Statically Oleophilic but Dynamically Oleophobic Smooth PDMS Brush Surface

摘要

Improvement of the liquid droplet motion on various solid surfaces is a key factor in a wide variety of advanced applications, such as microfluidics, micro-/nano-electromechanical systems (MEMS/NEMS), self-cleaning surfaces and window coatings. In the ongoing effort to control the motion of liquid droplets on solid surfaces, dynamic contact angles (CAs) rather than static CAs (θ_s) are more useful because the latter only provides information about the general surface energy of the solid surface, and in some cases, can result in incorrect understandings of the surface chemistry/topography. To understand the actual surface dewettability and the ease with which liquid droplets can move on the surface, it is necessary to determine the CA hysteresis. This is described in Equation 1,1 where F_S = kwγ_(LV) (cosθ_R - cosθ_A) (1) F_S is the force required to move the droplet, k is a constant that depends on the drop shape, w is the width of the drop and γLV is the liquid-vapor surface tension. θ_A and θ_R are the advancing and receding CAs, respectively, and cosθ_R - cosθA is the CA hysteresis. Equation 1 predicts that with greater differences between θ_A and θ_R, F_s also increases. If the CA hysteresis is high enough, the liquid droplets must be angled at larger tilt angles (TAs) and be considerably distorted from a section of a sphere to a section of a tapered ellipsoid, for the droplets to finally move. This droplet shape change has been described as the activation energy barrier to droplet movement. If the CA hysteresis is sufficient small, only slight distortion of the droplet is required and the activation energy barriers for movement are small. Subsequently, the liquid droplet moves smoothly at relatively low TAs, even if the CAs are low (regardless of the degree of hydrophobicity).