The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology, encapsulation, emulsification, biofilm formation, water remediation and the interface-driven assembly of materials. Common intuition corroborated by hydrodynamic theories suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation-dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.

The ability to dictate the motion of microscopic objects is an important challenge in fields ranging from materials science to biology. Field-directed assembly drives microparticles along paths defined by energy gradients. Nematic liquid crystals, consisting of rod-like molecules, provide new opportunities in this domain. Deviations of nematic liquid crystal molecules from uniform orientation cost elastic energy, and such deviations can be molded by bounding vessel shape. Here, by placing a wavy wall in a nematic liquid crystal, we impose alternating splay and bend distortions, and define a smoothly varying elastic energy field. A microparticle in this field displays a rich set of behaviors, as this system has multiple stable states, repulsive and attractive loci, and interaction strengths that can be tuned to allow reconfigurable states. Microparticles can transition between defect configurations, move along distinct paths, and select sites for preferred docking. Such tailored landscapes have promise in reconfigurable systems and in microrobotics applications.

In this article, we review both theoretical models and experimental results on the motion of micro-and nanoparticles that are close to a fluid interface or move in between two fluids. Viscous drags together with dissipations due to fluctuations of the fluid interface and its physicochemical properties affect strongly the translational and rotational drags of colloidal particles, which are subjected to Brownian motion in thermal equilibrium. Even if many theoretical and experimental investigations have been carried out, additional scientific efforts in hydrodynamics, statistical physics, wetting and colloid science are still needed to explain unexpected experimental results and to measure particle motion in time and space scales, which are not accessible so far.

We study the dynamics of individual polystyrene ellipsoids of different aspect ratios trapped at the air-water interface. Using particle tracking and in situ vertical scanning interferometry techniques we are able to measure translational drags and the protrusion in air of the ellipsoids. We report that translational drags on the ellipsoid are unexpectedly enhanced: despite the fact that a noticeable part of the ellipsoid is in air, drags are found larger than the bulk one in water.

By confining soft materials within tailored boundaries it is possible to design energy landscapes to address and control colloidal dynamics. Twist distortions in confined liquid crystals multiply configurations for particles-boundaries interactions.

Shaping liquid crystals (LCs) into arrays of defect patterns enables the design of composite materials with new stimuli‐responsive properties. Self‐assembled defect assemblies that may arise in layered smectic A (SmA) LCs such as focal conic domains (FCDs), exhibit remarkable optical features and abilities for ordering nanoparticles. However, such SmA defect patterns are essentially electrically irreversible, which currently limits their adjustability in a dynamic way. Here, in situ polymerization of the texture of SmA FCDs allows transferring them into more electrically responsive LC phases, such as nematic, making possible a dynamic switch between different textural and optical states of FCDs in a reversible manner with voltage. Moreover, the method readily enables to program the operating temperature range of the polymer/LC composite from its chemical composition, adapting the system to various potential uses. This approach may increment new applications of SmA defect patterns such as voltage‐tunable privacy layers and may further inspire the design of LC‐based nanostructured composite and hybrid materials with new functions that can be dynamically tuned with voltage.

International audienceColloids with aggregation and adhesion properties reversibly tunable by shift of pH, T, light etc. can be designed by deposition of stimuli-responsive polymer chains on the particle surface. The aim of this work was to investigate how to control the strength of temperature-triggered attraction by analysing self-aggregation kinetics and soft adhesion of colloids to a at substrate. In order to endow the colloids with reversible and temperature-controlled interactions , silica or polystyrene microbeads (d =200 nm and 1 µm) were coated by mixed solutions of poly(lysine)-grafted-polyethylenoxide (PLL-g-PEG, for steric repulsion) and PLL-g-PNIPAM (i.e. PLL with poly-N-isopropylacrylamide T-responsive side chains). PEG-coated particles were stable in suspension, while the presence of PNIPAM provoked, at T > T c = 32 ± 1 • C, reversible aggre-gation and/or adsorption on glass plates. Dynamic light scattering following a T-jump from 25 • C to 40 • C was used to measure the aggregation rate and corresponding stability ratio W. For a molar fraction of PLL-g-PNIPAM, f , ranging from 100% down to about 20%, particles aggregate rapidly with slowly increasing W. Below f ≈ 20%, W increases by 3 orders of magnitude. The real-time 2D tracking method was used to monitor (x, y) positions of particles in suspension above microscope glass slides during a T-triggered adsorption. In order to capture transitory dynamics near PNIPAM collapse transition, particles tracks were recorded within a Tramp of 10 • C/min from below to above T c. The particle-substrate interaction was found to hinder the near-wall diusion and provoke the soft adhesion, as revealed by observation of characteristic conned Brownian motion. Resulting connement potential stiness prole α(f) presents a crossover from constant to linearly increasing at f ≈ 20%. Altogether , the characteristic coverage f * ≈ 20% is interpreted as a crossover from discrete to continuous coverage pattern within the soft contact domain

In a recent article Toro-Mendoza et al. considered an elastic response of an interface in order to explain the enhanced lateral drag of solid particles straddling fluid interfaces we recently measured.