SP disks in 2d below (left) and above (right) the density where phase separation occurs.

1. Motility-driven phase separation of active particles M. Cristina Marchetti, Syracuse University, DMR 1004789 Clustering of active Janus colloids, L. Boquet, Lyon.(Theurkauff et al., PRL 2012). Collections of self-propelled particles have been used as minimal models for active systems, from bacterial suspensions to animal groups to self-catalytic Janus colloids (right). Activity eliminates microscopic detailed balance and yields novel complex dynamics. 50μm SP disks repel with force proportional to overlap δ SP disks in 2d below (left) and above (right) the density where phase separation occurs. (Fily and Marchetti, PRL 2012) Using numerical and analytical methods we have demonstrated that a collections of self-propelled (SP) disks in two dimensions with repulsive interactions phase separates even in the absence of any attractive forces. The phenomenon is reminiscent of strong clustering observed ubiquitously in active systems. It is distinct from phase separation in conventional liquids (that requires attractive interactions) and in active flocks (that requires aligning interactions breaking orientational symmetry). Our work points to the suppression of motility due to steric effect as a generic mechanism for phase separation in active systems.

2. Motility-driven phase separation of active particles M. Cristina Marchetti, Syracuse University, DMR 1004789 The PI’s group, from left: graduate student Shiladitya Banerjee, postdoc Silke Henkes, graduate student Kazage Utuje, postdoc Yaouen Fily (top), graduate student Xingbo Yang, senior researcher Moumita Das, Cristina Marchetti. New group webpage: https://mcmarche.expressions.syr.edu/ The PI has co-organized, with Susan Coppersmith (U-Wisconsin, Madison) and Clare Yu (UC-Irvine), a very successful Aspen Winter Conference on Patterns formation in biology. She is also co-organizer (with Iain Couzin, Princeton, Sriram Ramaswamy , IISc Bangalore, and Christoph Schmidt, Goettingen) of aKITP program on Active Matter planned for Jan 6-May 16, 2014.