Wetting in water–oil–nonionic amphiphile mixtures
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In a three-phase equilibrium of H2O –oil–amphiphile mixtures, the middle amphiphile rich phase may or may not wet the water/oil interface. For nonwetting middle phases, theory predicts a nonwetting→wetting transition upon approaching either one of the two critical endpoints. With respect to an experimental confirmation of this prediction, the situation appears to be controversial. In this paper, we have, therefore, studied the wetting behavior of the middle phase as it depends on the amphiphilicity of nonionic amphiphiles. We find that in mixtures with short-chain amphiphiles, the middle phase wets the water/oil interface in the entire three-phase interval, whereas with long-chain amphiphiles it (apparently) never wets. For medium-chain amphiphiles, however, one does find a nonwetting→wetting transition. On the basis of this result, we suggest that there exist four cases for the wetting behavior as a consequence of the dependence of the relations between the three interfacial tensions on amphiphilicity. The wetting behavior can be correlated with the evolution of the three-phase bodies from a tricritical point. Upon increasing amphiphilicity, their characteristic properties pass through maxima in the range of medium-chain amphiphiles, coinciding with the transition from always wetting to never wetting.Keywords:
Wetting transition
Superhydrophobicity of biological surfaces with micro/nanoscale hierarchical roughness has recently been given great attention and widely reported in many experimental studies due to the unique wettability. For example, the dual-scale structure of the lotus leaf not only shows high contact angle and low contact angle hysteresis but also presents good stability and mechanical properties. Though lots of experimental studies on the wettability of artificial hierarchical rough surface have been carried out, a thorough analysis on the contribution of micro- and nano-scaled roughness to the metastable wetting states and their transition is still lack. In this paper, a thermodynamic approach is applied to analyze all the wetting states (including four stable wetting states and five transition states) of a water droplet on a surface with micro/nanoscale hierarchical roughness, and the corresponding free energy expressions and apparent contact angle equations are deduced. The stable wetting states are confirmed by the principle of minimum free energy. And the calculated results by these state equations can fit well with the experimental results reported in the literature when compared with the previous models. Meanwhile, the influence of micro/nanoscale roughness on the stable wetting states and metastable-stable transition has been analyzed thermodynamically. It is found that there is a synergistic effect of micro and nanoscale roughness on wettability, which nlay result in many different wetting states. There are four wetting states during increasing relative pitch of a microscaled structure at a given nanoscaled structure, but two wetting states can be obtained as increasing relative pitch of nanoscaled structure at a given microscaled structure. The change of nondimensional energy and nondimensional energy barrier in the metastable-stable transition process of water droplet wetting micro and nanoscaled structure is quantitatively analyzed. Results indicate that the micro-scaled structure is never wetted in a special size range of the nanoscaled structure, and the special size range is of great significance to enhance superhydrophobic stability of the microscaled structure. Furthermore, the existence of microscaled structure decreases the transition energy barrier of water droplet wetting nanoscaled structure, which is helpful for understanding the experimental results reported in the literature. Finally, all possible stable wetting states of water droplet no a surface with micro/nanoscale hierarchical roughness are discribed in a wetting map. A design principle of superhydrophobic surface with micro/nanoscale hierarchical roughness is put forward, which is helpful to ensure the size of micro/nanoscale structure in the “stable superhydrophobic region” and to provide a theoretical guidance in the preparation of superhydrophobic surface.
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The Cassie–Wenzel (C–W) wetting transition has been extensively investigated; however, the wetting transition of water films on textured surfaces with different topologies, together with underlining mechanisms, is unsatisfactorily explored. In this study, the C–W wetting transition of water films on pillar-arrayed and striped surfaces is studied. The results show that, on pillar-arrayed surfaces, the free energy variation during the C–W wetting transition follows the classical wetting pathway. The free energy first increases with the intrusion of water into the asperities and then decreases after a water film touches the basal surface. However, on striped surfaces, there exist multiple partial wetting states with each one occupying a local energy-minimization configuration. Accordingly, the water film needs to overcome multiple energy barriers to realize the C–W wetting transition. Moreover, the effects of aspect ratio and intrinsic wettability of the two textured surfaces on the C–W wetting transition are discussed.
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When a liquid droplet is put onto a surface, two situations distinguishable by the contact angle may result. If the contact angle is zero, the droplet spreads across the surface, a situation referred to as complete wetting. On the other hand, if the contact angle is between 0° and 180°, the droplet does not spread, a situation called partial wetting. A wetting transition is a surface phase transition from partial wetting to complete wetting. We review the key experimental findings on this transition, together with simple theoretical models that account for the experiments.
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Wettablity is one of the important characteristics defining a given surface. Here we show that the effective interface potential method of determining the wetting temperature, originally proposed by MacDowell and Müller for the surfaces exhibiting the first order wetting transition, can also be used to estimate the wetting temperature of the second order (continuous) wetting transition. Some selected other methods of determination of the wetting temperature are also discussed.
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Ellipsometry measurements of the wetting behavior of different alkanes on water show a sequence of two wetting transitions: a first-order (discontinuous) transition followed by a critical (continuous) one. We report temperature-induced wetting transitions for different alkanes and a novel pressure-induced wetting transition for an alkane mixture. The experiments enable us to determine the global wetting phase diagram as a function of chain length and temperature which we subsequently calculate theoretically. The two transition lines are found to be approximately parallel, in accordance with basic theoretical arguments.
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