Constrained Drop Surfactometer for Studying Interfacial Structure and Rheology

Abstract

Measurements of surface tension and interfacial rheology of liquid-fluid surfaces play an important role in a variety of scientific and industrial fields, such as smart material, thin film, soft matter, microfluidics, and biophysics. Being a miniaturized experimental platform for studying surface phenomena, droplets hold great advantages over the traditional experimental methods, such as the classical Langmuir trough, in determining surface tension and interfacial rheological properties. The focus of this thesis was to develop a novel droplet-based experimental platform called the constrained drop surfactometer (CDS) for studying surface tension and interfacial rheology. Axisymmetric drop shape analysis (ADSA) was used as a numerical algorithm to determine the dynamic surface tension as a function of time and surface area variations. We first proposed a new dimensionless parameter, called the Neumann number, N e ≡ Δ ρ g R0H / γ, to replace the classical Bond number for evaluating the accuracy of ADSA upon reducing drop volume. We then developed a closed-loop ADSA (CL-ADSA) algorithm for determining and controlling droplet parameters, including the volume, surface area, and surface tension, in real-time. With the CL-ADSA, the CDS was transformed from a traditional surface tension measurement methodology to a sophisticated experimental platform for manipulating millimeter-sized single droplets in real-time. We have demonstrated the accuracy, robustness, versatility, and automation of this droplet manipulation technique. Finally, we engaged the combination of CDS and CL-ADSA in studying interfacial rheology. Understanding the interfacial rheology of complex fluids plays a central role in a range of applications such as food processing, detergency, coating, cosmetic, and pharmacology. For the first time, our methodological advance permitted direct control of surface area oscillated in a sinusoidal pattern, thus resulting in a precise evaluation of the surface dilational rheological properties of complex fluids, including surfactants and proteins. Our results showed that the CDS, together with the CL-ADSA, holds great promise for advancing the study of interfacial structure and rheology.