Rotational diffusion of partially wetted colloids at fluid interfaces

Rotational and translational drags of a Janus particle close to a wall and a lipid membrane

HYPOTHESIS

Measuring rotational and translational Brownian motion of single spherical particles reveals dissipations due to the interaction between the particle and the environment.

EXPERIMENTS

In this article, we show experiments where the in-plane translational and the two rotational drag coefficients of a single spherical Brownian particle can be measured. These particle drags are functions of the particle size and of the particle-wall distance, and of the viscous dissipations at play. We measure drag coefficients for Janus particles close to a solid wall and close to a lipid bilayer membrane.

FINDINGS

For a particle close to a wall, we show that according to hydrodynamic models, particle-wall distance and particle size can be determined. For a particle partially wrapped by lipid membranes, in absence of strong binding interactions, translational and rotational drags are significantly larger than the ones of non-wrapped particles. Beside the effect of the membrane viscosity, we show that dissipations in the deformed membrane cap region strongly contribute to the drag coefficients.

Sideways propelled bimetallic rods at the water/oil interface

The motion of self-propelling microswimmers is significantly affected by confinement, which can enhance or reduce their mobility and also steer the direction of their propulsion. While their interactions with solid boundaries have already received considerable attention, many aspects of the influence of liquid-liquid interfaces (LLI) on active particle propulsion still remain unexplored. In this work, we studied the adsorption and motion of bimetallic Janus sideways propelled rods dispersed at the interface between an aqueous solution of hydrogen peroxide and oil. The wetting properties of the bimetallic rods result in a wide distribution of their velocities at the LLI. While a fraction of rods remain immotile, we note a significant enhancement of motility for the rest of the particles with velocities of up to 8 times higher in comparison to those observed near a solid wall. Liquid-liquid interfaces, therefore, can provide a new way to regulate the propulsion of bimetallic particles.

3-D rotation tracking from 2-D images of spherical colloids with textured surfaces

Tracking the three-dimensional rotation of colloidal particles is essential to elucidate many open questions, e.g. concerning the contact interactions between particles under flow, or the way in which obstacles and neighboring particles affect self-propulsion in active suspensions. In order to achieve rotational tracking, optically anisotropic particles are required. We synthesise here rough spherical colloids that present randomly distributed fluorescent asperities and track their motion under different experimental conditions. Specifically, we propose a new algorithm based on a 3-D rotation registration, which enables us to track the 3-D rotation of our rough colloids at short time-scales, using time series of 2-D images acquired at high frame rates with a conventional wide-field microscope. The method is based on the image correlation between a reference image and rotated 3-D prospective images to identify the most likely angular displacements between frames. We first validate our approach against simulated data and then apply it to the cases of: particles flowing through a capillary, freely diffusing at solid-liquid and liquid-liquid interfaces, and self-propelling above a substrate. By demonstrating the applicability of our algorithm and sharing the code, we hope to encourage further investigations in the rotational dynamics of colloidal systems.

Effects of Shape on Interaction Dynamics of Tetrahedral Nanoplastics and the Cell Membrane

Cellular uptake of nanoplastics is instrumental in their environmental accumulation and transfer to humans through the food chain. Despite extensive studies using spherical plastic nanoparticles, the influence of the morphological characteristics of environmentally released nanoplastics is understudied. Using dissipative particle dynamics simulations, we modeled the interactions between a cell membrane and hydrophobic nanotetrahedra, which feature high shape anisotropy and large surface curvature seen for environmental nanoplastics. We observe robust uptake of nanotetrahedra with sharp vertices and edges by the lipid membrane. Two local energy minimum configurations of nanotetrahedra embedded in the membrane bilayer were identified for particles of large sizes. Further analysis of particle dynamics within the membrane shows that the two interaction states exhibit distinct translational and rotational dynamics in the directions normal and parallel to the plane of the membrane. The membrane confinement significantly arrests the out-of-plane motion, resulting in caged translation and subdiffusive rotation. While the in-plane diffusion remains Brownian, we find that the translational and rotational modes decouple from each other as the particle size increases. The rotational diffusion decreases by a greater extent compared to the translational diffusion, deviating from the continuum theory predictions. These results provide fundamental insights into the shape effect on the nanoparticle dynamics in crowded lipid membranes.

Dynamically shaping the surface of silica colloids

Surface roughness is an important design parameter to influence the processing of particle-based materials. Current methods to synthesize rough particles present some limitations, e.g. low yield, relative methodological complexity, requirements of multiple steps, or poor roughness control. Here, we thoroughly investigate a facile synthesis route where two silanes, tetraethyl orthosilicate (TEOS) and vinyltrimethoxysilane (VTMS), are added in one pot to form silica particles with controlled corrugated surfaces. We first show that the morphology of these particles can be defined by regulating the amount and ratio of the two silane precursors and by adjusting the concentration of ammonia during synthesis. We characterize the surface topography of the particles using atomic force microscopy and show a direct correlation between surface roughness and the synthesis conditions. Furthermore, we carry out an in situ observation of the evolution of surface morphology and propose a mechanism for surface structuring that hinges on the formation of silane droplets, followed by the preferential hydrolysis/condensation reaction of VTMS starting from the droplet surface and evolving towards the center. The exchange of liquid from the droplets through the VTMS shell leads to stress accumulation and wrinkling/buckling of the particles. Moreover, we explicitly show that osmotic imbalances between the inside and the outside of the droplets regulate their shrinking. We therefore demonstrate that exchanging solvents has a comparable impact to adjusting silane and ammonia content in defining the particle shape and that this synthesis route is highly dynamical. Finally, we demonstrate that it is possible to incorporate fluorescent dyes during synthesis to enable future studies on the impact of surface roughness on dynamic processes, including shear, via direct high-resolution imaging. Our findings show that the mechanism for wrinkling and buckling in colloidal silica particles follows a general scheme found in a broad range of systems, from liposomes and polymeric capsules to Pickering emulsion droplets.

Driven Engulfment of Janus Particles by Giant Vesicles in and out of Thermal Equilibrium

The interaction between Janus colloids and giant lipid vesicles was experimentally investigated to elucidate the dynamics and mechanisms related to microparticle engulfment by lipid vesicles. Janus (Pt–SiO₂ and Pt–MF, where MF is melamine formaldehyde) colloids do not spontaneously adhere to POPC or DOPC bilayers, but by applying external forces via centrifugation we were able to force the contact between the particles and the membranes, which may result in a partial engulfment state of the particle. Surface properties of the Janus colloids play a crucial role in the driven particle engulfment by vesicles. Engulfment of the silica and platinum regions of the Janus particles can be observed, whereas the polymer (MF) region does not show any affinity towards the lipid bilayer. By using fluorescence microscopy, we were able to monitor the particle orientation and measure the rotational dynamics of a single Janus particle engulfed by a vesicle. By adding hydrogen peroxide to the solution, particle self-propulsion was used to perform an active transport of a giant vesicle by a single active particle. Finally, we observe that partially engulfed particles experience a membrane curvature-induced force, which pushes the colloids towards the bottom where the membrane curvature is the lowest.

Interfacial viscoelasticity and jamming of colloidal particles at fluid-fluid interfaces: a review

Colloidal particles can be adsorbed at fluid-fluid interfaces, a phenomenon frequently observed in particle-stabilized foams, Pickering emulsions, and bijels. Particles adsorbed at interfaces exhibit unique physical and chemical behaviors, which affect the mechanical properties of the interface. Therefore, interfacial colloidal particles are of interest in terms of both fundamental and applied research. In this paper, we review studies on the adsorption of colloidal particles at fluid-fluid interfaces, from both thermodynamic and mechanical points of view, and discuss the differences as compared with surfactants and polymers. The unique particle interactions induced by the interfaces as well as the particle dynamics including lateral diffusion and contact line relaxation will be presented. We focus on the rearrangement of the particles and the resultant interfacial viscoelasticity. Particular emphasis will be given to the effects of particle shape, size, and surface hydrophobicity on the interfacial particle assembly and the mechanical properties of the obtained particle layer. We will also summarize recent advances in interfacial jamming behavior caused by adsorption of particles at interfaces. The buckling and cracking behavior of particle layers will be discussed from a mechanical perspective. Finally, we suggest several potential directions for future research in this area.

Interpretation of interfacial interactions between lenticular particles

HYPOTHESIS

The geometric features of charged particles at a fluid-fluid interface substantially affect their interfacial configurations and interparticle interactions (electrostatic and capillary forces). Because lenticular particles exhibit both spherical and nonspherical surface characteristics, an investigation of their interfacial phenomena can provide in-depth understanding of the relationship between the configuration and the interactions of these particles at the interface.

EXPERIMENTS

Three types of lenticular particles are prepared using a seeded emulsion polymerization method. Pair interactions at the oil-water interface are directly measured with optical laser tweezers. The numerical calculation of the attachment energy of the particle to the interface is used to predict their configuration behaviors at the interface.

FINDINGS

The lenticular particles are found to adopt either an upright or inverted configuration that can be determined stochastically. When the interface contacts the truncated boundary or the biconvex boundary, the local interface deformation-induced capillary attraction likely becomes dominant. The contact probability can be estimated on the basis of the attachment energy profile and related to the relative strengths of capillary attraction and electrostatic repulsion between two particles at the interface. Furthermore, possible artifacts in measurements of the pair interactions between nonspherical particles with optical laser tweezers are discussed, depending on their interfacial configurations.

Motion of micro- and nano- particles interacting with a fluid interface

In this article, we review both theoretical models and experimental results on the motion of micro- and nano- particles 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.

Spherical probes for simultaneous measurement of rotational and translational diffusion in 3 dimensions

Real time visualization and tracking of colloidal particles with 3D resolution is essential for probing the local structure and dynamics in complex fluids. Although tracking translational motion of spherical particles is well-known, accessing rotational dynamics of such particles remains a great challenge. Here, we report a novel approach of using fluorescently labeled raspberry-like colloids with an optical anisotropy to concurrently track translational and rotational dynamics in 3 dimensions. The raspberry-like particles are coated by a silica layer of adjustable thickness, which allows tuning the surface roughness. The synthesis and applicability of the proposed method is demonstrated by two types of probes: rough and smoothened. The accuracies of measuring Mean Squared (Angular) Displacements are also demonstrated by using these 2 probes dispersed in 2 different solvents. The presented 3D trackable colloids offer a high potential for wide range of applications and studies, such as probing the dynamics of crystallization, phase transitions, biological interactions and the effect of surface roughness on diffusion.

Brownian Diffusion of Individual Janus Nanoparticles at Water/Oil Interfaces

Janus nanoparticles could exhibit a higher interfacial activity and adsorb stronger to fluid interfaces than homogeneous nanoparticles of similar sizes. However, little is known about the interfacial diffusion of Janus nanoparticles and how it compares to that of homogeneous ones. Here, we employed fluorescence correlation spectroscopy to study the lateral diffusion of ligand-grafted Janus nanoparticles adsorbed at water/oil interfaces. We found that the diffusion was significantly slower than that of homogeneous nanoparticles. We carried out dissipative particle dynamic simulations to study the mechanism of interfacial slowdown. Good agreement between experimental and simulation results has been obtained only provided that the flexibility of ligands grafted on the nanoparticle surface was taken into account. The polymeric ligands were deformed and oriented at an interface so that the effective radius of Janus nanoparticles is larger than the nominal one obtained by measuring the diffusion in bulk solution. These findings highlight further the critical importance of the ligands grafted on Janus nanoparticles for applications involving nanoparticle adsorption at an interface, such as oil recovery or two-dimensional self-assembly.

Electrostatic pair-interaction of nearby metal or metal-coated colloids at fluid interfaces

In this paper, we theoretically study the electrostatic interaction between a pair of identical colloids with constant surface potentials sitting in close vicinity next to each other at the fluid interface. By employing a simplified yet reasonable model system, the problem is solved within the framework of classical density functional theory and linearized as well as nonlinear Poisson-Boltzmann (PB) theory. Apart from providing a sound theoretical framework generally applicable to any such problem, our novel findings, all of which contradict common beliefs, include the following: first, quantitative and qualitative differences between the interactions obtained within the linear and the nonlinear PB theories; second, the importance of the electrostatic interaction between the omnipresent three-phase contact lines in interfacial systems; and, third, the occurrence of an attractive electrostatic interaction between a pair of identical metal colloids. The unusual attraction we report largely stems from an attractive line interaction, which although scales linearly with the size of the particle can compete with the surface interactions and can be strong enough to alter the nature of the total electrostatic interaction. Our results should find applications in metal or metal-coated particle-stabilized emulsions where densely packed particle arrays are not only frequently observed but also sometimes required.

Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression

Langmuir monolayers of silica/gold Janus particles with two different degrees of amphiphilicity have been examined to study the significance of particle surface amphiphilicity on the structure and mechanical properties of the interfacial layers. The response of the layers to the applied compression provides insight into the nature and strength of the interparticle interactions. Different collapse modes observed for the interfacial layers are linked to the amphiphilicity of Janus particles and their configuration at the interface. Molecular dynamics simulations on nanoparticles with similar contact angles provide insight on the arrangement of particles at the interface and support our conclusion that the interfacial configuration and collapse of anisotropic particles at the air/water interface are controlled by particle amphiphilicity.