Session R42: Colloids and Interfaces

Curvature-Induced Potential for Colloidal Particles at an Oil-Water Interface

At the micrometer scale, surface tension plays a predominant role in the interactions that occur at fluid interfaces. For example, when a spherical colloidal particle is adsorbed onto a curved oil-water interface, the surface must deform in order to satisfy the requirement of constant contact angle. The energy cost of the deformation depends on the local curvature of the interface, and so a particle sitting on an interface of varying curvature will experience a potential which depends on the particle's position on the interface. We present results from an experiment in which a capillary bridge droplet creates an interface of varying Gaussian curvature. The shape of this interface is obtained by using confocal microscopy. One or more spherical microparticles are then introduced to the interface. We demonstrate that a curvature-induced potential exists for a single wetting particle, which attracts the particle to the most highly curved regions. By tracking the motion of the particle in 3D, we are able to calculate the forces acting on the particle. We can then compare these forces to theoretical and numerical predictions based on the shape of the interface.   

Self-pinning by colloids confined at a contact line

Colloidal particles suspended in a fluid usually inhibit complete wetting of the fluid on a solid surface and cause pinning of the contact line, known as self-pinning. We show differences in spreading and drying behaviors of pure and colloidal droplets using optical and confocal imaging methods. These differences come from spreading inhibition by colloids confined at a contact line. We propose a self-pinning mechanism based on spreading inhibition by colloids. We find a good agreement between the mechanism and the experimental result taken by directly tracking individual colloids near the contact lines of evaporating colloidal droplets.   

Holographic Imaging of Interfacial Mobility at Emulsion Interfaces

The difficulty of achieving nm resolution in the vertical direction has limited prior studies of nanoparticle mobility at the oil-water interface. This can be overcome by techniques of holographic imaging, implemented in this study and applied here to this problem. We have studied both homogeneous and Janus particles with emphasis on what determines the dynamics of surface pinning and desorption. Surprising dependence is found on conditions which govern kinetic depinning and the time scale for desorption.   

The Effect of Size, Morphology and Composition on Second Harmonic Light Scattering from Colloidal Particles

Second harmonic light scattering (SHS) is a coherent second-order optical technique that is specifically surface sensitive and can be performed in-situ [1]. It has also been recently shown to be sensitive to size, shape and composition of metallic (Ag) and dielectric (polystyrene) nano and microparticles with or without adsorbed molecular monolayers. An understanding of how the size, shape, composition, structure, charge and surface chemistry influence the nonlinear optical properties makes SHS a versatile in-situ probe of nano- and/or micro-particle whose importance span from plasmonics to biomedicine [2].\\[4pt] [1] S. Roke and G. Gonella. Annu. Rev. Phys. Chem. 2012, 63, 353.\\[0pt] [2] (a) S.-H. Jen, G. Gonella, H.-L. Dai. J. Phys. Chem. A 2009, 113, 4758; (b) S.-H. Jen, H.-L. Dai, G. Gonella. J. Phys. Chem. C 2010, 114, 4302; (c) W. Gan, G. Gonella, M. Zhang, H.-L. Dai. Phys. Rev. B 2011, 84, 121402; (d) G. Gonella, H.-L. Dai. Phys. Rev. B 2011, 84, 121402; (e) G.Gonella, W. Gan, B. Xu, H.-L. Dai. J. Phys. Chem. Lett. 2012, 3, 2877.   

Using DNA-directed crystals to template colloidal clusters

DNA is a versatile tool for directing the controlled self-assembly of nanoscopic and microscopic objects. We demonstrate a new, scalable method for producing highly ordered clusters of sub-micron colloidal microspheres at high yield. The basic idea is first to form a binary AB-type crystal using DNA-directed assembly, where a small fraction of the A species, A', contains a unique DNA sequence not present on the other A species. If the DNA domain of the A' and A particles that drive their interaction with the B species are identical, then the A' co-crystallize stiochiometrically as an 'impurity' into a well ordered AB lattice. Once formed, a soluble DNA strand is added to the crystals which binds the unique A' sequence and selectively stabilizes the A'-B bonds. When the crystals are then melted by heating, every A' particle yields a cluster surrounded by its nearest B neighbors. We will discuss the different clusters we have formed using this approach, as well as limits to yield and ordering in the clusters.   

Particle-Size Dependency of Single Molecule Properties in Surface-Tethered Particle Systems by Monte Carlo Simulation

We consider the behavior of a surface-tethered particle system, comprising a single colloid particle tethered to a flat surface by a single polymer chain. This study is relevant to the interpretation of tethered particle motion experiments, wherein the motion and position of the tethered particle are used as reporters on the conformational properties of the underlying polymer molecule. The dependency of the polymer dimensions on the relative size of the tethered particle at equilibrium is obtained by Monte Carlo simulations with both random walk and self-avoiding walk polymer models. Two local maxima are found in the expansion factors of the polymer tether as a function of particle size, with both models. Comparison of these two models shows that the particle-size effects are separable from expansion by self-excluded volume of the polymer. Furthermore, the non-monotonic behavior persists to very large particle sizes before the expected asymptotic gparallel plate h limit is reached. The maxima are revealed to be due to the rotational entropy of the junction between the polymer and particle.   

Liquid-vapor interface in two-dimensional colloid-polymer systems

The phase diagrams of two-dimensional aqueous colloid-polymer systems are determined experimentally. Mixtures of fluorescent polystyrene spheres and polyacrylamide are confined between a glass slide and a coverslip to construct a two-dimensional system. Liquid--vapor phase coexistence between a colloid-rich phase (colloid liquid) and a polymer-rich phase (colloid vapor) occurs at intermediate polymer concentrations, while vapor--solid phase coexistence between a polymer-rich liquid and a colloid-rich solid is observed at high polymer concentrations. For the interface between the coexisting liquid-vapor phases, the interfacial thickness and tension are measured using image analysis and Fourier analysis of the capillary waves. Close to the critical point, the fluctuations of the inteface become large and can no longer be decomposed into waves. It is also observed that the colloid-rich solid and liquid domains coarsen mainly by Ostwald ripening in a short time and long time regime.   

Tunable Soft Structure in Charged Fluids confined by Dielectric Interfaces

We study the deformation of the local structure in an electrolytic background by micro- and nanoscopic polarizable surfaces, and vice versa, the emergence of induced forces between two surfaces due to the cohesive properties of the background. The range and strength of these forces depend sensitively on the material properties of the charged fluid, and can be varied over decades, offering high tunability and, aided by accurate theory, control in experiments and applications. The attention is directed towards the electrolyte-induced forces between neutral boundaries, to distinguish correlational effects from simple ionic screening. The interplay of thermal motion, short range repulsions, and electrostatic forces is responsible for a typical ordered fluid state, a soft structure, that changes near polarizable interfaces and causes diverse attractions between fluctuation-confining walls that seem well exploited by microbiological systems. We use liquid state theory and classical density functional theory to accurately calculate these interactions and nuance the understanding of double-layer forces, relevant for colloid and emulsion stability, phase-transfer catalysis, and (interface-directed) self-assembly of nanomaterials.   

Interfacial free energy calculation of a binary hard-sphere fluid at a hard wall by Gibbs-Cahn Integration

The interfacial free energy, $\gamma$, of fluids at surfaces is a parameter that is central to a number of technologically important phenomena, such as wetting, nucleation and the stability and self assembly of colloidal particles in solution. In recent years, our group has developed techniques to determine $\gamma$ from atomistic simulation. In this work, we apply one of these methods, Gibbs-Cahn Integration, to determine $\gamma$ for a model two-component (binary) mixture of hard spheres. Molecular dynamics simulation is used to characterize a hard-sphere fluid mixture in a slit-pore confined geometry as packing fraction, mole fraction, and diameter ratio are varied. We find that recent theoretical predictions from the White Bear II classical density functional theory [Roth et al., J. Phys.: Condens. Matter, {\em 18}, 8413, (2006)] agree well with our computational results We also observe that, for this model system, the preferential adsorption of one particle species over the other contributes negligibly to the interfacial free energy at modest diameter ratios.   

Understanding the correlation function of the inhomogeneous hard-sphere fluid

We present a new functional for the correlation $g(r)$ at contact for an inhomogeneous distribution of hard spheres. This term is a key input into classical density functional theories developed using Statistical Associating Fluid Theory, a widely used approach for handling complex liquids in which hydrogen bonding plays an important role. We use a thermodynamic approach to derive an exact formula for the correlation expressed as a derivative of the free energy functional. We evaluate this approach using the approximate free energy of the ``White Bear'' version of Fundamental Measure Theory, and test our approach (and two previously published approximations) against correlation values found from Monte Carlo simulations.   

Testing ``Soft'' Fundamental Measure Theory for not-so-hard-sphere fluids

A standard approach for modelling in-homogeneous distributions of the hard-sphere fluid is the Fundamental Measure Theory (FMT) formulation of classical density functional theory. Due to the paucity of truly hard spheres in the real world, it seems advisable to consider interactions that are not quite so ``hard,'' a challenge tackled by the ``Soft'' FMT (SFMT) introduced by Schmidt [1]. We apply SFMT to a simple potential describing slightly penetrable spheres, that is, spheres at moderate temperatures---such that the spheres are thermally able to penetrate by a short distance. We compare the predicted equation of state with the result of Monte Carlo simulations, and also compare the free energy and density distribution near a hard wall with simulation.\\[4pt] [1] Schmidt, M. Phys. Rev. E 62(4), 4976 (2000)   

Nonlinear functional for solvation in Density Functional Theory

Density functional calculations of molecules and surfaces in a liquid can accelerate the development of many technologies ranging from solar energy harvesting to lithium batteries. Such studies require the development of robust functionals describing the liquid. Polarizable continuum models (PCM's) have been applied to some solvated systems; but they do not sufficiently capture solvation effects to describe highly polar systems like surfaces of ionic solids. In this work, we present a nonlinear fluid functional within the framework of Joint Density Functional Theory. The fluid is treated not as a linear dielectric, but as a distribution of dipoles that responds to the solute, which we describe starting from the exact free energy functional for point dipoles. We also show PCM's can be recovered as the linear limit of our functional. Our description is of similar computational cost to PCM's, and captures complex solvation effects like dielectric saturation without requiring new fit parameters. For polar and nonpolar molecules, it achieves millihartree level agreement with experimental solvation energies. Furthermore, our functional now makes it possible to investigate chemistry on the surface of lithium battery materials, which PCM's predict to be unstable.