Different molecular components can demix from each other leading to the formation of droplets. This so-called phase separation belongs to the class of phase transitions and can be observed in many forms in everyday life, for example when oil is added to water. In cells, phase separated droplets have been shown to be involved in the regulation of many biochemical processes but also disease related pathways. Moreover, during embryonic development droplets are positioned by different concentration levels of a regulator protein that are capable to regulate phase separation. Recent studies had shown that the positioning occurs via a process where droplets grow faster close to one boundary while dissolving at the other. However, it remained unknown whether the position to one or the other boundary occurs smoothly or if an abrupt mechanism underlies the positioning.

The team of biophysicists Samuel Krüger, Jens-Uwe Sommer, Frank Jülicher and Christoph Weber, group leader at the CSBD and the MPI for the Physics of Complex Systems (MPI-PKS), have now developed a general biophysical model that provides the underlying mechanism for the positioning of such droplets in complex mixtures. Christoph and his colleagues at the MPI-PKS, and the Leibniz Institute for Polymer Research have found that changing the interactions between the regulator protein and the droplet material affects the position of the droplets like a switch: they switch from a position in the region of large regulator concentration to the region of low regulator concentration, and back.

This switching of position represents a novel discontinuous phase transition, which was discovered by the scientists for the first time and now published in the New Journal of Physics. At such a discontinuous phase transition the properties of the system jump at a certain value of the parameters (e.g. interaction parameters).

Understanding the mechanism of the positioning of phases may open the possibility of applications in microfluidic systems, such as lab-on-chip devices. For example, in these systems, positioned components could be used to seal and open junctions at specific locations, or simply position chemicals that partition into the droplets.

“It is always exciting when a complex process such as the positioning of protein droplets in cells stimulates theory to find new physics, which could even be applied to other fields of science. This is one of the reasons why I enjoy working so much in this interdisciplinary environment!”, Christoph says.

Congratulations to everyone involved!

Publication:

Samuel Krüger, Christoph A. Weber, Jens-Uwe Sommer and Frank Jülicher: Discontinuous switching of position of two coexisting phases, New Journal of Physics, Volume 20, Published 27 July 2018