Session A51: Colloids I: Beyond Hard Spheres

Photonic Droplets Containing Transparent Aqueous Colloidal Suspensions with Optimal Scattering Properties

In recent years, there has been a growing interest in quasi-ordered structures that generate non-iridescent colors. Such structures have only short-range order and are isotropic, making colors invariant with viewing angle under natural lighting conditions. Our recent simulation suggests that colloidal particles with independently controlled diameter and scattering cross section can realize the structural colors with angular independence. In this presentation, we are exploiting depletion-induced assembly of colloidal particles to create isotropic structures in a milimeter-scale droplet. As a model colloidal particle, we have designed and synthesized core-shell particles with a large, low refractive index shell and a small, high refractive index core. The remarkable feature of these particles is that the total cross section for the entire core-shell particle is nearly the same as that of the core particle alone. By varying the characteristic length scales of the sub-units of such `photonic' droplet we aim to tune wavelength selectivity and enhance color contrast and viewing angle.   

Curvature-Induced Capillary Interaction between Spherical Particles at a Liquid Interface

Capillary interactions among particles adsorbed at a fluid interface are important in a variety of natural and technological systems but still pose many mysteries. Capillary interactions induced by buoyancy, referred to as the''Cheerios ''effect, have been studied for years. Here, we experimentally investigate how anisotropic interfacial shape affects capillary forces among millimeter-sized spheres. The Cheerios model predicts that particles with densities that are higher and lower compared to the fluids adsorbed at an initially flat interface will repel. Our experiments, however, clearly show that they can attract one another at the short range. We explain our results with a model, in which each sphere creates an anisotropic curvature at the position of the other sphere. To satisfy the constant contact-angle boundary condition, the interface is deformed with quadrupolar symmetry around each sphere. This quadrupolar deformation creates a short-ranged, attractive capillary force. The range of size and density ratios at which we observe a dominant short-range attraction is consistent with the model. Our results show how interfacial shape may be used to direct the assembly of interfacial particles.   

Using Micron-Sized Ellipsoids as a New Tool for Microrheology

Microrheology is a well-established technique, and in its simplest form it allows you to measure the viscosity of a fluid by examining the diffusion of microspheres, provided the diameter of the microspheres is known. We are developing a similar technique using ellipsoids, where the viscosity can be calculated without prior knowledge of the length and width of the ellipsoid. The asymmetry of ellipsoids provides a distinct advantage, allowing for the diffusion to be decomposed into two translational motions and one rotational motion. For each of these diffusive motions, we can measure a diffusion constant and relate the constant to the three unknowns: the length and width of the ellipsoid, and the viscosity. By measuring the three diffusion constants, we can determine the three unknowns. To verify this technique, we produce ellipsoids in the lab and suspend them in a viscous solution for three-dimensional imaging of the diffusion with a confocal microscope. ~We are able to get good agreement between the microrheological measurements and macroscopic viscosity measurements.   

Non-capillary binding of colloidal particles to liquid interfaces

We observe colloidal polystyrene particles binding reversibly to an oil-water interface through the combination of a repulsive electrostatic force and an attractive van der Waals force. Previously studied interactions of an aqueous colloidal particle and a liquid interface have generally fallen into two categories: 1) electrostatic repulsion indicated by the dependence on salt and 2) capillary adsorption where surface tension brings the particle in contact with both phases and is indicated by practically irreversible binding. With our technique of pushing individual colloidal particles towards a planar oil-water interface and observing their motion in three-dimensions with holographic microscopy we have observed both interactions. However, our observations indicate that under certain conditions the electrostatic repulsion, which is due to repulsive image charges, is weak enough for a particle to experience a van der Waals attraction while strong enough to prevent a particle from penetrating the interface and becoming bound through capillary action. We observe individual particles transition between repulsive and attractive interactions with the interface suggesting that these colloidal particles have a heterogeneous surface charge.   

Memory effects in soap film arrangements

We report experiments on soap film configurations in a triangular prism for which the shape factor can be changed continuously. Two stable configurations can be observed for a range of the shape factor $h$. A hysteretic behaviour is found, due to the occurence of another local minima in the free energy. Experiments demonstrate that soap films can be trapped in a particular configuration being different from a global surface minimization. This metastability can be evidenced from a geometrical model based on idealized structures. Depending on the configuration, providing clues on the structural relaxations taking place into 3D foams, such as T1 rearrangements. The composition of the liquid is also investigated leading to dynamical picture of the transition. (Phys. Rev. E 83, 021403 (2011))   

Droplet-based microfluidics and the dynamics of emulsions

Emulsions are complex fluids already involved for a long time in a wide-range of industrial processes, such as, for example, food, cosmetics or materials synthesis [1]. More recently, applications of emulsions have been extended to new fields like biotechnology or biochemistry where the compartmentalization of compounds in emulsion droplets is used to parallelise (bio-) chemical reactions [2]. Interestingly, these applications pinpoint to fundamental questions dealing with surfactant dynamics, dynamic surface tension, hydrodynamic interactions and electrohydrodynamics. Droplet-based microfluidics is a very powerful tool to quantitatively study the dynamics of emulsions at the single droplet level or even at the single interface level: well-controlled emulsions are produced and manipulated using hydrodynamics, electrical forces, optical actuation and combination of these effects. We will describe here how droplet-based microfluidics is used to extract quantitative informations on the physical-chemistry of emulsions for a better understanding and control of the dynamics of these systems [3].\\[4pt] [1] J. Bibette et al. Rep. Prog. Phys., 62, 969-1033 (1999)\\[0pt] [2] A. Theberge et al., Angewandte Chemie Int. Ed. 49, 5846 (2010)\\[0pt] [3] J.-C. Baret et al., Langmuir, 25, 6088 (2009)   

Pseudo-Steady Liquid Transport in Aqueous Foams during Filling of a Container

Various applications of aqueous foams involve filling a container or a column (e.g., fractionation), where the foam is formed and processed. However, existing studies in the literature do not treat the filling stage and only describe liquid transport within a static foam bed. We developed a theory that predicts liquid loss from the foam and the liquid distribution within its interior during the filling and post-filling stages. During the filling stage, the theory predicts that the foam reaches a pseudo-steady state characterized by a time-independent drainage rate and liquid fraction. The pseudo-steady-state liquid fraction appears above a thin, liquid-saturated boundary layer that exists at the bottom of the foam bed. During the post-filling stage, the theory predicts that the drainage rate decreases with time, similar to static foams beds studied by others. The theory compares well with our previously reported volume-averaged macroscopic model and drainage measurements for dry (high-expansion) foams. We will show that drainage during the filling stage is significant when the fill time is comparable to the intrinsic drainage time scale of the foam.   

Coarsening of Two Dimensional Foams on a Curved Surface

We report on foam coarsening and statistics of bubble distributions in a closed, two dimensional, hemispheric cell of constant curvature. Using this cell it is possible to observe individual bubbles and measure their coarsening rates. Our results are consistent with the modification to von Neumann's law predicted by Avron and Levine. We observed the relative frequencies of bubbles with a given number of sides and found a shortage of bubbles with few sides as compared to a flat two dimensional cell. We also measured the value of $m(n)$, the average number of sides of an $n$ sided bubble, and found general agreement with the Aboav-Weaire law, although there was greater deviation than for a flat cell.   

Analysis of emulsion stability in acrylic dispersions

Emulsions either micro or nano permit transport or solubilization of hydrophobic substances within a water-based phase. Different methods have been introduced at laboratory and industrial scales: mechanical stirring, high-pressure homogenization, or ultrasonics. In digital imaging, toners may be formed by aggregating a colorant with a latex polymer formed by batch or semi-continuous emulsion polymerization. Latex emulsions are prepared by making a monomer emulsion with monomer like Beta-carboxy ethyl acrylate ($\beta $-CEA) and stirring at high speed with an anionic surfactant like branched sodium dodecyl benzene sulfonates , aqueous solution until an emulsion is formed. Initiator for emulsion polymerization is 2-2'- azobis isobutyramide dehydrate with chain transfer agent are used to make the latex. If the latex emulsion is unstable, the resulting latexes produce a toner with larger particle size, broader particle size distribution with relatively higher latex sedimentation, and broader molecular weight distribution. Oswald ripening and coalescence cause droplet size to increase and can result in destabilization of emulsions. Shear thinning and elasticity of emulsions are applied to determine emulsion stability.   

Dynamics of charged particles in nonpolar solvent in response to an electric field

In nonpolar solvent, surfactant molecules aggregate to form charge-stabilizing reverse micelles. This enables surface charging of colloidal particles suspended in nonpolar solvent. We investigate the dynamics of such charged particles in response to an externally applied electric field. By combining microfluidics and confocal microscopy, we directly visualize the transport of particles between two parallel electrodes. We use direct visualization to measure the electrophoretic mobility of each particle and determine the effect of added surfactant on the measured mobility. In addition, we find that the presence of surfactant has a significant effect on the transport dynamics of the charged particles.   

Controlling Aggregation in Non-Polar Asphaltene Suspensions Through Electrostatics

Asphaltenes, the most aromatic and largest molecular weight components of petroleum fluids, can undergo a liquid-liquid phase transition in conditions including highly non-polar environments. Phase separation begins with molecular association and proceeds to and through the colloidal length-scale until complete sedimentation or deposition. Non-ionic polymeric dispersants can stabilize asphaltenes at the colloidal scale in non-polar suspensions. We perform a variety of experiments which suggest that stabilization occurs by adsorption of dispersant onto the asphaltenes, truncating the progress of precipitation. In particular, dynamic light scattering (DLS) and phase-analysis light scattering (PALS) measurements indicate that electrostatic repulsion is responsible for stabilizing asphaltene colloids against further aggregation. Aggregation time increases exponentially with dispersant concentration, as expected for particles interacting through a combination of attractive dispersion forces and repulsive electrostatics. However, contrary to current understandings of electrostatic stabilization in non-polar systems, the charges in colloidal asphaltene suspensions seem to arise from the asphaltene colloids themselves rather than from dispersant micelles.   

The Electric Double Layer Structure Around Charged Spherical Interfaces

We derive a formally simple approximate analytical solution to the Poisson-Boltzmann equation for the spherical system via a geometric mapping. Its regime of applicability in the parameter space of the spherical radius and the surface potential is determined, and its superiority over the linearized solution is demonstrated. In addition, the influence of nonuniform surface potential on the electric double layer structure is studied for large spheres in the weak potential limit.   

First experimental determination of permanent and induced electric dipolar moments of colloidal cellulose nanocrystals dispersed in apolar solvents

Scientists and industrialists show a growing interest for cellulose nanocrystals (CNCs) since these rod-like nanoparticles display excellent mechanical properties that make them perfect candidates for the design of high performance biobased composites. Furthermore, CNCs can be obtained as colloidal suspensions in apolar solvents that form chiral nematic (cholesteric) liquid crystals. Our aim is to obtain homogeneous unidimensionnal structures to enhance the optical and/or mechanical properties of CNCs-based architectures at a macroscopic scale. Using electric fields, CNCs suspensions from either cotton or tunicate were succesfully oriented in the direction of an electric field, in both AC and DC configurations. To probe the electric field induced orientation of the CNCs, a birefringence experimental set-up has been developped. While applying short electric DC field pulses, static and transient birefringence has been measured in diluted isotropic suspensions. From these measurements, we determined both the permanent and induced electric dipolar moments of the CNCs, whose effects appeared to be of the same order of magnitude. The results are discussed regarding to the CNC type and the apolar solvent used.