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.
In addition to critical Casimir forces between homogeneous surfaces, we experimentally study the interaction between colloidal particles and chemically patterned substrates immersed in a binary critical mixture. Chemical patterns are created by means of a focused ion beam or micro-contact-printing. Close to the critical point of the mixture, the particles are subjected to critical Casimir interactions with force components normal and parallel to the surface. Because the strength and sign of these interactions can be tuned by variations in the surface properties and the mixture´s temperature, critical Casimir forces allow the formation of highly ordered monolayers. In contrast to other aggregation mechanisms, critical Casimir forces are fully reversible which allows to thermally anneal the obtained structures. This will result in a largely reduced defect density being important for technical applications such as photonic or phononic crystals. Since critical Casimir forces are not restricted to micron-sized particles, the principle should be also applicable on smaller length scales.
Critical Casimir forces can be conveniently applied for tuning colloidal interactions. Both, the temperature of the solvent and adsorption preference of the confining geometries allow the control of strength and the sign of this type of interaction. When salt is added to a critical mixture, we observe that for symmetric boundary conditions attractive forces already arise several Kelvin below the critical temperature. This can be attributed to smaller particle wall distances, i.e. to larger critical Casimir forces. More interestingly, for antisymmetric boundary conditions a temperature dependent crossover from attractive towards repulsive interactions is observed. This unexpected behavior demonstrates the rich properties of colloidal interactions in critical mixtures.
Salt-induced changes of colloidal interactions in critical mixtures 
Ursula Nellen, Julian Dietrich, Laurent Helden, Shirish Chodankar, Kim Nygard, J. Friso van der Veen, and Clemens Bechinger, Soft Matter 7, 5360 (2011)
Tunability of Critical Casimir Interactions by Boundary Conditions 
U. Nellen, L. Helden, C. Bechinger. EPL 88, 26001 (2009) download
Critical Casimir effect in classical binary liquid mixtures 
A. Gambassi, A. Maciolek, C. Hertlein, U. Nellen, L. Helden, C. Bechinger and S. Dietrich, Phys. Rev. E 80, 061143 (2009) download
Direct measurement of critical Casimir forces 
C. Hertlein, L. Helden, A. Gambassi, S. Dietrich, C. Bechinger
Nature 451, 172 (2008) download