Mechanism Study on Transition of Cassie Droplets to Wenzel State after Meniscus Touching Substrate of Pillars

To understand the conditions and mechanism of droplet wetting transition from Cassie to Wenzel state (C-W) and furthermore to prohibit this transition are the key research contents about superhydrophobic materials. In this study, the C-W transition process after meniscus touching substrate (MTS) was divided into different stages. Then, the changes of droplet interfacial free energy (IFE) after MTS were analyzed. And the resistance on three-phase contact line (TPCL) was also investigated. Furthermore, based on the droplet IFE always changing from high to low, a criterion formula for C-W transition was derived so that the mathematical model was founded. The calculation results show that the smaller the pitch and/or diameter of pillars, the more difficult the MTS and C-W transition for an initial sessile Cassie droplet. Therefore, nanotextures can efficiently prevent the C-W transition. Besides, the MTS of droplets on short and high pillars can be realized with sag and TPCL depinning impalement, respectively. Additionally, the greater the intrinsic contact angle, the more unfavorable the droplet C-W transition. Moreover, micronano two-tier textures can effectively inhibit the C-W transition. Finally, the model results are in good agreement with the experimental measurements reported in literature, with 92% accuracy.