Active Matter Physics: combining physics with living things

The world of physics covers a wide range of length scale: from nanometer scale atoms all the way up to stars and galaxies. When speaking of physics, people tend to think of things that are not “alive”, such as material physics, particle physics, astronomy, etc.


Figure.1 Caption: Bird flock in vedanthangal (author: Vinoth Chandar) (

In the last decade, a new area of physics called active matter physics has arisen. To better understand active matter physics, it is helpful to introduce a similar field, soft matter physics. Soft matter physics mainly studies the group behavior of particles with the size of several micrometers. Behavior of a system at such a small size is largely determined by thermal dynamics. Glass, soft gels, granular material are all studied by soft matter physics.

Active matter physics, on the other hand, is very similar to soft matter physics, since it also mainly focuses on studying the group behavior of a system that arises from the interactions of “particles”. However, one of the crucial differences between active matter physics and soft matter physics is that active matter physics studies particles that are “active”, which means that by consuming energies from the environment, they can produce self-motility. Systems studied by active matter physics can range from bird flocks (Fig.1) to cytoskeleton structures inside cells (Fig.2).


Figure.2 caption: actin cytoskeleton of mouse embryo fibroblasts (author: Y tambe) (

Studying active matter physics provides another perspective for understanding the emerging behavior in a biological system. For instance, there has been work done on simulating cell motion in densely packed tissues. Physicists using tools from statistical mechanics have successfully simulated how cells move around inside tissues and have found that there is a transition where cell motility changes from liquid-like flowing into solid-like jiggling around its initial position. They are able to plot out the phase diagram of such transitions in a space determined by three parameters: cell moving speed, persistence time along a single cell track, and a shape index that characterizes the competition between cell-cell adhesion and cortical tension. Such results provide an insight into the understanding of similar solid-to-liquid transition observed in cancer progressions.

About the author: Yu Shi is a 4th year PhD student in the Department of Physics & Astronomy at Johns Hopkins University. He is in Prof. Daniel Reich’s lab, and his current works focus on studying dynamic properties of actin-myosin system inside cells using micro-patterned substrate.

Reference article:

1.     Dapeng Bi, X. Yang, M. C. Marchetti, M. L. Manning, “Motility-driven glass transitions in biological tissues,” Phys. Rev X, arXiv:1509.06578 (2015).

2.     M.C. Marchetti, J.F. Joanny, S. Ramaswamy ,  T.B. Liverpool, J. Prost,  Madan Rao,  and R. Aditi Simha,” arXiv:1207.2929v1 (2012)


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