Depletion interaction between colloids mediated by an athermal polymer blend.

We calculate the immersion energy of a colloid and the potential of the depletion interaction (DI) acting between colloids immersed in an athermal polymer blend. The developed theory has no limitations with respect to the polymer-to-colloid size ratios and polymer densities, covering, in particular, dense polymer blends. We demonstrate that in addition to the standard compressibility-induced mechanism of the DI there exists the mechanism relying on the correlations between compositional fluctuations specific to polymer blends. We quantitatively investigate this "compositional" mechanism of the DI and demonstrate that it causes significant contributions to the effective force acting between colloids. Further we show that relative significance of the contributions to the colloid immersion energy and the depletion potential caused by the above compositional mechanism strongly depends on the mass fractions of the polymer species and their size ratio. We find out that these contributions strongly affect the range of the DI, thus causing a significant increase in the absolute value of the second virial coefficient of the effective potential acting between colloids.