Congratulations to Susumu for his first publication in Soft Matter

Susumu has combined experiments, numerical simulations and scaling models to explain and describe a new hydrodynamic mechanism, which we call leaping, for the spreading of liquid drops on non-smooth and textured surfaces.

Similar to everyday life experience, where jumping and leaping to overcome obstacles increase the speed and efficiency of mobility, Susumu has shown that a liquid contact line may jump between texture posts and ridges and thus obtain speeds much larger than expected.

These insights improve our understanding of splashing, droplet impact, or in situations where time scales are imposed externally, such as wetting phenomena in the presence of vibrations. Moreover, our model helps in answering how asymmetric textured surfaces should be designed to direct the fast motion of liquids.

The article which of course is open access can be found here!

The animation below shows experiments (left) and numerical simulations (right) of a water drop spreading on skewed ridges.

Congratulations to Johan for publishing his first paper!

For the first time, we are able to observe in detail how turbulence interacts with hairy surfaces. Using large-scale computations, Johan Sundin has exposed nearly 10000 flexible filaments (attached to a wall) to a turbulent flow. Through these simulations, we can show that hairy surfaces in water and in air interact with near-wall turbulence very differently.

 

A hairy surface in water (say aquatic vegetation) will modify the nearby turbulent flow significantly, such that turbulent drag and entrainment is increased. This is because, the mass density of a hair is nearly the same as the water, which in turn results in a very fast and responsive bed that is able to “keep up” with the fastest turbulent time scales.

The same hairy surface in flow of air (say wind of plants), will be very slow compared to the dominating turbulent time scales. The hairs are too heavy to keep up with the turbulent flow; although the hairy surface in air deforms, the turbulence is left nearly unmodified.

Although the interaction of hairy surfaces and turbulence is very complex (both highly non-linear and multiscale), its characterization can thus be broken down to determining whether the time scale of the surface (which depends on the density ratio) is larger or smaller than the dominant time scale present in the turbulent flow.

The paper will appear soon in Journal of Fluid Mechanics. You can also find here on arXiv.