Université Paris Diderot
Active particles in an elastic medium and surface tension of active fluids
In this seminar, we will tackle two separate subjects whose common thread is active matter. In a first part I will analyze the organization properties of two-state particles embedded in an elastic medium under the influence of a state-controlling active drive. This is relevant to biophysics, when considering transmembrane proteins whose conformational changes are induced by ATP which, in turn, actively reshapes the lipid bilayer. Synthetic switchable objects inside liquid crystals have also been designed. Their shape is light-controlled and this triggers modifications in mediated interactions between objects. Our goal is to focus on the simplest model retaining the basic building blocks common to these systems (a fluctuating medium in which otherwise noninteracting objects are embedded, thermal fluctuations competing with an active drive). We explore the wealth of patterns these systems can give rise to.
In the second part of the seminar, I will address surface tension in active fluids. Since relying on a free energy-based definition is impossible I will propose a purely mechanical approach to surface tension in assemblies of self-propelled particles. I will explain that surface tension for active systems is a priori measuring-device dependent and I will find the culprit for this state of affairs. But I will also temper my stand by pointing to settings in which actual measurement may not be meaningless at all.