Critical Casimir Forces

When fluctuating fields are confined between two surfaces, long-ranged forces arise. Arguably, the most famous example is the quantum-electrodynamical Casimir force resulting from zero-point vacuum fluctuations between neutral, parallel, conducting plates. In 1978, Fisher and de Gennes realized that a thermodynamic analogue exists, the critical Casimir force, acting between surfaces immersed in a binary liquid mixture close to its critical point and generated by confinement of its concentration fluctuations. So far, indirect experimental evidence has been obtained by monitoring the thickness dependence of thin adsorbed films close to their critical temperatures. Here, we present a direct measurement of the critical Casimir force between a single colloidal sphere and a flat silica surface in the presence of a critical water – 2,6 lutidine mixture. Forces are determined using total internal reflection microscopy allowing in situ measurements with femto Newton resolution. Depending on the properties of the surfaces in contact with the critical mixture we observe attractive or repulsive forces in quantitative agreement with theoretical predictions. Since critical Casimir forces offer unprecedented temperature-dependent control over the magnitude and even the attractive versus repulsive nature of colloidal interactions, they open novel perspectives for the application of colloids as model systems.

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