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Register a new userCritical Casimir forces steered by patterned substrates
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Among the various kinds of effective forces in soft matter, the spatial range and the direction of the so-called critical Casimir force - which is generated by the enhanced thermal fluctuations close to a continuous phase transition - can be controlled and reversibly modified to an uncommonly large extent. In particular, minute temperature changes of the fluid solvent, which provides the near-critical thermal fluctuations, lead to a significant change of the range and strength of the effective interaction among the solute particles. This feature allows one to control, e.g., the aggregation of colloidal dispersions or the spatial distribution of colloids in the presence of chemically or topographically patterned substrates. The spatial direction of the effective force acting on a solute particle depends only on the surface properties of the immersed particles and can be spatially modulated by suitably patterned surfaces. These critical Casimir forces are largely independent of the specific materials properties of both the solvent and the confining surfaces. This characteristic universality of critical phenomena allows systematic and quantitative theoretical studies of the critical Casimir forces in terms of suitable representative and simplified models. Here we highlight recent theoretical and experimental advances concerning critical Casimir forces with a particular emphasis on the numerous possibilities of controlling these forces by substrate patterns.
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We study the normal and lateral effective critical Casimir forces acting on a spherical colloid immersed in a critical binary solvent and close to a chemically structured substrate with alternating adsorption preference. We calculate the universal scaling function for the corresponding potential and compare our results with recent experimental data [Soyka F., Zvyagolskaya O., Hertlein C., Helden L., and Bechinger C., Phys. Rev. Lett., 101, 208301 (2008)]. The experimental potentials are properly captured by our predictions only by accounting for geometrical details of the substrate pattern for which, according to our theory, critical Casimir forces turn out to be a sensitive probe.
Colloids immersed in a critical or near-critical binary liquid mixture and close to a chemically patterned substrate are subject to normal and lateral critical Casimir forces of dominating strength. For a single colloid we calculate these attractive or repulsive forces and the corresponding critical Casimir potentials within mean-field theory. Within this approach we also discuss the quality of the Derjaguin approximation and apply it to Monte Carlo simulation data available for the system under study. We find that the range of validity of the Derjaguin approximation is rather large and that it fails only for surface structures which are very small compared to the geometric mean of the size of the colloid and its distance from the substrate. For certain chemical structures of the substrate the critical Casimir force acting on the colloid can change sign as a function of the distance between the particle and the substrate; this provides a mechanism for stable levitation at a certain distance which can be strongly tuned by temperature, i.e., with a sensitivity of more than 200nm/K.
A recent Letter [Phys. Rev. Lett. 103, 156101 (2009)] reports the experimental observation of aggregation of colloidal particles dispersed in a liquid mixture of heavy water and 3-methylpyridine. The experimental data are interpreted in terms of a model which accounts solely for the competing effects of the interparticle electrostatic repulsion and of the attractive critical Casimir force. Here we show, however, that the reported aggregation actually occurs within ranges of values of the correlation length and of the Debye screening length ruled out by the proposed model and that a significant part of the experimental data presented in the Letter cannot be consistently interpreted in terms of such a model.
Critical Casimir forces emerge between objects, such as colloidal particles, whenever their surfaces spatially confine the fluctuations of the order parameter of a critical liquid used as a solvent. These forces act at short but microscopically large distances between these objects, reaching often hundreds of nanometers. Keeping colloids at such distances is a major experimental challenge, which can be addressed by the means of optical tweezers. Here, we review how optical tweezers have been successfully used to quantitatively study critical Casimir forces acting on particles in suspensions. As we will see, the use of optical tweezers to experimentally study critical Casimir forces can play a crucial role in developing nano-technologies, representing an innovative way to realize self-assembled devices at the nano- and microscale.
Motivated by recent experiments with confined binary liquid mixtures near their continous demixing phase transition we study the critical behavior of a system, which belongs to the Ising universality class, for the film geometry with one planar wall chemically structured such that there is a laterally alternating adsorption preference for the species of the binary liquid mixture. By means of Monte Carlo simulations and finite-size scaling analysis we determine the critical Casimir force and the corresponding universal scaling function.
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