(353d) Thin Film Drainage Between Micro-Drops Under Applied Cyclical Drives to Mimic Micro-Fluidic Pumping Scenarios

Our research has focused on the development of innovative methods to quantitatively study the nano-scale thin films between micro-drops or micro-bubbles during their collision and coalescence. The forces that control these collisions are crucial for understanding applications as wide ranging as purification steps in mineral and pharmaceutical processing to emulsion and foam stability in food and personal care products as well as micro-fluidic devices.  Previously, we have developed experimental and theoretical methods developed to study the dynamic interactions between two drops or bubbles using Atomic Force Microscopy (AFM) for single collisions. The agreement between experimental measurements and the quantitative modeling of the thin film drainage between deformable drops and bubbles has shown that the governing physics is dependent on a combination of equilibrium surface forces, hydrodynamic drainage forces and interfacial deformation at a wide range of collision speeds and varied solution conditions.  Both experimental measurements and quantitative modeling have been able to shed light on behavior as diverse as the charging mechanism for drops and bubbles to counter-intuitive observations in micro-fluidic devices such as coalescence of drops on separation. 

This talk will focus on what we can learn from the direct force measurement of sequential collisions of pairs of micro-drops in order to mimic pumping conditions in micro-fluidic devices.  A number of diverse velocity drives profiles were used to drive to drops together with repeated collisions including waveforms for sinusoidal, peristaltic and diaphragm pumping.  In most cases, it was shown that when equilibrium interactions of the drops were a meta-stable state, small perturbations in pumping conditions could cause drop coalescence.  This was further probed through using simple constant velocity approach and retract cycles where the approach and retract velocities were varied independently.  Agreement between quantitative modeling of these collisions and the experimental measurements allows for visualization of the thin film and pressures profiles during these processes.  In addition, the validated model was then used to produce drop stability maps as a function of pumping condition parameters.