Substrates that gradually change their properties along a particular direction can be used to provoke, steer, and sustain the motion of fluids without external energy supply, which is relevant for a range of applications, such as microfluidics, microfabrication, coatings, nanoscale actuation and energy conversion, and biology.

Research led by IP PAS in collaboration with various international partners from France (ENS Paris Saclay), Czechia (Central European Institute of Technology), USA (University of Virginia), and Bulgaria (Institute of Physical Chemistry of the Bulgarian Academy of Sciences) has led to a new substrate design based on polymer brushes with stiffness gradient along a specific direction. It was shown that this can cause spontaneous and unidirectional fluid motion in the direction of increasing brush stiffness without the requirement of an energy source, a phenomenon known as durotaxis.

The results have indicated that the grafting density of the brush chains and the droplet adhesion to the substrate are two key parameters for this substrate design, and moderate choices of these parameters can lead to the fastest fluid motion. Surprisingly, the droplet motion is not affected by the droplet viscosity or the length of the substrate brush polymers. This research has also unraveled the underlying mechanism of the durotaxis motion of the droplet for the brush substrate design, which suggests that the substrate roughness is the ultimate factor that determines the droplet motion. That is the droplet will move from rougher regions to those with less roughness, whose presence is a result of the stiffness gradient, thus minimizing its energy.

This research has been supported by the National Science Centre, Poland, under grant No. 2019/35/B/ST3/03426 and the PLGrid Infrastructure.