Effect of initial droplet shape on the tangential force required for spreading and sliding along a solid surface

Abstract

Listen

Translate article

Despite the extensive study of wetting in literature, there are still many unresolved issues regarding forced wetting. One of them refers to the effect of the initial droplet shape on the force required for spreading and sliding along a solid surface; an effect that has not ever been explicitly reported. Previous experimental works, in general, assume initially axisymmetric droplet shapes. In this study, experiments are performed with an innovative device, Kerberos, capable of subjecting sessile droplets at different tilting angles to varying centrifugal forces in order to explore the spreading/sliding behavior of droplets of different volumes and different initial shapes (including non-axisymmetric). A broad validation of the technique is achieved by the repeatability and consistency of measurements. Results for initially axisymmetric and non-axisymmetric droplets concerning contact angles, droplet length, droplet shape and velocity are presented and discussed. Furthermore, detailed results for the critical tangential accelerations required for the inception of spreading and sliding are presented showing the different sensitivity of these parameters on the droplet initial shape. Experimental results are employed to test the applicability of the well-known Furmidge equation for the retention force in the case of initially non-axisymmetric droplets. On this account, the initial length of droplets is found to be a more appropriate length scale in the Furmidge equation than the initial droplet width.