Suppression of chemotactic explosion by mixing

Chemotaxis plays a crucial role in a variety of processes in biology and ecology. In many instances, processes involving chemical attraction take place in fluids. One of the most studied PDE models of chemotaxis is given by Keller-Segel equation, which describes a population density of bacteria or mold which attract chemically to substance they secrete. Solution of Keller-Segel equation can exhibit dramatic collapsing behavior, where the density concentrates positive mass in a measure zero region. A natural question is whether presence of fluid flow can affect singularity formation by mixing the density thus making concentration harder to achieve. In this paper, we consider parabolic-elliptic Keller-Segel equation in two and three dimensions with additional advection term modeling ambient fluid flow. We prove that for any initial data, there exist incompressible fluid flows such that the solution to the equation stays globally regular. On the other hand, it is well known that when the fluid flow is absent, there exist initial data leading to finite time blow up. Thus presence of fluid can prevent the singularity formation. We discuss two classes of flows that have the explosion arresting property. Both classes are known as very efficient mixers. The first class are relaxation enhancing (RE) flow of [5]. These flows are stationary. The second class of flows are Yao-Zlatos optimal mixing flows [38], which are time dependent. The proof is based on the nonlinear version of the relaxation enhancement construction of [5], and on some variations of global regularity estimate for Keller-Segel model.