Bum Jun Park

Direct Measurements of the Effects of Salt and Surfactant on Interaction Forces between Colloidal Particles at Water−Oil Interfaces

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable. Averaging over several particle pairs was hence found to be necessary to obtain a reliable assessment of the effects of salt and surfactant. It has previously been suggested that the repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side also contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over longer times. Finally, measurements of three-body interactions provide insight into the anisotropic nature of the interactions. The unique time-dependent and anisotropic interactions between particles at the oil-water interface allow tailoring of the aggregation kinetics and structure of the suspension structure.

Hydrochromic conjugated polymers for human sweat pore mapping

Hydrochromic materials have been actively investigated in the context of humidity sensing and measuring water contents in organic solvents. Here we report a sensor system that undergoes a brilliant blue-to-red colour transition as well as ‘Turn-On’ fluorescence upon exposure to water. Introduction of a hygroscopic element into a supramolecularly assembled polydiacetylene results in a hydrochromic conjugated polymer that is rapidly responsive (<20 μs), spin-coatable and inkjet-compatible. Importantly, the hydrochromic sensor is found to be suitable for mapping human sweat pores. The exceedingly small quantities (sub-nanolitre) of water secreted from sweat pores are sufficient to promote an instantaneous colorimetric transition of the polymer. As a result, the sensor can be used to construct a precise map of active sweat pores on fingertips. The sensor technology, developed in this study, has the potential of serving as new method for fingerprint analysis and for the clinical diagnosis of malfunctioning sweat pores.

Equilibrium Orientation of Nonspherical Janus Particles at Fluid–Fluid Interfaces

We study the equilibrium orientation of nonspherical Janus particles at an oil-water interface. Two types of nonspherical Janus particles are considered: Janus ellipsoids and Janus dumbbells. To find their equilibrium orientation, we calculate and minimize the attachment energy of each Janus particle as a function of its orientation angle with respect to the oil-water interface. We find that the equilibrium orientation of the interface trapped Janus particles strongly depends on the particle characteristics, such as their size, aspect ratio, and surface properties. In general, nonspherical Janus particles adopt the upright orientation (i.e., the long axis of ellipsoids or dumbbells is perpendicular to the interface) if the difference in the wettability of the two sides is large or if the particle aspect ratio is close to 1. In contrast, Janus particles with a large aspect ratio or a small difference in the wettability of the two regions tend to have a tilted orientation at equilibrium. Moreover, we find that Janus ellipsoids, under appropriate conditions, can be kinetically trapped in a metastable state due to the presence of a secondary energy minimum. In contrast, Janus dumbbells possess only a primary energy minimum, indicating that these particles prefer to be in a single orientation. The absence of a secondary minimum is potentially advantageous for obtaining particle layers at fluid-fluid interfaces with uniform orientation. Our calculation provides a detailed guidance for synthesizing nonspherical Janus particles that can be used as effective solid surfactants for the stabilization of multiphasic fluid mixtures and the modification of the rheological properties of fluid interfaces.

Amphiphilic Janus particles at fluid interfaces

Janus particles are colloids that have both hydrophilic and hydrophobic faces. Recent advances in particle synthesis enable the generation of geometrically and chemically anisotropic Janus particles with high uniformity and precision. These amphiphilic particles are similar to molecular surfactants in many aspects; they self-assemble in bulk media and also readily attach to fluid interfaces. These particles, just like molecular surfactants, could potentially function as effective stabilizers for various multiphasic systems such as emulsions and foams. In particular, just as the shape and chemical composition have a significant impact on the surfactancy of molecular amphiphiles, the ability to control the shape and wetting properties of Janus particles could provide a unique opportunity to control their surface activity. In this review, we first examine the recent developments in using amphiphilic Janus particles as colloid surfactants to stabilize multiphasic mixtures such as emulsions. These results have motivated a number of detailed investigations aimed at understanding the behaviour of Janus particles at fluid–fluid interfaces at the microscopic level, which we highlight. This review also discusses the importance of controlling the shape of Janus particles, which has a drastic impact on their behaviour at fluid interfaces. We conclude this review by presenting outlook on the future directions and outstanding problems that warrant further study to fully enable the utilization of Janus particles as colloid surfactants in practical applications.

Janus particles at an oil–water interface

We study the behaviour of Janus particles at an oil–water interface. Amphiphilic Janus particles exhibit attractive interactions at the fluid–fluid interface. The attractive interactions are likely due to the pinning of contact line around the diffuse boundary between the two hemispheres. The undulation of the three-phase contact line around Janus particles leads to quadrupolar capillary interactions, which we confirm by measuring the interparticle forces. We also show that Janus particles with two negatively charged-hydrophilic hemispheres show repulsive interactions.

Attractive interactions between colloids at the oil–water interface

The effects of salts and surfactants on the interaction force between colloidal polystyrene latex particles confined to a decane–water interface are measured directly using optical tweezers. After adding 0.25 M NaCl, 0.25 M NaCl and 0.1 mM sodium dodecyl sulfate (SDS) to the aqueous sub-phase, or 25 μM sorbitan monooleate (SPAN 80) to the decane super-phase, the strong repulsive force between particles is reduced and an attractive force becomes significant. The magnitude and dependence of the attraction on particle separation is consistent with a capillary quadrupole interaction. Similar interaction forces between polystyrene latex doublet particles at a pristine interface are measured, however, the anisotropic particles exhibit only a long-range attraction that is approximately two orders of magnitude stronger than spherical colloids. These results confirm the presence of long-range capillary attractions and provide a guide for manipulating colloidal interactions with additives or particle shape at fluid interfaces to control suspension structure and surface rheology.

Thermodynamically stable emulsions using Janus dumbbells as colloid surfactants.

One of the most important properties of emulsions is their stability. Most emulsions stabilized with molecular surfactants tend to lose their stability over time via different mechanisms. Although the stability of emulsions stabilized with homogeneous particles have been shown to be superior to that of surfactant-stabilized emulsions, these Pickering emulsions nevertheless are only kinetically stable and thus can undergo destabilization. Janus particles that have two opposite wetting surfaces have shown promise in imparting emulsions with long-term stability because of their strong attachment to the oil-water interface. In this theoretical study, we consider thermodynamics of emulsion stabilization using amphiphilic Janus dumbbells, which are nonspherical particles made of two partially fused spherical particles of opposite wettability. These amphiphilic dumbbells are attractive candidates as colloid surfactants for emulsion stabilization because highly uniform Janus dumbbells can be synthesized in large quantities; thus, their application in emulsion stabilization can become practical. Our theoretical calculation demonstrates that Janus dumbbells can indeed generate thermodynamically stable Pickering emulsions. In addition, we also find that there exists a total oil-water interfacial area that results in the lowest energy state in the system, which occurs when Janus dumbbells available in the system are completely consumed to fully cover the droplet interfaces. We show that the geometry of dumbbells as well as the composition of the emulsion mixtures has significant influences on the average size of dumbbell-stabilized emulsions. We also investigate the effect of asymmetry of Janus dumbbells on the average droplet radius. Our results clearly show that amphiphilic Janus dumbbells provide unique opportunities in stabilizing emulsions for various applications.

Heterogeneity of the electrostatic repulsion between colloids at the oil–water interface

The pairwise and multi-body interaction forces between polystyrene particles at an oil–water interface are measured. The electrostatic repulsive force has the expected dependence on particle separation for a dipole–dipole interaction, Frep ∼ r−4, but exhibits a distribution of magnitudes in which the force depends on the particle pairs tested and sample preparation method. A gamma distribution accurately models this variation in the repulsion between pairs of particles. Despite this heterogeneity, the multibody interactions measured in small ensembles are pairwise additive. Good agreement is found for the two-dimensional equilibrium suspension structure between experiments and Monte Carlo simulations when a heterogeneous interaction potential is implemented in the latter. The heterogeneity and long-range of the repulsive interaction accounts for the lower apparent pair interaction potential derived from the suspension radial distribution function at dilute, but finite, surface concentrations when compared to the direct pair interaction measurements made with laser tweezers at nearly infinite dilution.

Configuration of nonspherical amphiphilic particles at a fluid–fluid interface

We present the equilibrium configuration of amphiphilic ellipsoids and amphiphilic dumbbells with asymmetric shape (i.e., unequal surface areas for apolar and polar sides) and surface wetting properties at an oil–water interface. The equilibrium configurations are obtained by minimizing the attachment energy of each amphiphilic particle as a function of orientation angle and vertical displacement of the particle with respect to the interface. We find that the orientation and vertical displacement of nonspherical amphiphilic particles are significantly influenced by their shape, aspect ratio, surface properties, and the location of the wettability separation line. In particular, due to the asymmetry in the particle geometry and wettability of these amphiphilic particles, we observe unique configurations that are not expected in symmetric Janus ellipsoids and dumbbells at fluid–fluid interfaces. In the case of amphiphilic ellipsoids, their orientation as a function of particle geometry and wettability can be divided into three regimes: upright orientation, tilted orientation, and coexistence of upright and tilted orientations due to the presence of secondary energy minima under appropriate conditions. In general, the secondary energy minimum is present when the aspect ratio of the ellipsoids becomes high. As for amphiphilic dumbbells, in addition to upright and tilted orientations, they can adopt intermediate orientations, especially when the size of the two spheres is significantly different, which leads to the detachment of one of the two spheres from the oil–water interface. This study provides guidelines for designing nonspherical amphiphilic particles with suitable geometry and wettability to tailor their properties as solid surfactants for emulsion stabilization and fluid interface modification.

Fabrication of unusual asymmetric colloids at an oil-water interface.

We present a novel method for creating asymmetrical particles with unusual, flattened shapes from colloidal latex microspheres pinned at an oil-water interface. The shape and degree of asymmetry are controlled by incubating particles for minutes to tens of minutes at an elevated temperature. Estimates of the surface energy and work account for the shape-change mechanism in which heated particles deform as they spread at the oil-water interface to minimize the contact between these immiscible phases.