulsion

The constituent particles of disordered colloidal dispersions compressed above the threshold for jamming flow with highly heterogeneous dynamics. Though this leads to the rich viscoelastic behavior that makes these materials so widespread, a clear description at the microscopic level has yet to emerge. We investigated the non-affine motion in a dense oil-in-water emulsion using a confocal rheometer, which can image individual droplets while applying a precisely controlled shear to the system. From these images, we identify the very fast rearrangements that accompany flow and determine the spatial and temporal distributions of the events as a function of the droplet volume fraction. In addition, it is possible to characterize the regions of the emulsion which are most susceptible to rearrangement.   

Wetting Colloidal Particles at a Curved Interfaces

At scales much smaller than the capillary length, surface tension plays a predominant role in the interactions that occur at an oil-water interface. When a spherical colloidal particle is adsorbed onto such an interface, two surface-tension-related effects occur: the adsorbed particle reduces the area of the interface, and the interface deforms in order to satisfy the requirement of constant contact angle at the three-phase contact line. If the interface is not flat or spherical, these effects depend on the position of the particle on the interface. In other words, the particle experiences an effective potential induced by the shape of the interface. [A. Wurger, PRE, 74, 041402 (2006).] We present results from an experiment in which a capillary bridge creates an interface of varying Gaussian curvature, onto which a colloidal particle is introduced. The shape of this interface is obtained by using confocal microscopy. We demonstrate that a shape-induced effective potential exists for this system, which attracts the wetting particle to the most curved regions. By tracking the motion of the particle in 3D, we are able to calculate the effective spring constant of this potential. We then compare our result to numerical and analytical predictions based on the geometry of the droplet.