Hi all,
In the Young's equation, σsg = σsl + σlg ⋅ cosθ.
When the surface tension of liquid( σlg ) is known, surface energy of solid(σsg) is known too.
After measure the contact angle between solid and liquid,
Is it possible that the interfacial tension between the liquid and the solid(σsl) be negative?
If a negative value is valid, does that mean it is exothermic when forming a solid-liquid interface?
I have read a paper: https://www.nature.com/articles/nmat1336a
However, I still cannot determine whether negative surface energy is reasonable or not.
Thank you very much.
Sincerely
If you just have a single solid material in equilibrium with a single liquid, or a gas/vacuum, then the surface energy is always positive.
If the surface energy would be negative, then molecules of the solid reduce their free energy when they go from being surrounded by other solid molecules, to being surrounded by the fluid. This means that the solid will want to make as much fluid/solid interface as it can, to lower its energy. The best way to do this is for the molecules of solid to dissolve into the fluid (as then each molecule of solid is completely surrounded by fluid, and there's a huge area solid/fluid interface). I.e. a solid with a negative surface energy should spontaneously vaporise/dissolve.
Another way to see this, is that for simple solid/vacuum interfaces, the surface energy scales like U/a^2, where U is cohesion energy between molecules in the solid, and a is molecule size (e.g. intro chapter of Capillarity and Wetting by de Gennes et al). So negative surface energy -> negative cohesion energy.
It's more complicated when you don't just have simple solid/fluid interfaces. i.e. if you have molecules that can adsorb to the interface, then in principle, you could have negative surface energies. It's certainly unusual though.
Robert Style It is also possible to create negative surface energy using electrostatics.
For example, consider a conductive membrane sitting on a conductive solid covered with a layer of dielectric with constant thickness. When a voltage V is applied across the conductive membrane and the conductive solid, the electrical potential energy is -1/2*\epsilon_0*\epsilon_r/h*V^2*A, where \epsilon_r and h are the dielectric constant and thickness of the dielectric respectively. This electric energy is mathematically equivalent to a negative surface energy and can be used in creating droplets.
For details, see our recent papers: Article Mechanics, modeling, and shape optimization of electrostatic...
,Article Electro‐Elastic Wetting: A Method to Migrate and Deform Droplets
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