McHale Glen, Ledesma-Aguilar Rodrigo, Neto Chiara
Institute for Multiscale Thermofluids, School of Engineering, The University of Edinburgh, Edinburgh EH9 3FB, U.K.
School of Chemistry and the University of Sydney Nano Institute, The University of Sydney, Sydney, New South Wales 2006, Australia.
Langmuir. 2023 Aug 8;39(31):11028-11035. doi: 10.1021/acs.langmuir.3c01313. Epub 2023 Jul 24.
In 1948, Cassie provided an equation describing the wetting of a smooth, heterogeneous surface. He proposed that the cosine of the contact angle, θ, for a droplet on a composite surface could be predicted from a weighted average using the fractional surface areas, , of the cosines of contact angles of a droplet on the individual component surfaces, i.e., cos θ = cos θ + cos θ. This was a generalization of an earlier equation for porous materials, which has recently proven fundamental to underpinning the theoretical description of wetting of superhydrophobic and superoleophobic surfaces. However, there has been little attention paid to what happens when a liquid exhibits complete wetting on one of the surface components. Here, we show that Cassie's equation can be reformulated using spreading coefficients. This reformulated equation is capable of describing composite surfaces where the individual surface components have negative (droplet state/partial wetting) or positive (film-forming/complete wetting) spreading coefficients. The original Cassie equation is then a special case when the combination of interfacial tensions results in a droplet state on the composite surface for which a contact angle can be defined. In the case of a composite surface created from a partial wetting (droplet state) surface and a complete wetting (film-forming) surface, there is a threshold surface area fraction at which a liquid on the composite surface transitions from a droplet to a film state. The applicability of this equation is demonstrated from literature data including data on mixed self-assembled monolayers on copper, silver, and gold surfaces that was regarded as definitive in establishing the validity of the Cassie equation. Finally, we discuss the implications of these ideas for super-liquid repellent surfaces.
1948年，卡西提出了一个描述光滑、非均质表面润湿情况的方程。他指出，复合表面上液滴的接触角θ的余弦值，可以通过使用各组分表面上液滴接触角余弦值的分数表面积 进行加权平均来预测，即cosθ = cosθ + cosθ。这是早期针对多孔材料方程的推广，最近已被证明是支撑超疏水和超疏油表面润湿理论描述的基础。然而，当液体在其中一个表面组分上表现出完全润湿时会发生什么，却很少受到关注。在这里，我们表明卡西方程可以用铺展系数重新表述。这个重新表述的方程能够描述单个表面组分具有负（液滴状态/部分润湿）或正（成膜/完全润湿）铺展系数的复合表面。当界面张力的组合导致复合表面上出现可定义接触角的液滴状态时，原始的卡西方程就是一个特例。对于由部分润湿（液滴状态）表面和完全润湿（成膜）表面构成的复合表面，存在一个临界表面积分数，在该分数下复合表面上的液体从液滴状态转变为膜状态。从文献数据（包括关于铜、银和金表面上混合自组装单分子层的数据，这些数据被认为是确定卡西方程有效性的决定性数据）中证明了该方程的适用性。最后，我们讨论了这些观点对超液体排斥表面的意义。
