Motion of Drops with Different Viscosities on Micro‐Nanotextured Surfaces of Varying Topography and Wetting Properties

Superhydrophobic surfaces are extensively investigated in the literature, yet the phenomenon of drop motion on such surfaces and the corresponding friction properties of surfaces with different topography are not sufficiently analyzed. Here, drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). The threshold force required to initiate drop movement is probed, the drop motion (velocity and acceleration) is analyzed, and the friction force on each surface is calculated. It is evident that as roughness increases, the threshold force to initiate 20 µL drop motion decreases; the lowest value for water is 17.9 ± 4.0 µN. For glycerol, the lowest threshold force value is 22.3 ± 5.9 µN. The results also indicate that this threshold force required for the initiation of the drop motion seems to be higher than that when the drop starts moving. Finally, this force (being proportional to the contact line) is expected to be about half smaller for 5 µL droplets. Water drops obtain higher velocities and accelerations by an order of magnitude compared to glycerol drops, which is attributed to the combinational effect of the higher hysteresis and the larger contact line of glycerol drops. Drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). It is evident that as roughness increases, the threshold force to initiate drop movement decreases and that the surface friction properties depend on the topography characteristics (i.e., height and spacing).