  In presence of non-adsorbing polymers, colloidal particles experience
ubiquitous attractive interactions induced by depletion forces. Here, we
measure the depletion interaction between a pair of microtubule filaments using
a method that combines single filament imaging with optical trapping. By
quantifying the dependence of filament cohesion on both polymer concentration
and solution ionic strength, we demonstrate that the minimal model of
depletion, based on the Asakura-Oosawa theory, fails to quantitatively describe
the experimental data. By measuring the cohesion strength in two- and three-
filament bundles we verify pairwise additivity of depletion interactions for
the specific experimental conditions. The described experimental technique can
be used to measure pairwise interactions between various biological or
synthetic filaments and complements information extracted from bulk osmotic
stress experiments.
