Complex surfaces and the collapse mechanism

Here we take a look at recent work in the Journal of Physics: Condensed Matter special issue on complex fluids at structured surfaces. Advances in controlling and exploiting the wetting and adsorption properties of complex fluids, such as liquid crystals, ionic liquids, colloids and active matter, have been fostered by impressive technical achievements allowing the fabrication of tailored surfaces with a well-controlled distribution of micro- or nano-scale features. J R Panter and H Kusumaatmaja investigated the effect of surface structure on the liquid collapse transition. Read on to find out more from the authors themselves:

Any surface structure designed for superhydrophobicity is afflicted by the collapse transition. The often-delicate, suspended state in which the liquid sits atop a vapour-filled solid texture demonstrates a range of remarkable properties, from droplet-mediated self-cleaning to reduction in viscous drag in fluid flow. Upon collapse however, these are lost and can be difficult to recover as the liquid penetrates into the surface texture.

Increasing the energetic barrier of this collapse is therefore a key design focus in producing commercially viable liquid-repellent surfaces. One promising design is the so-called re-entrant geometry shown in the figure, which allows a suspended state to exist even for pressurized or low surface tension liquids. Currently these structures can be time-consuming and expensive to fabricate, and the narrow pillar section can lead to weak mechanical strength.  Prior to manufacturing, it would therefore be valuable to use computational methods to assess which structural features increase the wetting transition barrier most effectively, whilst maintaining a texture resistant to impacts and abrasion. However, it is not yet clear how the wetting transition proceeds, never mind about how it can be affected by the surface structure.

Diagonal slices through the reentrant system, showing the liquid-vapour interface position at different stages along the Base Contact (top) and Pillar Contact (bottom) pathways. 3D images at the top of the energy barrier are shown. Taken from J. Phys. Cond. Mat. 29 084001 © IOP Publishing, All Rights Reserved.

In our recent article, we utilise a phase field model to observe and rationalise the fundamental impacts of the reentrant surface geometry on the wetting state stabilities and the collapse transitions. By surveying a broad range of liquid pressures and surface wettabilities, we begin to build a framework for successful surface structure design, even under the challenging conditions of high liquid pressure or low surface tension. Such surfaces are particularly attractive for reducing drag in marine shipping and efficiently transporting volatile liquid fuels.

We find that the reentrant cap structure leads to two morphologically distinct collapse mechanisms: Base Contact and Pillar Contact, with the lowest energy mechanism selected by changing the pillar height. We highlight the latter as a particularly important consideration for future surface design, as Pillar Contact places a hard limit on the maximum energetic barrier obtainable by increasing the height of the surface geometry.

A key outcome of the pressure-wettability survey is the identification of technologically exciting regions where liquid condensation is supressed within the surface texture – important for suspending low surface tension liquids for long time periods. In other regions, extensive vapour cavitation can be harnessed to produce surface textures with no stable collapsed state, demonstrating the tantalising prospect of perfect superhydrophobicity.

Building on the foundations of this work, we are now aiming to examine how the wetting transitions are modified when the interface spans multiple posts, and to examine wetting on increasingly complex structural geometries.

For more special issue articles on complex fluids at structured surfaces, click here.

About the Authors

Jack Panter is a PhD student in the Durham University, studying methods for exploring complex energy landscapes in soft matter systems, with a special focus on surface wetting.

Halim Kusumaatmaja is a lecturer at Durham University. He obtained his PhD in Theoretical Physics from the University of Oxford, and he was previously a postdoctoral research fellow at the Max Planck Institute of Colloids and Interfaces and at the University of Cambridge. His research is in theoretical soft and biological matter, with current focus on wetting and interfacial phenomena, membrane biophysics, and colloidal self-assembly.

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Categories: Journal of Physics: Condensed Matter, JPhys+

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