Mixing enhancement by degenerate modes in electrically actuated sessile droplets

• Published in: Sensors and actuators. B, Chemical Vol. 232; pp. 318 - 326
• Main Authors: Bansal, Shubhi, Sen, Prosenjit
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2016
• Subjects: Actuation; Axisymmetric; Contact; Degenerate modes; Droplet dynamics; Droplets; Electric potential; Electrowetting; Mathematical analysis; Mathematical models; Mixing; Non-axisymmetric oscillations; Voltage; Electrowetting; Droplet dynamics; Non-axisymmetric oscillations; Mixing; Degenerate modes
• ISSN: 0925-4005; 1873-3077
• DOI: 10.1016/j.snb.2016.03.109

•Non-axisymmetric degenerate modes of droplet oscillations actuated by electrowetting were studied.•A region of instability marked by actuation parameters required to get non-axisymmetric mode was determined experimentally.•A parametric oscillator model was developed using Mathieu equation, which matched with the experimental observations.•The contact angle and radius were found to increase and decrease together for consecutive non-axisymmetric modes.•Mixing in parametric oscillations occurred in 393 milliseconds and was 37 times faster than diffusive mixing. The dependence of oscillation dynamics of a sessile droplet on the actuation parameters (voltage and frequency) in AC electrowetting which leads to the manifestation of non-axisymmetric oscillation patterns were investigated through experiments and theoretical modeling. The symmetrical nature of the electrowetting force leads to a circular three phase contact line for low actuation voltages. At higher actuation voltages, despite of symmetrical actuation force the contact line showed a transition from axisymmetric to non-axisymmetric oscillations. We found a good match between the experimentally determined region in the actuation parameter space where non-axisymmetric modes are dominant and the theoretically modeled parametric instability region derived from the Mathieu equation. The results showed that these non-axisymmetric modes are degenerate sectoral modes defined by the spherical harmonic functions. In contrast to axisymmetric oscillations, for non-axisymmetric oscillations the variation of contact angle and base radius remained in-phase between successive resonant modes. Finally, mixing by these parametric oscillations was investigated and the best mixing time was approximately 2% of the diffusive mixing time.