Water droplet movements on a variety of organosilane modified wettability gradient surfaces were first examined. These gradient surfaces were generated by the contact printing (CP) of octadecyltrichlorosilane (OTS) or octadecylmethyldichlorosilane (OMDS) on silicon wafer surfaces. The experimental results showed that a water droplet as small as a few pecoliters could move toward the higher wettability region on these gradient surfaces. As the droplet size or the gradient scale increased, the droplet velocity increased. The study also confirmed that of the two factors to cause the resistances in droplet movement, contact angle hysteresis (CAH) was always the predominated factor, while the interfacial friction only became more important when the wettability gradient size scaled down to sub-millimeters.
To predict the contribution of CAH and interfacial friction in resisting droplet motion, the modes of droplet movement on the gradient surfaces should first be determined. However, under the current experimental conditions (small droplets and short droplet traveling time); it was too challenging to obtain the droplet motion modes on the wettability gradient surface. Alternatively, tracer particles were suspended in large water drops that moved down on inclined OTS surfaces, also generated by CP, and the internal fluidity was deduced from the movement of the tracer particles. The results showed that the motion of the water droplet is a combination of sliding, slipping, and rolling.
Furthermore, to evaluate the effect of drop size on the drop motion mode, water drop movements on an inclined dimethydichlorosilane (DDS) surface having a low water CAH (i.e. CAH of about 5° as compared to ~ 20° for OTS or OMDS surfaces) were studied. It was experimentally observed, by including tracer particles inside the water drops during the drop movements, which at a lower inclined angle, rolling had a greater contribution to the drop motion; while at a higher inclined angle, sliding contributed more to the drop movement. An analysis based on the ratio of the rotational torque to that of the sliding torque of a water drop on an inclined surface captures the basic trends of the experimental results. Experimental results also showed that a smaller drop exhibits a larger rolling contribution to the drop motion. Under a small ratio (< about 18) of the torques after by considering the effects of both inclined angle and drop size, a complete rolling becomes possible; conversely, a large enough ratio (> about 36) of the torques leads to no rolling with small or even no slipping. When the degree of rolling and the degree of the interfacial slipping sum up to a value less than unity; the sliding of the drop likely accounts for the remaining portion of the droplet velocity.