arXiv subjects

Andrew B. Schofield

Publications and source records attributed to Andrew B. Schofield.

12 recordsLinked to original sources

arXiv2025

Impact of particle-size polydispersity on the quality of thin-film colloidal crystals

Size polydispersity in colloidal particles can disrupt order in their self-assembly, ultimately leading to a complete suppression of crystallization. In contrast to various computational studies, few experimental studies systematically address the effects of size polydispersity on the quality of colloidal crystals. We present an experimental study of structural order in thin films of crystals vertically dried from colloidal dispersions with a systematically varying polydispersity. As expected, an increase in polydispersity leads to a deterioration in order with significant drops in the local bond-orientational order at 8% and 12% polydispersity. Our results align with previously suggested models of epitaxial-like growth of 2D layers during convective assembly. Our results can offer critical insights into the permissible limits for achieving colloidal crystals from more polydisperse systems such as those synthesized through more sustainable methods.

cond-mat.soft↗

arXiv2024

Programmable self-assembly of core-shell ellipsoids at liquid interfaces

Ellipsoidal particles confined at liquid interfaces exhibit complex self-assembly behaviour due to quadrupolar capillary interactions induced by meniscus deformation. These interactions cause particles to attract each other in either tip-to-tip or side-to-side configurations. However, controlling their interfacial self-assembly is challenging because it is difficult to predict which of these two states will be preferred. In this study, we demonstrate that introducing a soft shell around hard ellipsoidal particles provides a means to control the self-assembly process, allowing us to switch the preferred configuration between these states. We study their interfacial self-assembly and find that pure ellipsoids without a shell consistently form a "chain-like" side-to-side assembly, regardless of aspect ratio. In contrast, core-shell ellipsoids transition from "flower-like" tip-to-tip to "chain-like" side-to-side arrangements as their aspect ratios increase. The critical aspect ratio for transitioning between these structures increases with shell-to-core ratios. Our experimental findings are corroborated by theoretical calculations and Monte Carlo simulations, which map out the phase diagram of thermodynamically preferred self-assembly structures for core-shell ellipsoids as a function of aspect ratio and shell-to-core ratios. This study shows how to program the self-assembly of anisotropic particles by tuning their physicochemical properties, allowing the deterministic realization of distinct structural configurations.

cond-mat.soft↗

arXiv2022

Contactless Interfacial Rheology: Probing Shear at Liquid-Liquid Interfaces without an Interfacial Geometry via Fluorescence Microscopy

Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solid-like behaviour and interfacial viscosities for fluid-like interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear.

cond-mat.soft↗

arXiv2021

Bicontinuous soft solids with a gradient in channel size designed for energy storage applications

We present examples of bicontinuous interfacially jammed emulsion gels ("bijels") with a designed gradient in the channel size along the sample. These samples are created by quenching binary fluids which have a gradient in particle concentration along the sample, since the channel size is determined by the local particle concentration. A gradient in local particle concentration is achieved using a two-stage loading process, with different particle volume fractions in each stage. Confocal microscopy and image analysis were used to quantitatively measure the channel size of the bijels. Bijels with a gradient in channel size of up to 2.8%/mm have been created. Such tailored soft materials could act as templates for energy materials optimised for both high ionic transport rates (high power) and high interfacial area (high energy density), potentially making them useful in novel energy applications.

cond-mat.soft↗

arXiv2021

Complex High Internal-Phase Emulsions that can Form Interfacial Films with Tuneable Morphologies

High internal phase emulsions (HIPEs) are considered as an important functional material and have been the focus of intense development effort, but it has not been possible to alter their fundamental attributes at either the microcosmic or macroscopic level, which severely limits their practical applications in various areas. In this work, we report a general strategy for creating complex HIPEs and we additionally show a route to forming HIPE films at liquid interfaces. Double HIPEs and Janus HIPEs are both realized for the first time. They feature complex microscopic patterns with a short-range anisotropy, and exhibit non-Newtonian pseudoplastic flow behavior. By taking advantage of their response to a high-pH subphase, interfacial films can be successfully obtained, which are tunable in thickness and morphologies under compression.

cond-mat.soft↗

arXiv2020

Interaction between Nearly Hard Colloidal Spheres at an Oil-Water Interface

We show that the interaction potential between sterically stabilized, nearly hard-sphere [poly(methylmethacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)] colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting that models previously developed for charged particles at liquid interfaces are not necessarily applicable to sterically stabilized particles. Interparticle potentials, $U(r)$, are extracted from radial distribution functions [$g(r)$, measured by fluorescence microscopy] via Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results are then validated by particle tracking in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA data is better described by a screened monopole only rather than a functional form having a screened monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface, i.e., the charge of the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here, we show that a screened $\frac{1}{r^2}$ term can also be used to fit our data. Finally, we present measurements for poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an additional contribution to the interaction potential. This is in line with our optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are slightly charged.

cond-mat.soft↗

arXiv2013

Eliminating cracking during drying

When colloidal suspensions dry, stresses build up and cracks often occur - a phenomenon undesirable for important industries such as paint and ceramics. We demonstrate an effective method which can completely eliminate cracking during drying: by adding emulsion droplets into colloidal suspensions, we can systematically decrease the amount of cracking, and eliminate it completely above a critical droplet concentration. Since the emulsion droplets eventually also evaporate, our technique achieves an effective function while making little changes to the component of final product, and may therefore serve as a promising approach for cracking elimination. Furthermore, adding droplets also varies the speed of air invasion and provides a powerful method to adjust drying rate. With the effective control over cracking and drying rate, our study may find important applications in many drying and cracking related industrial processes.

physics.flu-dyn↗

arXiv2010

Brambilla, et al. Reply

Brambilla, et al. Reply: van Megen and Williams (vMW) question our recent claim that dense colloidal hard spheres enter at large volume fraction ϕa dynamical regime not observed in earlier work and not described by the mode-coupling theory (MCT) of the glass transition.

cond-mat.soft↗

arXiv2010

Drying of complex suspensions

We investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed. A simple model coupling the Laplace pressure to Darcy's law correctly estimates both the threshold radius separating these two behaviors, and the rate of large-droplet evacuation. Finally, we use these systems to make novel hierarchical structures.

cond-mat.soft↗

arXiv2009

Probing the equilibrium dynamics of colloidal hard spheres above the mode-coupling glass transition

We use dynamic light scattering and computer simulations to study equilibrium dynamics and dynamic heterogeneity in concentrated suspensions of colloidal hard spheres. Our study covers an unprecedented density range and spans seven decades in structural relaxation time, $\ta$, including equilibrium easurements above $ϕ_{\rm c}$, the location of the glass transition deduced from fitting our data to mode-coupling theory. Instead of falling out of equilibrium, the system remains ergodic above $ϕ_{\rm c}$ and enters a new dynamical regime where $\ta$ increases with a functional form that was not anticipated by previous experiments, while the amplitude of dynamic heterogeneity grows slower than a power law with $\ta$, as found in molecular glass-formers close to the glass transition.

cond-mat.soft↗

arXiv2009

Particle dynamics in colloidal suspensions above and below the glass-liquid re-entrance transition

We study colloidal particle dynamics of a model glass system using confocal and fluorescence microscopy as the sample evolves from a hard-sphere glass to a liquid with attractive interparticle interactions. The transition from hard-sphere glass to attractive liquid is induced by short-range depletion forces. The development of liquid-like structure is indicated by particle dynamics. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. As samples enter the attractive liquid region, particle speed during these motional events increases by about one order of magnitude, and the particles move more cooperatively. Interestingly, colloidal particles in the attractive liquid phase do not exhibit significantly larger displacements than particles in the hard-sphere glass.

cond-mat.soft↗

arXiv2008

Dynamics of drying in 3D porous media

The drying dynamics in three dimensional porous media are studied with confocal microscopy. We observe abrupt air invasions in size from single particle to hundreds of particles. We show that these result from the strong flow from menisci in large pores to menisci in small pores during drying. This flow causes air invasions to start in large menisci and subsequently spread throughout the entire system. We measure the size and structure of the air invasions and show that they are in accord with invasion percolation. By varying the particle size and contact angle we unambiguously demonstrate that capillary pressure dominates the drying process.

physics.flu-dyn↗