Ordering of binary colloidal crystals by random potentials

Abstract

Structural defects are ubiquitous in condensed matter, and not always a nuisance. For example, they underlie phenomena such as Anderson localization and hyperuniformity, and they are now being exploited to engineer novel materials. Here, we show experimentally that the density of structural defects in a 2D binary colloidal crystal can be engineered with a random potential. We generate the random potential using an optical speckle pattern, whose induced forces act strongly on one species of particles (strong particles) and weakly on the other (weak particles). Thus, the strong particles are more attracted to the randomly distributed local minima of the optical potential, leaving a trail of defects in the crystalline structure of the colloidal crystal. While, as expected, the crystalline ordering initially decreases with increasing fraction of strong particles, the crystalline order is surprisingly recovered for sufficiently large fractions. We confirm our experimental results with particle-based simulations, which permit us to elucidate how this non-monotonic behavior results from the competition between the particle-potential and particle-particle interactions.Perfect crystalline structures are not commonly found in Nature, because, even in the absence of impurities, structural defects occur spontaneously and disrupt the periodicity of the crystalline lattice [1]. For example, when a melt is cooled down, multiple crystallites grow with degenerate orientations [2]. Since the coarsening time of these crystallites diverges with size, structural defects appear and prevent the emergence of global order [3,4]. While the existence of these defects is a challenge when growing single crystals, it can also be an opportunity when engineering the properties of materials; FIG. 1. Colloidal crystals with tunable degree of disorder. Final configurations obtained in (a-c) experiments and (d-f) simulations, for different molar fractions χ of strong particles. The weak (silica) particles are light gray, and the strong (polystyrene) particles are dark gray.indeed, control over defects enables the development of solid-state devices with fine-tuned mechanical resilience, optical properties, and heat and electrical conductivity [5][6][7][8][9]. In atomic crystals, engineering structural defects is an experimental challenge for two reasons [10]: first, current visualization techniques at the atomic scale do not provide a high spatial or time resolution [11,12]; second, no current technique can control the density of defects in a systematic manner [13]. The first challenge can be overcome studying colloidal crystals as models for atomic systems [14,15], where colloidal particles can be individually tracked using standard digital video microscopy techniques [16][17][18]. Here, we demonstrate that the second challenge can be solved combining a binary colloidal mixture and an optical random potential generated by a speckle light pattern. This permits us to control the density of structural defects in the resulting 2D colloidal crystal and ...

Roadmap for optical tweezers

“…Historically, the atom-cooling community was the first to adopt speckle light fields to trap small particles [38]. The last decade has experienced a revival of the technique to tackle questions in mesoscopic physics, where both static and time-varying speckle optical potentials have been used to study the emergence of anomalous diffusion (from subdiffusion to superdiffusion) in colloidal dispersions [36,[39][40][41][42], to tune effective dispersion forces between small colloidal particles [43], to perform standard microfluidic operations, such as particle guiding and sorting [35], to assemble 2D crystal-like and glassy colloidal materials [44], to control collective behaviour in active matter [45], and to reproduce first-passage statistics [46].…”

Steering self-organisation through confinement

“…56,57 Heterogeneity can be introduced, e.g., by generating disordered optical potentials exploiting the formation of speckle patterns when light propagates through complex media. 58,59 With the advance of wave modulation techniques, similar experiments could be extended to shorter length scales (e.g. electron microscopy) 60 and larger length scales (e.g.…”

Monte Carlo simulation of particle size separation in evaporating bi-dispersed colloidal droplets on hydrophilic substrates

“…In some applications, solutions containing mixtures of particles of different sizes are of interest. As an example, we will mention only some applications: evaporative lithography [1][2][3][4] , diagnostics in medicine 5 , development of biosensors 6 , formation of supraparticles [7][8][9][10] and nanocomposites 11 , creation of photonic crystals 7,12,13 , production of color filters for displays 14 , nanosphere lithography [15][16][17][18] and inkjet printing 19 . Mixtures of particles are also used in other technologies for the formation of structures that are not associated with evaporation, for example, the Langmuir-Blodgett method 20,21 , spin-coating 22 , transfer of a particle monolayer from the liquid-air interface onto an inclined substrate in the process of liquid discharge from a glass container with a tap 23 , etc.…”