Droplet Manipulation with Feedback Control.

Abstract

Droplet manipulation has an important role in a wide range of scientific and industrial applications, such as thin film, soft matter, microfluidics, biophysical simulations, interfacial reactions, and interfacial rheology. We developed a feedback-controlled closed-loop axisymmetric drop shape analysis (CL-ADSA) to manipulate millimeter-sized droplets, based on the experimental methodology constrained drop surfactometer (CDS). With CL-ADSA, we extended the CDS from a surface tension measurement tool to a sophisticated droplet manipulation instrument. We demonstrated the usefulness of this methodology in three practical applications, control of droplet volume by automatically compensating natural evaporation, precise control of surface area variations for high-fidelity biophysical simulations of natural pulmonary surfactant, and steady control of surface pressure for in situ Langmuir-Blodgett transfer from droplets. Furthermore, we developed an arbitrary waveform generator for controlled oscillation of a droplet/bubble. We demonstrated this methodology in three ways. First, we demonstrated arbitrary waveform generation by controlling the volume and surface area of a water droplet to sine, triangle, square, and sawtooth waveforms. Second, we evaluated the accuracy of the arbitrary waveform generation through a coefficient of determination analysis. Third, we apply arbitrary waveform generation to study the interfacial dilational rheology of adsorbed surfactant films. Our studies suggest the feedback control system developed in this thesis hold great promise for advancing droplet manipulation in a variety of material and surface science applications, such as thin-film fabrication, self-assembly, and biophysical study of pulmonary surfactants.