A depletion force is an effective interaction caused by exclusion of smaller particles or macromolecules from regions around larger objects. Overlap of the excluded regions increases the accessible volume for the smaller species. At fixed reservoir osmotic pressure , the associated pair free energy is times the overlap volume in the ideal dilute-depletant approximation. It is an entropic attraction for two large hard particles. Surface adsorption can introduce different interactions, so adding an arbitrary polymer does not by itself guarantee depletion attraction.
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The depletion force is a type of effective force that arises in a system of particles due to the presence of larger particles or obstacles that restrict the available volume for smaller particles. This force is often observed in colloidal systems, polymer solutions, and biological systems where different sizes of particles are present. When smaller particles (often called "depletants") are introduced into a suspension of larger particles, they tend to occupy the available space between the larger particles.