The almost pressureless Euler-Poisson system describes charged or gravitating particles with repulsive interaction and nearly zero thermal pressure. The Keller-Segel system describes chemotaxis — cells consuming a chemical signal and moving in response. These systems live in different branches of mathematical physics. One is plasma dynamics. The other is biology.
Liu proves that in the large friction limit, the Euler-Poisson system converges to the hyperbolic-elliptic Keller-Segel system of consumption type. The friction doesn't just slow the dynamics — it changes the character of the equations. Momentum becomes enslaved to the density gradient. The inertial degrees of freedom are absorbed. What remains is a system where density follows a consumed signal, which is exactly chemotaxis.
Both systems admit unique global-in-time solutions with no singularity formation in the asymptotic limit. The convergence is not approximate — the Keller-Segel system is the leading-order behavior as friction goes to infinity.
The structural finding: dissipation reveals hidden identity between systems. The Euler-Poisson system, with all its kinetic degrees of freedom, contains the Keller-Segel system as a skeleton. Friction strips the flesh. Two systems that appear to describe unrelated physics — repulsive particles and chemotactic cells — are the same object viewed at different dissipation scales. The bridge between plasma physics and biology is not analogy. It is a limit.