Nonadditivity of critical Casimir forces

Paladugu, Sathyanarayana; Callegari, Agnese; Tuna, Yazgan; Barth, Lukas; Dietrich, S.; Gambassi, Andrea; Volpe, Giovanni

doi:10.1038/ncomms11403

Cited by 79 publications

(116 citation statements)

References 48 publications

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“…Adjusting the parameters of the theoretical potential, it is possible to match the experimental distribution P eq (r) at t = 0. By doing so, we have obtained, for each value of ∆T , the correlation length ξ of the order parameter fluctuations [9] and the Debye screening length ℓ D of the electrostatic interaction as fit parameters. Specifically, we adopt for V C the theoretical prediction…”

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Controlling the dynamics of colloidal particles by critical Casimir forces

et al. 2019

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Critical Casimir forces can play an important role for applications in nano-science and nanotechnology, owing to their piconewton strength, nanometric action range, fine tunability as a function of temperature, and exquisite dependence on the surface properties of the involved objects. Here, we investigate the effects of critical Casimir forces on the free dynamics of a pair of colloidal particles dispersed in the bulk of a near-critical binary liquid solvent, using blinking optical tweezers. In particular we measure the time evolution of the distance between the two colloids to determine their relative diffusion and drift velocity. Furthermore, we show how critical Casimir forces change the dynamic properties of this two-colloid system by studying the temperature dependence of the distribution of the so-called first-passage time, i.e., of the time necessary for the particles to reach for the first time a certain separation, starting from an initially assigned one. These data are in good agreement with theoretical results obtained from Monte Carlo simulations and Langevin dynamics. PACS numbers: 05.40.Jc, 68.35.RhCritical Casimir forces (CCFs) arise in a binary liquid mixture close to its critical point [1][2][3][4][5]. Upon approaching the critical point, fluctuations of the composition of the mixture emerge. If these critical fluctuations are confined between neighboring objects (e.g., two colloids, or a colloid and a planar surface), they lead to effective forces between these objects. These socalled CCFs were first predicted theoretically in 1978 by M. E. Fisher and P. G. de Gennes [1] in analogy to quantum-electrodynamical (QED) Casimir forces [6]. Only recently they have been measured directly [7-9] and proved to be relevant for soft matter [10][11][12]. These CCFs have been enjoying significant interest both from basic research and because they are promising candidates for applications in nano-science and nano-technology, in order to manipulate objects (e.g., by controllable periodic deformations of chains), to assemble devices (e.g., via the self-assembly of colloidal molecules [13,14]), and to drive machines (e.g., by powering rotators [15]) at the nanoand micro-meter scale. In fact, their piconewton strength and nanometric ranges of action match the requirements of nano-technology. Furthermore, these forces show an exquisite dependence on the temperature of the environment and on the chemical surface properties of the objects involved [4,5,8,16,17]. For example, if density fluctuations are confined between particles with the same surface property (e.g., hydrophilic), attractive CCFs take hold, while they are repulsive between particles with opposite surface properties (e.g., hydrophilic vs. hydrophobic particles). With the exception of Ref. [18], until now, the experimental studies have focused on the time-

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Controlling the dynamics of colloidal particles by critical Casimir forces

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“…In Ref. [39], using a system of three optically trapped [40] spherical colloidal particles, immersed in a critical binary mixture, the authors demonstrated experimentally the theoretically predicted nonadditivity of the fluctuation-induced interactions. Other experimental setups which exploit a sphere-plate or sphere-sphere configurations immersed in a critical fluid include the interaction of spherical colloids with chemically patterned substrates [41,42] which theoretical description is discussed in Refs.…”

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Sign change in the net force in sphere-plate and sphere-sphere systems immersed in nonpolar critical fluid due to the interplay between the critical Casimir and dispersion van der Waals forces

Valchev

Dantchev

2017

Phys. Rev. E

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We study systems in which both long-ranged van der Waals and critical Casimir interactions are present. The last arise as an effective force between bodies when immersed in a near-critical medium, say a nonpolar onecomponent fluid or a binary liquid mixture. They are due to the fact that the presence of the bodies modifies the order parameter profile of the medium between them as well as the spectrum of its allowed fluctuations. We study the interplay between these forces, as well as the total force (TF) between a spherical colloid particle and a thick planar slab, and between two spherical colloid particles. We do that using general scaling arguments and mean-field type calculations utilizing the Derjaguin and the surface integration approaches. They both are based on data of the forces between two parallel slabs separated at a distance L from each other, confining the fluctuating fluid medium characterized by its temperature T and chemical potential µ. The surfaces of the colloid particles and the slab are coated by thin layers exerting strong preference to the liquid phase of the fluid, or one of the components of the mixture, modeled by strong adsorbing local surface potentials, ensuring the socalled (+, +) boundary conditions. On the other hand, the core region of the slab and the particles, influence the fluid by long-ranged competing dispersion potentials. We demonstrate that for a suitable set of colloids-fluid, slab-fluid, and fluid-fluid coupling parameters the competition between the effects due to the coatings and the core regions of the objects involved result, when one changes T , µ or L, in sign change of the Casimir force (CF) and the TF acting between the colloid and the slab, as well as between the colloids. This can be used for governing the behavior of objects, say colloidal particles, at small distances, say in colloid suspensions for preventing flocculation. It can also provide a strategy for solving problems with handling, feeding, trapping and fixing of microparts in nanotechnology. Data for specific substances in support of the experimental feasibility of the theoretically predicted behavior of the CF and TF have been also presented.

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“…These media will enhance or reduce the vdW potential via manybody interactions [24,25]. Note that evidence for threebody dispersion forces has recently been found for the critical Casimir effect [26].…”

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Enhanced Chiral Discriminatory van der Waals Interactions Mediated by Chiral Surfaces

et al. 2017

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We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic and chiral response to non-trivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waals interaction is discriminatory with respect to enantiomers of different handedness and could be used to separate enantiomers. We also suggest a specific geometric configuration where the electric contribution to the van der Waals interaction is zero, making the chiral component the dominant effect.PACS numbers: 34.20. Cf, 33.55.+b,42.50.Nn Introduction. Casimir and van der Waals (vdW) forces are electromagnetic interactions between neutral macroscopic bodies and/or molecules due to the quantum fluctuations of the electromagnetic field [1][2][3]. In particular, the attractive vdW potential between two electrically polarisable particles was first derived by Casimir and Polder using the minimal-coupling Hamiltonian [2]. Molecules can also exhibit magnetic [4][5][6][7] and chiral polarisabilities [4,8,9] and their contribution to the vdW force can be repulsive.The aim of this work is the study of the interaction between chiral molecules in the presence of a chiral magneto-dielectric body. Chiral molecules lack any center of inversion nor plane of symmetry. Hence they exist as two distinct enantiomers, left-handed and righthanded, which are related to space inversion. Due to their low symmetry they have distinctive interactions with light. In a chiral solution the refractive indices for circularly polarized light of different handedness are different. Hence a chiral solution can rotate the plane of polarization of light with an angle related to the concentration of the solution (optical rotation) [10][11][12], or absorb left-and right-circularly polarised light at different rates (circular dichroism) [13]. All of these phenomena are related to the optical rotatory strength, defined in terms of electric (d nk ) and magnetic (m nk ) dipole moment matrix elements [14]:

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Nonadditivity of critical Casimir forces

Cited by 79 publications

References 48 publications

Controlling the dynamics of colloidal particles by critical Casimir forces

Controlling the dynamics of colloidal particles by critical Casimir forces

Sign change in the net force in sphere-plate and sphere-sphere systems immersed in nonpolar critical fluid due to the interplay between the critical Casimir and dispersion van der Waals forces

Enhanced Chiral Discriminatory van der Waals Interactions Mediated by Chiral Surfaces

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