A Drunken Trajectory: Tracking Active Matter Systems

Look!  Up in the sky!  It's a bird … It's a plane … It's active matter!  Like birds and planes (and superman), a core feature of "active matter" systems is their ability to move, to self-propel, to be active.  By virtue of their self-motion, active matter systems of all scales trace out a trajectory similar to that of a drunken person: straight, directed movements punctuated by sudden changes in direction.  This drunken trajectory engenders a unique mechanical "swim pressure" that dictates the motion and collective behavior of active matter.  We provide experimental proof of this new concept by trapping active microswimmers in various degrees of confinement in both external acoustic traps and in phospholipid liposomes.  We fabricate soft, flexible materials that can expand, elongate, and/or steer on command by loading the material with swimming microorganisms and self-propelled synthetic particles.  These materials may one day be used to create micro- or nanomechanical devices and motors that could have multiple applications in medicine (e.g., focused drug delivery), biophysics, microelectronics, and other fields.