Depletion interaction between cylindrical inclusions in polymer brushes

Inclusions in mobile brushes experience apparent (depletion) attraction, which arises from a tendency to minimize the volume of depletion zones around the inclusions, thereby to maximize the entropy of the surrounding polymers. Here, we study the brush-induced depletion attraction between cylindrical inclusions using molecular dynamics simulations and the Asakura-Oosawa theory. Our considerations find that the correlation blobs defined in the brush environment serve as the fundamental units of the attraction. In tall brushes, however, the entropy of the overgrown polymer competes with the depletion attraction between the inclusions. As a result, the brush-induced depletion interaction displays non-monotonic variations with the brush height. Our study not only expands the repertoire of colloid-polymer mixtures to depletion interactions in brushes, but also suggests the brush-induced depletion interaction as a previously unappreciated mechanism for glycocalyx-induced protein cluster formation on cell surfaces.