Dynamic wetting of drop spreading over asymmetric sawtooth surface structures

A liquid drop spreading over a dry solid surface is ubiquitous in nature and the technological importance is increasing. While there is lack of understanding in the mechanism of early rapid spreading, it has been shown that a friction related to the contact line movement plays an important role in hindering the spreading. The hindrance by line friction can be magnified by micro-scale roughness as it increases the effective surface area. However, the role of actual geometry of the roughness has remained unclear. In this work, we have formulated a toy model that accounts for the roughness geometry and applied to the case of asymmetric sawtooth structures in which the surface area is the same, but the dynamic contact angle varies depending on the spreading direction. By series of spreading experiments, the imbalanced growth of the spreading radius between the two opposite directions perpendicular to the sawtooth alignment is found to be well quantified by the toy model. The results reveal that the engineered surface geometry can realize directional spreading. Furthermore, the toy model validates the mechanism of the early spreading as a time extension of the contact line movement caused by the local departure of the contact angle from its static value in terms of roughness shapes.