Wetting phenomena

Wetting occurs when a surface gets in touch with an interface between two immiscible fluids. Below you find examples of our research in the area of wetting phenomena.

Intermediate wetting states on superhydrophobic surfaces

Together with cooperation partners from the Technion/Israel, we have studied the wetting states of hierarchical superhydrophobic surfaces, consisting of an array of micropillars that is decorated with nanoparticles. We find a multitude of intermediate states between the classical Cassie and Wenzel states. The transition from the Cassie state to these intermediate states is partially reversible. A summary of the main results can be found here.

B. Rofman, S. Dehe, V. Frumkin, S. Hardt, and M. Bercovici, Intermediate states of wetting on hierarchical superhydrophobic surfaces, Langmuir 36 (2020), 5517−5523.

Non-contact electrostatic manipulation of droplets on liquid-infused surfaces

Liquid-infused surfaces have unique liquid-repelling properties. In comparison with conventional superhydrophobic surfaces, liquid-infused surfaces provide stable wetting states and pronounced self-healing properties. We have established the manipulation of highly mobile droplets sitting on silicone-oil infused surfaces by exploiting a non-uniform electric field between a grounded substrate and a non-touching pin electrode placed above it. Droplets are attracted towards the pin electrode, and translational velocities can exceed 1 cm/s.

Reference: N. Sinn, M. T. Schür, and S. Hardt, No-contact electrostatic manipulation of droplets on liquid-infused surfaces: Experiments and numerical simulations, Applied Physics Letters 114 (2019), 213704. DOI: 10.1063/1.5091836

Evaporation of droplets on surfaces with wettability patterns

On a surface with hydrophilic and hydrophobic stripes, an evaporating droplet breaks up if the wettability contrast between the different regions is high enough. A liquid bridge forms on the hydrophobic stripe, and when the width of the bridge reaches a critical value, it becomes unstable and breaks up rapidly. We have studied this process based on experiments, numerical simulations, and a heuristic analytical model.

Reference: M. Hartmann and S. Hardt, Stability of evaporating droplets on chemically patterned surfaces, Langmuir 35 (2019) 4868−4875. DOI: 10.1021/acs.langmuir.9b00172

Stability of holes in liquid films

If you spill liquid over a surface, it forms a film with a thickness of the order of its capillary length. If the surface is bounded and you did not spill enough liquid, a hole will form. The stability of the hole depends on its size. There is a threshold size below which it collapses. For a water film, the collapse is dominated by inertia. We studied the stability and collapse of holes in liquid films in detail and compared the experimental results with mathematical models and simulations.

Reference: C. Lv, M. Eigenbrod, and S. Hardt, Stability and collapse of holes in liquid layers, Journal of Fluid Mechanics 855 (2018), 1130-1155. DOI:10.1017/jfm.2018.680