Session L14: Colloids IV: Colloids and Interfaces

The equilibrium colloidal crystal/colloidal liquid interface

We use confocal microscopy to study an equilibrated crystal-liquid interface in a colloidal suspension. The surface shows spatial fluctuations due to capillary waves. Local measurements of the structure and dynamics near the rough surface reveal that the intrinsic surface, while meandering in space, is locally sharply defined. Examining different quantities finds slightly different widths of this intrinsic surface. In terms of the particle diameter $d$, this width is either $1.3d$ (based on structural information) or $2.4d$ (based on dynamics), both not much larger than the particle size.   

Janus particles at the liquid-liquid interface

Dipolar Janus particles (negatively charged on one side, positively charged on the other), deposited on PDMS droplets in water, are studied in real time by fluorescence and phase contrast microscopy. Crystals form, under some conditions with long-range hexagonal order, but this self-assembled structure depends strongly on particle size and ionic strength of the water phase. Their provocative translational and rotational dynamics is studied using single-particle tracking.   

Short-time self-diffusion of nearly hard spheres at an oil-water interface

Optical microscopy and multi-particle tracking are used to study hydrodynamic interactions of monodisperse polymethylmethacrylate (PMMA) spheres at a decalin-water interface. The short-time self-diffusion coefficient measured at low surface coverage has the form, $D^S_S(n) = \alpha D_0 (1- \beta n)$, where $n$ is the area fraction occupied by the particles and $D_0$ is the Stokes-Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of $\alpha$ are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of $\beta$ differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface.   

Observing the three-dimensional motion of colloids at an oil-water interface

Our experimental system allows us to place micron-sized colloids at a flat oil-water interface.~ Using digital holographic microscopy we track the motion of particles at the interface in all three dimensions.~ Of particular interest is the out-of-plane motion of an adsorbed particle.~ I will present data of such motion and what it reveals regarding the energy and length scales of a particle attached to an interface.~ Introducing a laser tweezer and customized colloids (such as core-shell particles) into our experiment allows us to further investigate this system.~   

Self-assembled Capillary Arrows

Anisotropic particles adsorbed at a water-air interface are known to aggregate due to capillary interactions. We show that the packing configuration of a pair of prolate ellipsoids critically depends on their relative size and/or aspect ratio mismatch. While identical particles simply pack side-by-side, particles of slightly different sizes are observed to systematically self-assemble into characteristic \textit{arrows}, i.e. with a finite angle between their axes. The occurrence of such arrows cannot be explained within the far-field approximation of interacting polar quadrupoles. A numerical analysis is worked out which allows us to explore the near-field characteristics of the capillary interaction. Results clearly show the destabilization of the side-by-side configuration due to a size mismatch, in agreement with experimental observations.   

Impact of Surfactant Sorption Kinetics on Microscale Tipstreaming

A microfluidic flow focusing system has been used to synthesize submicron sized droplets via a thread formation mode of drop breakup. This process utilizes the interaction of fluid motion and surfactant transport to draw out a thin thread, which then fragments into a stream of tiny droplets whose sizes are orders of magnitude smaller than the size of the device. In this work, we use a homologous series of C$_{n}$E$_{8}$ (n = 10, 12 and 14) surfactants to probe the impact of surfactant sorption kinetics on this process. To characterize the effects of these surfactants on the thread formation process, we measure the relevant timescales for the formation of a cone-like interface, the drawing and disintegration of a fine thread, and the period with which the process repeats. We then relate these timescales to the characteristic timescales for transport of surfactants to the oil-water interface. These measurements and simple scaling analyses suggest ways to extend the thread length and optimize the overall yield of submicron droplets.   

Network Formation at the Air-Water Interface

A series of diacetylene end functionalised peptides have been designed to form beta-sheet rich monolayers at the air-water interface and their structure, rheological properties and ability to polymerize in response to UV light have been studied using a Langmuir trough and dilatational rheology. Surface pressure-area isotherms as well as compression-expansion cycles reveal all our peptide monolayers organise into the three distinct organisational states typically observed for surfactants at the air-water interface: gaseous (G), liquid expanded (LE) and liquid condensed (LC) and the limiting area per molecule suggests the alternating amphiphilic character of the peptide causes the molecule to orient with its long axes parallel to the air-water interface. The presence of the diacetylene group enhances surface activity and stability over time. Here we will discuss how peptide sequence, UV exposure strength and time, as well as peptide concentration (and hence organisation) influence the kinetics of network formation, and the morphology and mechanical properties of the final network formed.   

Kinetically Controlled Adsorption to Freshly Formed Interfaces

The coefficients of diffusion, adsorption, and desorption are fundamental properties of surfactant molecules and should be independent of the nature in which they are applied.~ However, the approaches currently used to obtain these parameters are highly context dependent and can lead to unphysical trends such as a concentration dependent diffusion coefficient and large mismatches between predicted and observed dynamic behavior. In pendant drop studies one is restricted to diffusion or mixed controlled adsorption at small concentrations, but in reality to get at the kinetic coefficients it would be more advantageous to probe the kinetic controlled regime. Recently it was shown that a characteristic length scale, R$_{D-K,}$ governs the transition from diffusion controlled adsorption to kinetically controlled adsorption for spherical interfaces. If the spherical interface has a radius smaller than R$_{D-K}$ the adsorption process is kinetically limited.~ This paper uses a micro-tensiometer to probe the adsorption dynamics to micron diameter spherical interfaces to test the transition from diffusion limited to kinetic limited adsorption. Using this method we measure kinetic adsorption constants directly. We also describe a new timescale for diffusion, which better describes the adsorption of surfactants onto spherical interfaces.   

Structures formed by colloidal particles on a droplet at small particle number

We discuss 3D imaging studies of the self-assembled structures formed by small numbers ($N\sim$ 10) of micron-sized polymethylmethacrylate (PMMA) colloids pinned to the surface of a $\sim$10 micron oil droplet in an aqueous solution. In the low $N$ limit, these structures are governed by the interactions between the constituent colloidal particles on a given droplet. We prepare these droplets using a capillary microfluidic device. Since the droplets are not density matched to the continuous phase, we study them with a time-averaged zero gravity apparatus, based on a rotary stage. Specifically, we image the 3D structures formed by the colloidal particles on the droplets using digital holographic microscopy (DHM). DHM records the 2D interference patterns, or holograms, formed by light scattered from the colloidal particles and unscattered light. Subsequent analysis of the holograms, based on the Lorenz-Mie solution for light scattering by spheres, allows us to determine the 3D particle positions with time resolution limited by the camera frame rate.   

Interfacial rheology in complex flow

Multiphase liquid systems are omnipresent in and essential to everyday life, e.g. foods, pharmaceutics, cosmetics, paints, oil recovery, etc. The morphology and stability of such systems depend on dynamic interfacial properties and processes. Typical methods utilized to measure such interfacial properties often employ drops that are much larger and flows that are much simpler than those encountered in typical processing applications. A microfluidic approach is utilized to measure dynamic structure and kinetics in multiphase systems with drop sizes comparable to those encountered in applications and flow complexity that is easily adjustable. The internal circulation and deformation of an aqueous droplet in clear mineral oil is measured using particle tracers and a detailed shape analysis, which is capable of measuring sub-micron deviations in drop shape. Deformation dynamics, detailed drop shape, interfacial tension, and internal circulation patterns and velocities are measured in Poiseuille and transient elongational flows. Flow kinematics are adjusted by varying the microchannel geometry, relative drop size, and drop height. The effects of confinement on interfacial dynamics and circulation patterns and velocities are also explored.   

Reverse coffee-ring effect

When a coffee drop dries on a solid surface, it commonly leaves a ring-like deposit along the edge, known as the coffee-ring effect. We present a reverse motion of particles in drying droplets, opposite to the coffee-ring effect. We reveal that the particle motion, initially toward the edge by the typical coffee-ring effect, is reversed to the droplet center owing to the capillary interaction generated by the droplet surface. The reverse coffee-ring effect always occurs whenever the capillary interaction prevails over the net outward force by the coffee- ring effect. The interaction predicts an inverse power-law time growth of moving distance from the edge, depending mostly on particle size and contact angle. The reverse coffee-ring effect may contribute to multiple ring formation by sweeping particles toward the center. We prove the mechanism with real-time optical, confocal, and X-ray microscopic observations of colloidal fluids.   

Measurements of contact forces at the bottom of a droplet pile

We measure the contact forces at the bottom of a container of frictionless liquid droplets as a function of compression and of distance to the container wall. Glass cylinders are used to contain 20-micron-radius droplets of silicon oil; Brownian motion is not significant for this size. Reflection interference contrast microscopy is used since we are particularly interested in contacts with the bottom surface. By looking at the Newton's Ring interference pattern, we measure the deformation of each droplet, which arises from gravity and pressure from the whole pile transmitted through droplet contacts. We also measure the radius of each droplet and thereby obtain the vertical contact force. We vary the pile height to change the compressive stress and then measure the corresponding forces, probability distributions, and correlations of rearrangements. The results elucidate the roles that friction and confining walls play in granular systems and also shed light on force chains in bulk of the material.   

Sticking colloids to liquid-liquid interfaces one by one

We investigate the dynamics of placing individual colloidal particles ($\sim $2 microns) onto a flat oil-water interface using optical tweezers. By monitoring the strength and position of the trap, we are able to measure the forces acting on a particle as it encounters the liquid-liquid interface. Digital holographic microscopy affords us three dimensional position information at high frame rates ($>$ 500fps), allowing us to probe short timescale behavior. We vary parameters such as particle surface chemistry, dissolved ion concentration, and pH in order to pursue questions about the nature of interface penetration dynamics.   

Controlled Crystal Growth and Solid-Liquid Interface in temperature-sensitive colloidal systems

We use temperature-sensitive colloidal NIPA systems to study crystal growth at the ``atomic scale''. By applying a temperature gradient we are able to control the growth of large colloidal single crystals. We visualize the nucleation of these crystals and solidification at the crystal-liquid interface in three dimensions by using confocal microscopy. Trajectories of particles on both the crystal and liquid side of an advancing interface are determined. These elucidate the mechanism of particle assembly at the interface of a growing crystal. At later stages of crystal growth, the interface becomes stationary, and we use the fluctuations of the stationary interface to determine the interface stiffness. Our data suggests a strong anisotropy of the interface tension. These microscopic observations provide unique insight into the mechanism of solidification.   

A Surface Plasmon Resonance Investigation of How Water Meets a Hydrophobic Surface.

By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low- density region forms near the surface. We have employed the surface-sensitive, quantum-optical technique of Surface Plasmon Resonance (SPR) to verify the existence of this region at the boundary.