Drop shape analysis

Surface and interfacial tensions are fundamental physicochemical properties governing the behavior of fluid interfaces. From the stability of emulsions, foams, and thin liquid films to pulmonary surfactant function, microfluidics, wetting, coating, drug delivery, and advanced materials processing, interfacial forces dictate how fluids deform, spread, transport mass, and dissipate energy across length scales spanning nanometers to millimeters. Despite their ubiquity in nature and technology, accurate characterization of surface and interfacial tensions remains experimentally and computationally challenging, particularly for dynamic systems far from equilibrium.

For more than half a century, axisymmetric drop shape analysis (ADSA) developed by Neumann and coworkers has served as the gold standard for tensiometry. By fitting experimentally observed droplet profiles to numerical solutions of the Young–Laplace equation, ADSA enables highly accurate measurements of surface and interfacial tensions from sessile and pendant drops. The method elegantly transforms droplet geometry into physicochemical information, establishing ADSA as a cornerstone technique in colloid and interface science.

We have developed a novel closed-loop axisymmetric drop shape analysis (CL-ADSA) integrated into constrained drop surfactometry (CDS) for real-time control of droplets. CL-ADSA transforms droplet tensiometry from an iterative offline analysis method into a real-time intelligent measurement platform. We have demonstrated the feasibility of CL-ADSA for automated droplet volume regulation through compensation of natural evaporation, precise control of surface area variations for high-fidelity biophysical simulations of natural pulmonary surfactant, and stable control of surface pressure for in situ Langmuir–Blodgett (LB) transfer from droplets. Furthermore, we have demonstrated the capability of CL-ADSA to continuously oscillate droplet surface area according to prescribed waveforms, including small-amplitude harmonic oscillations. This novel functionality enables quantitative evaluation of interfacial dilatational rheological properties.