Active versus Passive Hard Disks against a Membrane: Mechanical Pressure and Instability

Area of Science:

• Physics
• Soft Matter Physics
• Statistical Mechanics

Background:

• Understanding the mechanical properties of active matter systems is crucial for various fields, including robotics and biological systems.
• Active Brownian particles, such as self-propelled disks, exhibit unique behaviors distinct from their passive counterparts.
• The concept of an equation of state, fundamental in equilibrium thermodynamics, is challenged in non-equilibrium active systems.

Purpose of the Study:

• To experimentally investigate the mechanical pressure exerted by passive isotropic and self-propelled polar disks on flexible membranes.
• To compare the behavior of active and passive disk systems in terms of their mechanical pressure and interaction with confining structures.
• To determine if an equation of state exists for active disk systems interacting with membranes.

Main Methods:

• Experimental setup involving passive isotropic and self-propelled polar disks.
• Utilizing two different flexible unidimensional membranes as confining structures.
• Inferring mechanical pressure from the deformation of the membranes.

Main Results:

• Passive isotropic disks exerted mechanical pressure consistent with the equilibrium equation of state for hard disks, irrespective of the membrane.
• Mechanical pressure from self-propelled disks was highly dependent on the specific membrane used, indicating it is not a state variable.
• An instability, similar to theoretical predictions for active Brownian particles against a soft wall, was observed when self-propelled disks were on both sides of the membrane.

Conclusions:

• The mechanical pressure of active disk systems does not follow a universal equation of state.
• The interaction between self-propelled disks and membranes is complex and system-dependent.
• Active matter systems exhibit unique phenomena like membrane instability, highlighting deviations from equilibrium physics.