Interfacial flow of a surfactant-laden interface under asymmetric shear flow

Thermocapillary migration of a drop with a thermally conducting stagnant cap

Hypothesis The thermocapillary migration of a spherical drop with a stagnant cap in the presence of a constant applied temperature gradient can be strongly affected by the finite thermal conductivity of the stagnant cap. Numerics The heat conduction of the stagnant cap is analytically modeled. The effects of the additional interfacial stresses generated by the disturbances to the local temperature field due to the presence of the cap at the fluid-fluid interface and the corresponding velocity of migration of the drop are evaluated by solving for the temperature and hydrodynamic field equations in and around the drop. An asymptotic model is derived to predict the terminal velocity in the presence of an infinitely conducting stagnant cap. Findings The effects of the surface conductivity and size of the stagnation region alongside the bulk thermal conductivities and viscosities of the drop and surrounding media are evaluated. The terminal velocity of the drop is shown to have a monotonic dependence on the conductivity of the stagnant cap. The bounds to the terminal velocity increment due to the stagnant cap are derived. These bounds can be of significance to multiphysics problems involving particle laden drops, Pickering emulsions and other multi-phase technologies where the conductivity of the surface adsorbents is non-negligible.

Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration

HYPOTHESIS

Gradients in the concentration of amphiphiles play an important role in many non-equilibrium processes involving complex fluids. Here we explore if non-equilibrium interfacial behaviors of thermotropic (oily) liquid crystals (LCs) can amplify microscopic gradients in surfactant concentration into macroscopic optical signals.

EXPERIMENTS

We use a milli-fluidic system to generate gradients in aqueous sodium dodecyl sulfate (SDS) concentration and optically quantify the dynamic ordering of micrometer-thick nematic LC films that contact the gradients.

FINDINGS

We find that the reordering of the LCs is dominated by interfacial shearing by Marangoni flows, thus providing simple methods for rapid mapping of interfacial velocities from a single optical image and investigating the effects of confinement of surfactant-driven interfacial flows. Additionally, we establish that surface advection and surfactant desorption are the two key processes that regulate the interfacial flows, revealing that the dynamic response of the LC can provide rapid and potentially high throughput approaches to measurement of non-equilibrium interfacial properties of amphiphiles. We also observe flow-induced assemblies of microparticles to form at the LC interface, hinting at new non-equilibrium approaches to microparticle assembly. We conclude that dynamic states adopted by LCs in the presence of surfactant concentration gradients provide new opportunities for engineering complex fluids beyond equilibrium.

Interfacial Behavior of Particle-Laden Bubbles under Asymmetric Shear Flow

The behavior of moving bubbles has mostly been studied in an axisymmetric flow field. To extend the knowledge to practical conditions, we investigate the interfacial and hydrodynamic properties of bubbles under asymmetric shear forces. Experiments are performed with a buoyant bubble at the tip of a capillary placed in a defined shear flow in the presence of surfactants, nanoparticles, and glass beads. The response of the interface to the surrounding asymmetric flow is measured under successive reduction of the surface area. Profile analysis tensiometry is utilized to investigate the dynamic surface tension and the surface rheology of the surfactant- and nanoparticle-laden interfaces. Microscopic particle image and tracking velocimetry are used to study the bulk flow and the interfacial mobility of the buoyant bubble. According to our results, the rotational component of the shear flow provokes an interfacial flow, which redistributes the adsorbed surfactants and particles at the interface. In the presence of NPSCs, a contiguous network of particles forms at the interface through densification of surface structures. We show that this interconnected nanoparticle network eventually stops the interfacial flow and decreases the mobility of the glass beads at the interface. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field. Our findings can serve as a basis to formulate boundary conditions for refined modeling and to predict the hydrodynamics of bubbles and droplets.