Stochastic buckling of a colloidal chain assembled by critical Casimir forces

Mechanical instabilities occur when structures spontaneously deform or change conformation under stress, and are relevant for all materials, at scales from ~10meters to nanometers. So far, they have been mostly considered in a pure mechanical context, where thermal fluctuations are negligible. However, these latter become important when the mechanical structures become small as is the case e.g. for filaments or membranes in soft and biological materials. Here, we study the buckling of a colloidal chain in the presence of thermal noise and unveil a new mechanical critical transition at the chain buckling points. We assemble short colloidal chains by critical Casimir forces. These solvent-mediated forces allow us to vary the attractive particle interactions and thus tune the chain stiffness with temperature. Two laser tweezers are used to hold the chain at its ends, and to push on it. When compressing the chain to a critical state we observe buckling reminiscent of macroscopic structures, yet the thermal fluctuations introduce a crucial new element: multiple buckling modes occur in sequences, accompanied by a divergence of bending fluctuations when a new buckling point is reached.