Forced spreading behavior of droplets undergoing low frequency vibration

Abstract

The profile of a droplet sitting on a flat surface is related to the contact angle, when the surface is vibrated the droplet oscillations result in a fluctuation in the contact angle over time. Depending on the amplitude of vibration this can be accompanied with an oscillation of the position of the contact line, this motion occurs when the instantaneous contact angle exceeds the advancing angle or falls below the receding angle. In this work we demonstrate that a net spreading can occur over multiple cycles. The result is that the final contact angle, after the cessation of vibration, is much reduced from the initial value. This has applications in the uniform filling of wells, such as in a microplate. The degree of forced spreading is shown to depend on the mode of oscillation, rather than purely the amplitude of vibration, with cases being demonstrated whereby an increase in amplitude decreases the degree of spreading due to an axisymmetric oscillation changing into a ‘spinning’ mode. Four regions of behavior are identified as a function of vibrational amplitude. In the lowest amplitude state no spreading is observed. The second region exhibits modest spreading resulting from axisymmetric oscillation. Upon further increase in amplitude, a spinning mode occurs in which no spreading results. Finally, the fourth region is characterized by a complex oscillation in which large scale spreading can be achieved.