Dewetting Transitions on Superhydrophobic Surfaces: When Are Wenzel Drops Reversible? Jonathan B. Boreyko and C. Patrick Collier* Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6493, United States * S Supporting Information ABSTRACT: On superhydrophobic surfaces, drops in the Wenzel state can be switched to the suspended Cassie state in some cases but in other cases are irreversibly impaled in the surface roughness. To date, the question of when dewetting transitions are possible for Wenzel drops has not been resolved. Here, we show that pinned Wenzel drops being stretched out-of-plane cannot reduce their contact angle below a critical value where unstable pinch-off occurs, preventing dewetting for Wenzel drops that exhibit receding contact angles beneath this critical pinch-off angle. Dewetting transitions are therefore only possible for Wenzel drops with moderately large receding contact angles (θ r ≈ 90°), which requires low surface roughness for one-tier surfaces or a Partial Wenzel wetting state for two-tier surfaces. ■ INTRODUCTION Superhydrophobic surfaces are typically designed to suspend water in the mobile Cassie state; however, it is well-known that wetting transitions to an impaled Wenzel state frequently occur even when the Cassie state is energetically favorable. 1 Conditions known to trigger wetting transitions include mechanical compression, 2,3 underwater submersion beyond a critical hydrostatic pressure, 4,5 electrowetting, 6 evaporation to a higher Laplace pressure, 7,8 drop impact, 9,10 mechanical vibration, 11 or deceleration during drop deposition. 12 Perhaps even more problematic are condensate drops that nucleate from within the surface roughness, which typically form in a fully impaled Wenzel state on microscale-only roughness 3,13,14 and in a partially wetting state on nanoscale or hierarchical roughness. 15-18 Utilizing superhydrophobic surfaces with robust hierarchical roughness can maximize the energetic favorability of the Cassie state and delay the occurrence of wetting transitions, 19-23 but it is becoming increasingly clear that Wenzel states are still common in many systems. Despite the endemic problem of superhydrophobic surfaces exhibiting Wenzel drops due to wetting transitions or condensation, the possibility of dewetting back to the suspended Cassie state has been largely ignored. This disregard of dewetting transitions was not borne out of neglect; rather, several reports convincingly demonstrated that the Cassie to Wenzel wetting transition was irreversible even when the Wenzel drop was mechanically forced 3,11 or when an electrowetting voltage was released. 6,24,25 A Wenzel to Cassie transition was accomplished by vaporizing a drop’s liquid-solid interface via high-temperature heating, 26,27 but this method is impractical for many systems and does not preserve the drop’s volume. A resurgent interest in dewetting was stoked by a recent report that deposited drops and condensate trapped in a Wenzel state on a lotus leaf could fully transition to the stable Cassie state when subjected to mechanical vibration. 16 A follow-up report using microfabricated surfaces found that hierarchical roughness is crucial for enabling dewetting transitions: Wenzel drops fully impaled in one-tier or two-tier roughness were indeed irreversible, but partial Wenzel drops wetting only one of two tiers could be effectively dewetted to the Cassie state. 28 Similar results were observed for hierarchical superhydrophobic surfaces submerged in water baths, where the partial Wenzel state could be dewetted to the Cassie state via negative pressure 29 or electrolysis. 30,31 The irreversibility of Wenzel drops on certain super- hydrophobic surfaces has previously been attributed to the energetic favorability of the Wenzel state, 3,11,29 but recently, dewetting transitions have been obtained even when the Cassie state is metastable. 27,32 Another common hypothesis is that an energy barrier prevents dewetting, 19,29,33 but this cannot explain why a Wenzel drop fails to dewet even when subjected to a strong out-of-plane force. 28 It is also unclear when dewetting is restricted to the Partial Wenzel state exclusive to hierarchical surfaces, 28,29,31 as several recent reports were able to obtain a Wenzel to Cassie transition for drops impaled in one-tier surfaces. 27,32,34 Therefore, it remains poorly understood when a dewetting transition from a Wenzel to Cassie state is possible. Received: May 29, 2013 Revised: August 12, 2013 Published: August 12, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 18084 dx.doi.org/10.1021/jp4053083 | J. Phys. Chem. C 2013, 117, 18084-18090