ramé-hart Newsletter

                   

May 2025
 

A wetting transition refers to a change in the wetting behavior of a liquid on a solid surface, typically characterized by a sudden (or even gradual) change in the contact angle between the liquid and the surface triggered by a variety of factors such as temperature, surface roughness, or chemical composition.

There are three wetting states:

1. Complete wetting: The contact angle is zero and the drop spreads out over the entire surface.

2. Partial wetting: The contact angle angle is greater than 0° but less than 180°. The drop spreads partially and is somewhere between complete wetting and non-wetting. This is the common state.

3. Non-wetting: The contact angle is 180° and the drop does not spread at all.

The Three Wetting States

In short, a transition from any state to another would be considered a wetting transition. A common example is when a sessile is deposited on a solid surface and exhibits partial wetting. The surface and drop are then heated up and the drop suddenly drops to a complete wetting state.

For example, if a drop of a liquid metal such is gallium is deposited on a metal oxide surface and heat is then applied, the oxide layer can break down triggering a transition to complete wetting when the critical wetting temperature is reached. In short, when the liquid–vapor surface tension decreases more rapidly with temperature than the solid–vapor and solid–liquid interfacial tensions it can cause the balance of interfacial tensions (as described by Young’s equation) to favor complete wetting.

While most wetting transitions are triggered by a change in surface energy, there are examples of other transitions that can be driven by other forces. In the case of the Cassie-to-Wenzel transition, for instance, a drop can fall from a (nearly) superhydrophobic non-wetting state to a partial wetting state when the drop falls off the nanoscopic asperities and into the nooks and crannies of the nanotextured surface. Unlike most wetting transitions that rely on changes in interfacial tension, the Cassie-to-Wenzel transition is driven by external mechanical factors (such as vibration) to overcome a metastable energy barrier.¹

In conclusion, wetting transitions play a critical role in understanding how liquids interact with solid surfaces under varying physical and chemical conditions. Whether driven by changes in surface energy, temperature, or mechanical forces, these transitions — from complete wetting to partial wetting, non-wetting to partial, or through topography-assisted shifts like the Cassie-to-Wenzel transition — reveal the complex interplay between interfacial tensions and surface structure. Appreciating these behaviors is important not only for surface science but also for designing advanced materials and coatings with controlled and predictable wetting properties. And for surface and material scientists, ramé-hart is the global leader in contact angle goniometers, delivering precision instruments built to perform and stand the test of time.

Notes
¹ Edward Bormashenko, Progress in understanding wetting transitions on rough surfaces, Advances in Colloid and Interface Science, Volume 222, 2015, Pages 92-103, ISSN 0001-8686, https://doi.org/10.1016/j.cis.2014.02.009.

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Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
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