Interfacial Viscoelastic Response in Electrically Deformed Picoliter Droplets of Synthetic Urine

Hypothesis: Adsorption of amphiphilic proteins at liquid–air interfaces modifies both interfacial tension and dilatational viscoelasticity. We hypothesize that electrically triggered oscillations of picoliter droplets provide a sensitive mechanical probe of these coupled bulk–interfacial effects. Experiments: Synthetic urine was supplemented with bovine serum albumin at concentrations from 0.0001 to 50 mg mL − 1 . Short electrohydrodynamic (EHD) pulses were applied to pendant picoliter droplets to induce underdamped post-jet oscillations, recorded by high-speed imaging. Oscillation frequency and decay time were extracted and interpreted using a reduced mechanical model that couples bulk viscosity with interfacial tension, dilatational elasticity, and surface viscosity. Independent pendant-drop tensiometry and step-strain dilatational rheology measurements provided surface tension and viscoelastic moduli. Findings: Increasing BSA concentration produces a monotonic decrease in both oscillation frequency and decay time. These trends reflect a reduction in surface tension and a concurrent increase in interfacial elasticity and viscosity due to protein adsorption. A single global coupling length enables quantitative agreement between measured dynamics and model predictions across regimes spanning low to high Boussinesq number. The results demonstrate that EHD-induced droplet oscillations constitute a robust, picoliter-scale method to excite and quantify interfacial viscoelasticity in protein-laden fluids. This approach provides a non-contact platform to investigate adsorption-driven interfacial mechanics in complex biofluids.