arXiv subjects

Job H. J. Thijssen

Publications and source records attributed to Job H. J. Thijssen.

10 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

Changing the flow profile and resulting drying pattern of dispersion droplets via contact angle modification

Spilling tea or coffee leads to a tell-tale circular stain after the droplet dries, known as the "coffee ring effect". The evaporation of suspension droplets is a complex physical process, and predicting and controlling the particle deposit patterns from sessile droplet evaporation are essential for many industrial processes, such as ink-jet printing and crop-care applications. In this article, we systematically investigate the effect of surface wettability on the evaporation dynamics of a particle-laden droplet, focussing on the contact line stick-slip behaviour, the hydrodynamic flow of the suspended particles, and the resulting particle deposit after evaporation. We use substrates with different wettabilities, ranging from hydrophilic to hydrophobic, and quantify the internal flow during the evaporation by tracking (fluorescent) tracer particles. We find that the internal flow shifts from a predominantly outward flow towards the contact line for low contact angles to an inward flow for large contact angles. Correspondingly, the particle deposit transitions from the typical coffee-ring pattern to a central stain as the substrates's hydrophobicity increases. Finally, we corroborate these experimental findings with dynamic density functional theory, modelling the droplet evaporation process and stick-slip behaviour of the contact line. Our investigation suggests that the wettability of the substrate can significantly alter hydrodynamic flow within drying droplets, thereby affecting the resulting particle deposit.

physics.flu-dyn↗

arXiv2024

The interplay between topography and contact line pinning mechanisms on flat and superhydrophobic surfaces

Wettability of a surface depends on both surface chemistry and topography. To move a three-phase contact line, a de-pinning force needs to be applied, which is of practical importance in various applications. However, a unified understanding and description of the de-pinning force on both flat and superhydrophobic surfaces is still lacking. This study aims to bridge the existing gap in our understanding of the three-phase contact line pinning on flat and microstructured superhydrophobic surfaces. The findings indicate that a general model, based on two different pinning mechanisms, can describe the pinning force on both flat and microstructured surfaces. We compare the general model against experimental data from literature, as well as our experiments on flat and microstructured surfaces coated with a liquid-like layer of grafted polymer chains. While this theoretical framework can be useful for designing micro-engineered surfaces on which the contact line behaviour is important, it also provides a potential experimental strategy to distinguish the contribution of defects from that of molecular re-orientation to contact line pinning on a given solid material.

physics.flu-dyn↗

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↗

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↗

arXiv2019

Debye-Hückel potential at an interface between two media

Electrostatic interactions between point charges embedded into interfaces separating dielectric media are omnipresent in soft matter systems and often control their stability. Such interactions are typically complicated and do not resemble their bulk counterparts. For instance, the electrostatic potential of a point charge at an air-water interface falls off as $r^{-3}$, where $r$ is the distance from the charge, exhibiting a dipolar behaviour. This behaviour is often assumed to be generic, and is widely referred to when interpreting experimental results. Here we explicitly calculate the in-plane potential of a point charge at an interface between two electrolyte solutions with different dielectric permittivities and Debye screening lengths. We show that the asymptotic behaviour of this potential is neither a dipole, which characterises the potential at air-water interfaces, nor a screened monopole, which describes the bulk behaviour in a single electrolyte solution. By considering the same problem in arbitrary dimensions, we find that the physics behind this difference can be traced to the asymmetric propagation of the interaction in the two media. Our results are relevant, for instance, to understand the physics of charged colloidal particles trapped at oil-water interfaces.

cond-mat.soft↗

arXiv2019

Soft meets hard -- how does freeze-thaw cycling affect the microstructure of particle-stabilised emulsions?

The freeze-thaw cycling of particle-stabilised emulsions can alter the emulsion structure and stability. This could have significant consequences for using particle stabilisation in industrial applications where increased stability is generally desirable. It is therefore important to characterise the behaviour and stability of these composites under the influence of freeze-thaw cycles. Water-in-oil Pickering emulsions stabilised by poly(methyl methacrylate) particles were subjected to freeze-thaw cycles of the continuous phase under two different conditions - uniform and non-uniform freezing. Confocal microscopy was used to study the emulsion behaviour and structure during these processes. The effect of droplet size and cooling rate on uniformly frozen emulsions was also considered. The final structure of the emulsion after a single freeze-thaw cycle is strongly dependent on the freezing method. Uniformly frozen emulsions show crumpled droplet structures, while non-uniformly frozen emulsions have a non-uniform structure containing foam-like regions not observed in uniform freezing. Droplet size has little effect on the final structure of uniformly frozen emulsions, which we attribute to the Laplace pressure in the droplets being orders of magnitude smaller than the pressure exerted on the droplets by the growing oil crystals. Cooling rate also has little effect as droplets become surrounded and trapped by oil crystals rapidly after samples reach the oil freezing temperature, irrespective of the speed at which they reached that temperature. When compared to surfactant-stabilised emulsions undergoing the same process, we find emulsion structure is recoverable in the surfactant case, whereas particle-stabilised emulsions are irreversibly altered.

cond-mat.soft↗

arXiv2012

Particle-stabilized oscillating diver: a self-assembled responsive capsule

We report the experimental discovery of a self-assembled capsule, with density set by interfacial glass beads and an internal bubble, that automatically performs regular oscillations up and down a vial in response to a temperature gradient. Similar composites featuring interfacial particles and multiple internal compartments could be the solution to a variety of application challenges.

cond-mat.soft↗

arXiv2011

Demixing, remixing and cellular networks in binary liquids containing colloidal particles

We present a confocal-microscopy study of demixing and remixing in binary liquids containing colloidal particles. First, particle-stabilized emulsions have been fabricated by nucleation and growth of droplets upon cooling from the single-fluid phase. We show that their stability mainly derives from interfacial particles; the surplus of colloids in the continuous phase possibly provides additional stability. Upon heating these emulsions, we have observed the formation of polyhedral cellular networks of colloids, just before the system remixes. Given a suitable liquid-liquid composition, the initial emulsions cross the binary-liquid symmetry line due to creaming. Therefore, upon heating, the droplets do not shrink and they remain closely packed. The subsequent network formation relies on a delicate balance between the Laplace pressure and the pressure due to creaming/remixing. As high concentrations of colloids in the cell walls inhibit film thinning and rupture, the networks can be stabilized for more than 30 minutes. This opens up an avenue for their application in the fabrication of advanced materials.

cond-mat.soft↗