Interfacial flows in active matter and energy processes

Abstract

Interfacial flows, occurring where two fluid phases meet, have rich dynamics and are crucial to many industrial applications. Combining both theoretical and experimental tools, this thesis explores interfacial flows in five settings: (1) a spontaneous dripping mechanism in viscous films that has not been studied before, (2) effects of the inner-flow field in diffusiophoretic droplets, (3) the interplay of chemical and hydrodynamic interactions between two autophoretic particles, (4) the Rayleigh-Taylor instabilities of an electrically conductive liquid film in magnetic fields inspired by applications in fusion devices, and (5) the airborne transmission of pathogens via speech-driven jets.

Unlike conventional jet and drop breakup, which forms a cylindrical liquid column, the dripping of a viscous film will form a liquid annulus, which then ruptures and heals due to surface tension and the inner surface forms a retracting tip. We apply a one-dimensional model to analyze the healing dynamics, which predicts a universal thinning curve and shows good agreement with experimental measurements. The shape of the tip is documented to be conical and the retraction speed is determined by the balance of viscous and capillary stresses.

In the second part of the thesis, the diffusiophoresis of a charged drop in an electrolyte solution is investigated both analytically and experimentally. The flow inside the drop is found to be driven by the tangential electric stress at the interface and directly influences the diffusiophoretic speed of the drop.

We then extend the theory of one-body diffuiophoresis to consider both chemical and hydrodynamic interactions between two particles undergoing self-diffusiophoresis by adsorbing or desorbing solutes on particle surfaces. We find that when the sorption fluxes are large, the ion concentration near the particle surfaces, and consequently the Debye length, is strongly modified, resulting in a nonlinear dependence of the phoretic speed on the sorption flux.

The stability of a thin conductive liquid film flowing under a solid wall is an important problem that occurs in many proposed fusion device designs. In the last part of the thesis, we discuss the linear stability of an electrically conductive liquid film of finite thickness in the presence of a strong magnetic field. The magnetic field is found to decrease the unstable growth rate for both finite and infinite conductivity fluids but only narrows the unstable modes for infinite conductivity fluids.

It is now recognized that aerosol transport contributes to the transmission of the SARS-CoV-2 virus. In the last chapter of the thesis, we improve existing social distancing guidelines for airborne pathogens, which are typically given in terms of distance with vague statements about contact times. Also, estimates of inhalation of virus in a contaminated space usually assume a well-mixed environment, which is realistic for some, but not all, situations. In particular, we consider a local casual interaction of an infected individual and a susceptible individual, both maskless, account for the air flow and aerosol transport characteristics of speaking and breathing, and propose social distancing guidelines that involve both space and contact time, based on a conservative model of the interactions.