Free energy barrier in wetting parallel-structured surfaces

Parallel structures with various simple cross-section geometries, such as trapezoid, rectangle, and circle, can be patterned on surfaces to control wettability. Free energy barriers that hinder a system from reaching its global minimum of free energy can influence the wetting behaviors. Specifying the forming conditions of energy barriers help evaluate metastable contact angles and contact angle hysteresis. Therefore, a thermodynamic model relating free energy with the continuous movement of three-phase contact line is needed to evaluate the energy barrier. This paper reports an integrated thermodynamic model for various parallel structures with simple geometries. Forming conditions of energy barriers are derived from the thermodynamic model, and validated using experimental data by electrospun microfibers and existing theoretical works. For sharp-edge structures with an internal angle φ and an intrinsic contact angle θY, the initiating and ending apparent contact angles for energy barriers are θY+φ and θY–φ, respectively, in noncomposite state, but this range may be discontinuous when θY<φ/2 or θY>180˚–φ/2. For round structures, an energy barrier may form at every structure in noncomposite state. In our experiment, aligned electrospun microfibers (with a mean radius of 0.9 µm) can pin the droplet at the first advancing energy barrier and achieve contact angles greater than 150˚.