Oscillatory flow in the vicinity of pinned-contact capillary surfaces

Abstract

A one-dimensional computational model is shown to predict the dynamics of the double-droplet system well for a certain range of physical parameters. The double-droplet system in an all liquid environment was demonstrated by using an oil-based ferrofluid immersed in water, actuated by an oscillating magnetic field. The shape of the ferrofluid droplet interface under a magnetic field was shown to have a prolate shape, similar to shapes observed in electric fields. While actuating an immersed ferrofluid double-droplet system a new technique for extracting interfacial tension was developed through experiments and simulations. To understand the fluid flow at different frequencies and amplitudes throughout a period of oscillation, PIV was performed in an all-liquid system. Velocity profiles are presented at cross-sections, with variations dependent on the distance from the pinned-contact capillary surface. The velocity profiles tended to resemble an analytic solution for laminar fluid flow in an infinitely long pipe, where there is a lag in velocity between the centerline and near the wall as the frequency was increased.

Capillary surfaces formed by a liquid protruding from circular openings were studied primarily through experiments. Most of the systems investigated involved at least one coupled droplet pair (double-droplet) with its contact line pinned, and the surrounding fluid was either a gas or another liquid. In the case of the latter, the scale was larger, which facilitated some of the measurements. These studies were motivated by the application of the double-droplet system in fast adaptive optics, microscale actuators and pumps, and adhesion devices. The responses and characteristics of the systems were observed through experimental measurements and most results have been compared with computational and theoretical approximations. The double-droplet system can be used as a variable focus liquid lens, by manipulating the radii of curvature of pinned-contact drops. In a gas/liquid system (water in air), fast responses on the order of 100 Hz are achieved with pressures less than 10 Pa, and utilized for variations of the focal distance while the system is oscillated. Optical characteristics of the water double-droplet system in air were studied parametrically.