We investigate the behavior of colloidal particles immersed in a binary liquid mixture of water and 2,6-lutidine in the presence of a chemically patterned substrate. 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 mixtures temperature, critical Casimir forces allow the formation of highly ordered monolayers but also extend the use of colloids as model systems.PACS numbers: 82.70. Dd, 68.35.Rh, 81.16.Dn Analogous to the geometrical confinement of quantumelectrodynamical (QED) vacuum fluctuations between two parallel metallic plates [1], the constraint of concentration fluctuations in fluid mixtures close to their critical point gives rise to critical Casimir forces acting on the confining surfaces [2]. The range of this interaction is set by the bulk correlation length ξ of the mixture which diverges when approaching the critical point. Therefore, critical Casimir forces are sensitive to minute changes in temperature. Despite several quantitative measurements of such forces [3,4], it was only recently when the amplitude of measured critical Casimir forces in quantumand classical liquids has been directly compared to theoretical predictions [5,6,7,8]. Direct force measurements of a single colloidal particle above a flat surface and immersed in a critical water-lutidine mixture demonstrated, that critical Casimir interactions can easily exceed multiples of the thermal energy k B T [8]. Accordingly, they offer a versatile opportunity to control the pair interaction in colloidal suspensions by weak temperature changes [9,10]. Apart from their exquisite temperature dependence, critical Casimir forces respond sensitively to the chemical properties of the confining surfaces. Depending on whether both surfaces preferentially attract the same mixture's component or not (symmetric or asymmetric boundary conditions), attractive or repulsive critical Casimir forces arise [8,11,12].So far, experimental investigations of critical Casimir interactions were limited to homogeneous surfaces (in contrast to QED Casimir forces [13]) where the corresponding forces act perpendicular to the confining walls. However, when one or both surfaces are chemically patterned, also lateral critical Casimir forces have been predicted [14] In this Letter we experimentally study the interaction between colloidal particles and chemically patterned substrates immersed in a binary critical mixture. Close to the critical point lateral critical Casimir forces lead to the formation of highly ordered colloidal assemblies whose structure is controlled by the underlying chemical pattern. This may suggest a novel route for templated growth of colloidal crystals. At higher particle concentrations, additional critical Casimir forces between nearby particle surfaces arise and eventually lead to the formation of three-dimensional, face...
We study theoretically and experimentally the solvent-mediated critical Casimir force acting on colloidal particles immersed in a binary liquid mixture of water and 2,6-lutidine and close to substrates which are chemically patterned with periodically alternating stripes of antagonistic adsorption preferences. These patterns are experimentally realized via microcontact printing. Upon approaching the critical demixing point of the solvent, normal and lateral critical Casimir forces generate laterally confining effective potentials for the colloids. We analyze in detail the rich behavior of the spherical colloids close to such substrates. For all patterned substrates we investigated, our measurements of these effective potentials agree with the corresponding theoretical predictions. Since both the directions and the strengths of the critical Casimir forces can be tuned by minute temperature changes, this provides a new mechanism for controlling colloids as model systems, opening encouraging perspectives for applications.Comment: Invited contribution to Molecular Physics Special Issue on Bob Evans' 65th birthda
We investigate the diffusion of a colloidal particle in a tilted periodic potential created by means of ten rotating optical tweezers arranged on a circle. Because of the viscous drag, the trap rotation leads to the onset of a tilting force in the corotating reference frame, so that in that frame the system can be described as an overdamped Brownian particle in a tilted periodic potential. The excellent agreement of the velocity and diffusion coefficient as a function of rotating frequency with theoretical predictions allowed us to extract the main parameters characterizing the system-the coefficient of free thermal diffusion and the potential corrugation depth-from the experimental results.
We present an experimental and theoretical study of the phase behavior of a binary mixture of colloids with opposite adsorption preferences in a critical solvent. As a result of the attractive and repulsive critical Casimir forces, the critical fluctuations of the solvent lead to a further critical point in the colloidal system, i.e. to a critical colloidal-liquid-colloidal-liquid demixing phase transition which is controlled by the solvent temperature. Our experimental findings are in good agreement with calculations based on a simple approximation for the free energy of the system.
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