Ofer Manor

Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions.

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.

Double flow reversal in thin liquid films driven by megahertz-order surface vibration

Arising from an interplay between capillary, acoustic and intermolecular forces, surface acoustic waves (SAWs) are observed to drive a unique and curious double flow reversal in the spreading of thin films. With a thickness at or less than the submicrometre viscous penetration depth, the film is seen to advance along the SAW propagation direction, and self-similarly over time t1/4 in the inertial limit. At intermediate film thicknesses, beyond one-fourth the sound wavelength λℓ in the liquid, the spreading direction reverses, and the film propagates against the direction of the SAW propagation. The film reverses yet again, once its depth is further increased beyond one SAW wavelength. An unstable thickness region, between λℓ/8 and λℓ/4, exists from which regions of the film either rapidly grow in thickness to exceed λℓ/4 and move against the SAW propagation, consistent with the intermediate thickness films, whereas other regions decrease in thickness below λℓ/8 to conserve mass and move along the SAW propagation direction, consistent with the thin submicrometre films.

Effects of Internal Flow and Viscosity Ratio on Measurements of Dynamic Forces between Deformable Drops

A model that has been shown to give very accurate predictions of dynamic forces between deformable emulsion drops and bubbles is used to quantify the effects of internal flow and viscosity ratio on the hydrodynamic interaction in such systems. The results demonstrate that direct force measurement using an atomic force microscope can readily differentiate whether the interfaces of drops of different viscosities respond as immobile (no-slip) or fully mobile (no tangential shear stress) boundaries.

Soft matter: from shapes to forces on the nanoscale

Soft matter deforms in response to imposed external forces. Here we demonstrate how dynamic surface forces are linked to far-field deformations. This offers a new paradigm for determining forces between soft particles in colloidal systems. The particular example we use to illustrate this concept is that of a fluid drop interacting with a solid wall through hydrodynamic drainage flow coupled with repulsive or attractive dissimilar electrical double layer interactions. The force can be deduced from a simple analysis of the drop surface geometry outside the interaction zone.

Spreading dynamics of a partially wetting water film atop a MHz substrate vibration

We use both theory and experiment to study the response of partially wetting films of water and surfactant solutions to a propagating MHz vibration in the solid substrate in the form of a Rayleigh surface acoustic wave (SAW). The SAW invokes a drift of mass in the liquid film, which is associated with the Schlichting boundary layer flow (also known as the Schlichting streaming). We study thin films that are governed by a balance between the drift and capillary stress alone. We demonstrate weak capillary contributions, such as for silicon oil films, support dynamic wetting and lead to the spreading of the liquid over the solid substrate along the path of the SAW. Strong capillary contributions, such as for water films, support however a concurrent dynamic wetting and dewetting along the path of the SAW, such that the film displace along the solid substrate. In addition, such films may support the formation of a capillary train-wave that propagate along the the same path. We further note the mechanism for film dynamics we discuss here is different to the more familiar Eckart streaming mechanism, which is associated with a film thickness that is greater than the wavelength of the sound leakage off the SAW and usually observed to support the motion of drops. The thickness of the films we discuss here is small in respect to the wavelength of the sound leakage, rendering contributions from the Eckart streaming, acoustic radiation pressure, and the attenuation of the SAW small.A MHz vibration, or an acoustic wave, propagating in a solid substrate may support the convective spreading of a liquid film. Previous studies uncovered this ability for fully wetting silicon oil films under the excitation of a MHz Rayleigh surface acoustic wave (SAW), propagating in a lithium niobate substrate. Partially wetting de-ionized water films, however, appeared immune to this spreading mechanism. Here, we use both theory and experiment to reconsider this situation and show partially wetting water films may spread under the influence of a propagating MHz vibration. We demonstrate distinct capillary and convective (vibrational/acoustic) spreading regimes that are governed by a balance between convective and capillary mechanisms, manifested in the non-dimensional number θ3/We, where θ is the three phase contact angle of the liquid with the solid substrate and We ≡ ρU2H/γ; ρ, γ, H, and U are the liquid density, liquid/vapour surface tension, characteristic film thickness, and the characteristic velo...

The deposition of colloidal particles from a sessile drop of a volatile suspension subject to particle adsorption and coagulation

Electrical double layer and van der Waals (DLVO) forces are known to determine the morphology of the deposit of colloidal particles following the evaporation of the carrier liquid. It is assumed that the adsorption of particles to the solid substrate and their coagulation in the liquid are the mechanisms connecting DLVO forces to the morphology of the deposit. We use theory to test this assertion. We model the deposition of particles from a volatile drop while accounting for the contribution of adhesion and coagulation. The rate of both mechanisms is connected to DLVO forces via the interaction-force boundary layer and the Smoluchowski theorems, respectively. We present analytical solutions for the morphology of the deposit, accounting for particle adsorption and pair-limited coagulation, and a corresponding numerical analysis for the case where particle adhesion and coagulation are concurrent. We conclude that larger aggregates of particles are found near the edge of the drop at the expense of the smaller ones in the absence of adhesion. The adhesion of particles to the substrate smears the deposit, rendering large aggregates to appear near the center of the drop. The analysis is in agreement with a previous experiment when accounting for the corresponding DLVO forces.

Terminal velocity and mobile surface species in rising microbubbles

The terminal velocity of rising microbubbles is a sensitive function of the bubble size and the surface concentration of mobile insoluble surfactants at the gas/liquid interface due to the Marangoni effect. With a model that allows for surface convection and diffusion, we delineate the regimes when the terminal velocity varies between the fully mobile (Hadamard-Rybczynski) and the fully immobile (Stokes) behavior at low Reynolds numbers. Results are presented in a universal form to facilitate conversion from bubble rise terminal velocity to trace amounts of surface contaminants.

Free films of a partially wetting liquid under the influence of a propagating MHz surface acoustic wave

We use both theory and experiment to study the response of thin and free films of a partially wetting liquid to a MHz vibration, propagating in the solid substrate in the form of a Rayleigh surface acoustic wave (SAW). We generalise the previous theory for the response of a thin fully wetting liquid film to a SAW by including the presence of a small but finite three phase contact angle between the liquid and the solid. The SAW in the solid invokes a convective drift of mass in the liquid and leaks sound waves. The dynamics of a film that is too thin to support the accumulation of the sound wave leakage is governed by a balance between the drift and capillary stress alone. We use theory to demonstrate that a partially wetting liquid film, supporting a weak capillary stress, will spread along the path of the SAW. A partially wetting film, supporting an appreciable capillary stress, will however undergo a concurrent dynamic wetting and dewetting at the front and the rear, respectively, such that the film wil...

Diminution of contact angle hysteresis under the influence of an oscillating force.

We suggest a simple quantitative model for the diminution of contact angle hysteresis under the influence of an oscillatory force invoked by thermal fluctuations, substrate vibrations, acoustic waves, or oscillating electric fields. Employing force balance rather than the usual description of contact angle hysteresis in terms of Gibbs energy, we highlight that a wetting system, such as a sessile drop or a bubble adhered to a solid substrate, appears at long times to be partially or fully independent of contact angle hysteresis and thus independent of static friction forces, as a result of contact line pinning. We verify this theory by studying several well-known experimental observations such as the approach of an arbitrary contact angle toward the Young contact angle and the apparent decrease (or increase) in an advancing (or a receding) contact angle under the influence of an external oscillating force.

Effect of non-homogeneous surface viscosity on the Marangoni migration of a droplet in viscous fluid

Marangoni migration of a single droplet in an unbounded viscous fluid under the additional effect of variable surface viscosity is studied. The surface tension and the surface viscosity depend on concentration of dissolved species. Cases of the motion induced by the presence of a point source and by a given constant concentration gradient are considered. The dependence of the migration velocity on the governing parameters is computed under quasi-stationary approximation. The effect of weak advective transport is studied making use of singular perturbations in the Peclet number, Pe. It is shown that, when the source is time dependent a Basset-type history term appears in the expansion of the concentration and, as a result, the leading order correction to the flow and to the migration velocity is of O(Pe(1/2)). If the source of active substance driving the flow is steady, the effect of convective transport on the migration is weaker.