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The Edge Contact Angle, Capillary Condensation, and Meniscus Depinning

Understanding of phase behavior of fluids which are subjects of confining solid objects of a particular geometry is one of the most challenging contemporary tasks in the theory of phase transitions. A recent contribution to this topic was done by Dr. Malijevsky in a collaboration with Prof. A. Parry from Imperial College London. The paper was published in the prestigious journal devoted to physical chemistry, Physical Review Letters. The work showed some novel phenomena which occur in a simple geometry formed by a wall of finite (microscopic) length H which is brought a distance L away from a macroscopic planar surface. It has been shown that such a system exhibits very rich phase behaviour due to a competition of various interfacial phenomena, such as capillary condensation, wetting, corner filling, and depinning, depending on the value of the aspect ratio a=L/H. In particular, for sufficiently long capillaries with a<2/π, the condensation is of type I involving menisci that are pinned at the top edges at the open ends of the capillary which they meet at the edge contact angle. For intermediate capillaries, with 2/π, depending on the contact angle the condensation may be of type I or of type II, which is characterized by menisci which are unpinned and the liquid overspill out of the capillary. Finally, for short capillaries with condensation is always of type II. In all cases, condensation is completely suppressed for sufficiently large contact angles. It was also shown that there is an accompanying continuous phase transition associated with meniscus depinning from the upper edges. Furthermore, finite-size scaling theory has been developed taking into account fluctuation effects and the predictions have been supported by fully microscopic Density Functional Theory calculations.

Fig. Three possible regimes of capillary condensation between capillary gas (CG) and capillary liquid (CL) which can be of type I or type II. Also shown are the loci of the continuous meniscus depinning transition (dashed line)

  • Alexandr Malijevský and Andrew O. Parry: Edge Contact Angle, Capillary Condensation, and Meniscus Depinning. Phys. Rev. Lett. 2021, 127(11), 115703. DOI: 10.1103/PhysRevLett.127.115703
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