Active matter: self-propelled particles and active fluids

7 problems

• How self-propulsion changes the glass transition of dense active matter

  Open

  Dense crowds of self-propelled particles, such as packed cell layers or active colloids, become glassy and nearly stop moving, much like a cooling liquid. Whether this active glass follows the same physics as an ordinary glass with an effective temperature, or a different one, is not known.

• Do two-dimensional dry active nematics keep quasi-long-range order?

  Contested

  Active nematics are layers of elongated self-driven units that line up without a preferred head or tail. Whether such a layer stays orientationally ordered over large distances, or activity always creates enough defects to destroy the order, is disputed.

• Is active turbulence described by universal statistical laws?

  Contested

  Dense bacteria and gels of motor proteins produce chaotic swirling flows even though their inertia is negligible (Reynolds number near zero). Whether these flows obey universal statistical laws, as ordinary turbulence does, is unknown.

• Predicting the bubble cutoff and microphase threshold in active phase separation

  Partially resolved

  In ordinary phase separation small droplets dissolve and big ones grow until one large region of each phase remains. In active systems small gas bubbles inside the dense phase persist, and a theory that predicts their largest size and when they take over from bulk coexistence is missing.

• Predicting MIPS coexistence densities from particle-level mechanics

  Partially resolved

  Self-propelled particles that separate into a crowd and a gas do not obey the equilibrium rules that fix the densities of the two phases. A complete replacement rule that predicts those densities from particle properties is not established.

• Universality class of the motility-induced phase separation critical point

  Contested

  Self-propelled particles that only repel each other can still separate into a dense crowd and a dilute gas, called motility-induced phase separation (MIPS). Whether the critical point where the two phases merge behaves like the ordinary liquid-gas (Ising) critical point is disputed.

• Exact scaling exponents of two-dimensional compressible polar flocks

  Contested

  Flocks of self-propelled particles that align with neighbours can move together in two dimensions, which equilibrium physics forbids for comparable systems. The exact laws governing the fluctuations of such a flock are disputed.