Experimental study on the evaporation of sessile droplets excited by vertical and horizontal ultrasonic vibration

- Behavior of evaporating sessile droplets excited by ultrasonic vibration is studied.
- Static apparent, receding and advancing, as well as dynamic contact angles are reported.
- Excitation by vibration results in an increase in the evaporation rate.
- Droplets under horizontal vibration evaporate faster than those under vertical vibration.
- The left and right contact angles of horizontally-vibrating droplets oscillate with a 180° phase shift.

Interaction between sessile droplets and solid surfaces is a fundamental science and engineering problem, with ubiquitous presence in various applications. In this paper, we study the effect of imposing vertical and horizontal ultrasonic vibration (40 kHz) on dynamics (oscillations) and evaporation of sessile droplets of dimethylformamide (DMF), isopropyl alcohol (IPA) and water on Teflon and glass substrates. There is no or very few works considering dynamics and evaporation aspects of excited droplets, simultaneously. The theory concerning the force balance and pinning/depinning in pristine and excited sessile droplet systems is elucidated. Time varying left and right contact angles and contact radius are measured for the duration of the droplet lifetime, where the stick-slip phenomena are observed and interpreted for various liquids. Imposing substrate vibration results in significant decrease in droplet lifetime and affects the behavior of the stick-slip mechanism. Droplets excited by horizontal vibration have the shortest lifetime. It is also experimentally shown that in the case of vertical vibration, the left and right contact angles oscillate in-phase, whereas in the case of horizontal vibration, there is 180° phase difference between left and right contact angles.