Direct measurement of critical Casimir forces

Abstract

When fluctuating fields are confined between two surfaces, long range forces arise. A famous example is the quantum electrodynamical Casimir force that results from zero point vacuwn fluctuations confuted between two conducting metal plates'. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture dose to its critical point and arises from the confiuement of con centra tion fluctuations within the thin film of fluid separating the sur faces 2 • So far, all ell:peri.mental evidence for the existence of this effect has been indirece 5 • Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6 lutidi.ne near its critical point. We use total internal reflection microscopy to detemti.ue in situ the forces between the sphere and the surface, with femtonewton resolution 6 • Depending on whether the adsorp tion preferences of the sphere and the surface for water and 2,6 lutidi.ne are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems.The simple act of confining a fluid can give rise to new ph en om en a not observed in the bulk. An intriguing example is the critical Casi.urir force predicted to occur in binary fluid mixtures close to their critical point; like other critical phenomena, it is characterized by universal scaling functions that depend only on the internal symmetries of the system rather than on its specific material properties' . Colloidal particles suspended in binary liquids offer a particularly interesting setting for the experintental observations of such forces. At suffi ciently small particle distances, concentration fluctuations of the solvent become confined between neighbouring colloidal surfaces and modify the pair interaction 8 • If tlte Casinlir interaction strengtlt is comparable to the thermal energy, drastic changes in the phase behaviour are expected. Reversible flocculation of silica colloids in water 2,6 lutidi.ue mixtures close to the critical point has in fact been observed 9 , and critical Casinlir forces may be invoked to explain tills phenomenon. However, flocculation was observed even far away from tlte critical point where critical fluctuations are negligible, so it cannot serve as conclusive evidence for the presence of Casi.utir forces 10 " 11 • Our experiments, ainted at directly measuring critical Casi.utir forces, use a single colloidal sphere and a planar surface immersed in a binary liquid mixture of water and 2,6 lutidine. Forces are deter mined using total internal reflection microscopy (TIRM, see Methods) 6 which allows itt situ measurements with femtonewton resolution 12 (Fig. 1). The binary liquid mixture bas a lower critical demixing point at Tc = ...

An Optical Conveyor for Molecules

“…[1][2][3][4] While the relaxation time scale of mass diffusion on the micrometer scale is on the order of milliseconds, heat diffusion of water is still typically 3 orders of magnitude faster. Our idea here is to use these faster thermal fields to confine and control the position of molecules despite their thermal motion.…”

Hydrodynamics in bridging and aggregation of two colloidal particles in a near-critical binary mixture

“…The adsorption-induced interaction decays exponentially as exp(−ℓ/ξ) for ℓ ξ if the solvent is at the critical composition without phase separation 8,10,11 , as was measured directly 32 . Some authors 22,27 used this exponential expression to explain the observed aggregation.…”

“…In real experiments, charges usually appear on the colloid surfaces in aqueous mixtures 14,15,[18][19][20][21][22]32 . Supposing weak ionization, we neglect the effect of charges on the adsorption-induced interaction.…”

Critical and near-critical phase behavior and interplay between the thermodynamic Casimir and van der Waals forces in a confined nonpolar fluid medium with competing surface and substrate potentials

“…In the present article we will report theoretical results dealing with the critical Casimir effect. Let us note, that the critical Casimir effect has been already directly observed, utilizing light scattering measurements, in the interaction of a colloid spherical particle with a plate [42] both of which are immersed in a binary liquid mixture. The effect has been also studied in 4 He [43], [44], as well as in 3 He- 4 He mixtures [45] in the context of forces that determine the properties of a film of a substance in the vicinity of its bulk critical point.…”