Capillary condensation, i.e. a finite-size shift in the liquid-vapour phase boundary of fluids in homogeneous slits relative to their bulk counterpart, is a well-known phenomenon; on a macroscopic level it is described by the Kelvin equation which tells us that the shift is inversely proportional to the width L of the slit. However, a recent progress in experimental technologies enable to modify solid surfaces already on a molecular level, which in turn allows for a design of heterogeneous surfaces that induce novel phenomena which are interesting fundamentally and also from an application point of view. Recently, a group of Dr. Malijevský in a collaboration with Imperial College London published a paper in a prestigious journal Physical Review Letters where they described theoretically phase behaviour of fluids confined within a chemically heterogeneous slit, in which case both plates forming the slit are decorated by adjacent and nanoscopically thin stripes of width H which are more wettable than the rest of the plates. This model not only allows for a formation of a capillary gas and a capillary liquid but also exhibits a locally condensed state, in which case the fluid condenses in the vicinity of the stripes which thus become connected by a liquid-like bridge.
In this study, the modified Kelvin equation describing the stability of all the three phases in the model heterogeneous slit has been derived and eventually verified numerically using methods of statistical mechanics. In particular, for the maximum contrast slit, in which case the stripes tend to be completely wet (Young’s contact angle θ=0), while the rest of the plates is superhydrophobic with Young’s contact angle θ=π, the location of the triple point at which all the three phases coexist was found to obey a very simple and elegant rule L/H=8/π. This result, which does not depend on temperature, the fluid model nor the slit materials and is thus fully universal has been verified numerically down to nanoscale. This study provides a first step towards a systematic and fundamental description of phase behaviour of fluids confined by heterogeneous surfaces which fills a gap in the theory of phase transitions and is also relevant for modern technologies, such as micro- or nanofluidics.
Láska, M., Parry, A.O., Malijevský, A. Three-Phase Fluid Coexistence in Heterogenous Slits. Physical Review Letters. 2020, 124(11), 115701. ISSN 0031-9007.
DOI: 10.1103/PhysRevLett.124.115701
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