Optical Measurement of Electrically Induced Resonance in Sessile Liquid Metal Drops: A Method for Mechanical Characterization

This paper addresses an innovative approach for characterizing the mechanical properties of liquid metals using the sessile drop method as a simpler alternative to acoustic or magnetic levitation. Although this method requires careful consideration of wetting dynamics, it offers new insights into determining key properties like viscosity and surface tension. Shape oscillations of sessile liquid metal drops are investigated by employing AC electrowetting to trigger mechanical resonance through electrostatic actuation of the contact angle, despite the metal's high electrical conductivity. Resonances are achieved at low frequencies ranging from 10 to 500 Hz. As a model system, millimeter-sized drops of galinstan (a non-toxic liquid alloy at room temperature) used. Surface tension and viscosity are tracked over time to monitor surface aging due to unavoidable surface oxidation, with micrometric oscillations generated from AC electrowetting and captured by a custom-built interferometer. The analysis reveals stable values for the viscosity (0.05 – 0.06 Pa.s), while surface tension progressively increases from 1 to 5 N/m due to oxidation. These results align with typical properties of liquid metals, underscoring the potential of this technique for efficient albeit simple mechanical characterization.