Shape-Induced Deformation, Capillary Bridging, and Self-Assembly of Cuboids at the Fluid–Fluid Interface

The controlled assembly of anisotropic particles through shape-induced interface deformations is shown to be a potential route for the fabrication of novel functional materials. In this article, the shape-induced interface deformation, capillary bridging, and directed self-assembly of cuboidal-shaped hematite particles at fluid–fluid interfaces are reported. The multipolar nature of the interface distortions is directly visualized using high-resolution scanning electron microscopy and 3D optical surface profiling. The nature of the interface deformations around cuboidal particles vary from monopolar to octupolar types depending on their orientation and position with respect to the interface. The deformations are of either hexapolar or octupolar type in the face-up orientation, quadrupolar or monopolar type in the edge-up orientation, and monopolar type in the vertex-up orientation. The particles adsorbed at the interface interact through the interface deformations, forming capillary bridges that lead to isolated assemblies of two or more particles. The arrangement of particles in any assembly is such that the condition for capillary attraction is satisfied, that is, in accordance with predictions based on the nature of interface deformations. At sufficient particle concentrations, these isolated structures interact to form a percolating network of cuboids. Furthermore, the difference in the nature of the assembly structures formed at the air–water interface and in the bulk water phase indicates that the interfacial assembly of these particles is controlled by the capillary interactions

Control over Coffee-Ring Formation in Evaporating Liquid Drops Containing Ellipsoids

A control over the nature of deposit pattern obtained after the evaporation of solvent from a sessile drop containing dispersed materials has been demonstrated to have applications in materials engineering, separation technology, printing technology, manufacture of printed circuit boards, biology, and agriculture. In this article, we report an experimental investigation of the effect of particle shape and DLVO (Derjaguin–Landau–Verwey–Overbeek) interactions on evaporation-driven pattern formation in sessile drops. The use of a model system containing monodisperse particles where particle aspect ratio and surface charge can be adjusted reveals that a control over the nature of deposit pattern can be achieved by tuning the particle–particle and particle–substrate interactions. A clear coffee-ring formation is observed when the strength of particle–particle repulsion is higher than the particle–substrate attraction. However, complete suppression of ringlike deposits leading to a uniform film is achieved when particle–substrate and particle–particle interactions are attractive. Results illustrate that for the system of submicron ellipsoids that are hydrophilic, the nature of deposit patterns obtained after evaporation depends on the nature of interactions and not on particle shape

Emulsions Stabilized by Silica Rods via Arrested Demixing

A binary liquid–liquid mixture with a lower critical solution temperature (LCST) when heated above a critical temperature undergoes demixing. During the initial phase of demixing process, high-energy liquid–liquid interfaces are created before both liquids eventually phase separate. By incorporating well-characterized colloidal silica rods in a homogeneous one-phase liquid–liquid mixture of lutidine/water (L/W) before inducing phase separation, we show that colloidal rod stabilized Pickering emulsions can be obtained. We show that the droplet size of Pickering emulsions can be tuned by varying particle concentration, and the droplet size distribution follows the prediction of the limited coalescence model

Evaporation of Sessile Drops Containing Colloidal Rods: Coffee-Ring and Order–Disorder Transition

Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along the three-phase contact line–a phenomena popularly known as the coffee-ring or the coffee stain effect. The formation of such rings as well as their suppression is shown to have applications in particle separation and disease diagnostics. We present an experimental study of the evaporation of sessile drops containing silica rods to elucidate the structural arrangement of particles in the ring, an effect of the addition of surfactant and salt. To this end, the evaporation of aqueous sessile drops containing model rod-like silica particles of aspect ratio ranging from ∼4 to 15 on a glass slide is studied. We first show that when the conditions such as (1) solvent evaporation, (2) nonzero contact angle, (3) contact line pinning, (4) no surface tension gradient driven flow, and (5) repulsive particle–particle/particle–substrate interactions, that are necessary for the formation of the coffee-ring are met, the suspension drops containing silica rods upon evaporation leave a ring-like deposit. A closer examination of the ring deposits reveals that several layers of silica rods close to the edge of the drop are ordered such that the major axis of the rods are oriented parallel to the contact line. After the first few layers of ordered arrangement of particles, a random arrangement of particles in the drop interior is observed indicating an order–disorder transition in the ring. We monitor the evolution of the ring width and particle velocity during evaporation to elucidate the mechanism of the order–disorder transition. Moreover, when the evaporation rate is lowered, the ordering of silica rods is observed to extend over large areas. We demonstrate that the nature of the deposit can be tuned by the addition of a small quantity of surfactant or salt

A Model for the Prediction of Droplet Size in Pickering Emulsions Stabilized by Oppositely Charged Particles

Colloidal particles irreversibly adsorb at fluid–fluid interfaces stabilizing what are commonly called “Pickering” emulsions and foams. A simple geometrical model, the limited coalescence model, was earlier proposed to estimate droplet sizes in emulsions. This model assumes that all of the particles are effective in stabilization. The model predicts that the average emulsion drop size scales inversely with the total number of particles, confirmed qualitatively with experimental data on Pickering emulsions. In recent years, there has been an increasing interest in synthesizing emulsions with oppositely charged particles (OCPs). In our experimental study, we observed that the drop size varies nonmonotonically with the number ratio of oppositely charged colloids, even when a fixed total number concentration of colloids is used, showing a minimum. We develop a mathematical model to predict this dependence of drop size on number ratio in such a mixed particle system. The proposed model is based on the hypothesis that oppositely charged colloids form stable clusters due to the strong electrostatic attraction between them and that these clusters are the effective stabilizing agents. The proposed model is a two-parameter model, parameters being the ratio of effective charge of OCPs (denoted as <i>k</i>) and the size of the aggregate containing <i>X</i> particles formed due to aggregation of OCPs. Because the size of aggregates formed during emulsification is not directly measurable, we use suitable values of parameters <i>k</i> and <i>X</i> to best match the experimental observations. The model predictions are in qualitative agreement with experimentally observed nonmonotonic variation of droplet sizes. Using experiments and theory, we present a physical insight into the formation of OCP stabilized Pickering emulsions. Our model upgrades the existing Wiley’s limited coalescence model as applied to emulsions containing a binary mixture of oppositely charged particles

Tailoring Pickering Double Emulsions by in Situ Particle Surface Modification

Fundamental studies on the formation and stability of Pickering double emulsions are crucial for their industrial applications. Available methods of double emulsion preparation involve multiple tedious steps and can formulate a particular type of double emulsion, that is, water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). In this work, we proposed a simple single-step in situ surface modification method to stabilize different types of double emulsions using hematite and silica particle systems which involves the addition of oleic acid. In the emulsification studies, we use (i) a combination of hematite and oleic acid, which is termed the binary system, and (ii) a mixture of hematite and silica particles together with oleic acid, which is designated as the ternary system. The wettability of hematite particles is tuned by direct or sequential addition of oleic acid to the water–decane medium. The direct surface modification (which involves the addition of a known quantity of oleic acid to the oil–water mixtures at once) of hematite particles in both binary and ternary systems shows transitional phase inversion from oil-in-water (o/w) to water-in-oil (w/o) emulsions. However, sequential surface modification results in the transition of a single emulsion to double emulsions. In the case of the binary system, the sequential surface modification of the hematite-particle-stabilized o/w emulsion can be converted into double emulsions of o/w/o type. However, in the case of the ternary system, i.e., in the presence of silica particles, sequential surface modification of hematite particles stabilizes both single (o/w) and double (w/o/w and o/w/o) emulsions. The critical concentration of oleic acid required to form a double emulsion is observed to be dependent on the ratio of the surface area of the silica particle to the total surface area of particles (S) and mixing protocols. A study of the size distribution of oil and water droplets of double emulsions shows that droplet size can be controlled by oleic acid concentration and magnitude of S. The arrangements of the particles at interfaces are visualized by SEM imaging. In this way, we developed an easy and novel single-step method of double emulsion preparation and provide a strategy to tailor the formation of different types of emulsions with a single/binary particle system by sequential in situ surface modification of the particles

Aggregation and Stabilization of Colloidal Spheroids by Oppositely Charged Spherical Nanoparticles

Heteroaggregation of colloids is an important yet complex physical process involving colloidal/nanosized particles and is relevant in river delta formation, paper-making, water treatment, blood flocculation, and so on. Despite the earlier studies on oppositely charged spherical colloids, heteroaggregation of colloids of different shapes is less explored. In this regard, we report an experimental study to investigate the colloidal stability of mixture of positively charged spheroidal hematite and negatively charged spherical silica nanoparticles. In this study, pH and surface area ratio (silica to hematite, <i>S</i>_S–H) are varied to tune the colloidal stability/instability of the suspension. At pH 6.5 and low <i>S</i>_S–H, the silica particles adsorb onto the hematite particles and reduce the effective charge of the latter, leading to aggregation and resulting in unstable dispersions. At higher S_S–H, adsorption of silica on hematite leads to overcharging and charge reversal, which leads to a stable dispersion. Similar experiments were performed at pH 2.4 and 3.5, and the crossover from unstable to stable dispersion is observed as a function of <i>S</i>_S–H. Calculation of Derjaguin, Landau, Verwey, and Overbeek (DLVO) interaction between particles in the binary mixture, as a function of pH and <i>S</i>_S–H, based on the aggregate size and zeta potential, explains the transition from unstable to stable dispersion. The size and zeta potential of heteroaggregates in the dispersion were analyzed by dynamic light scattering (DLS) technique. Adsorption of silica nanoparticles on hematite particles was visualized by scanning electron microscopy (SEM). The study provides a framework based on DLVO interactions to stabilize or destabilize a colloidal dispersion of nonspherical particles by controlled addition of oppositely charged spherical colloids, which is a feat that is not possible with simple salt. The stability ratio (<i>W</i>) calculated from DLVO interactions demark the unstable–stable dispersion regions, which is found to be in agreement with the experimental results

Tailoring Pickering Double Emulsions by in Situ Particle Surface Modification

Fundamental studies on the formation and stability of Pickering double emulsions are crucial for their industrial applications. Available methods of double emulsion preparation involve multiple tedious steps and can formulate a particular type of double emulsion, that is, water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). In this work, we proposed a simple single-step in situ surface modification method to stabilize different types of double emulsions using hematite and silica particle systems which involves the addition of oleic acid. In the emulsification studies, we use (i) a combination of hematite and oleic acid, which is termed the binary system, and (ii) a mixture of hematite and silica particles together with oleic acid, which is designated as the ternary system. The wettability of hematite particles is tuned by direct or sequential addition of oleic acid to the water–decane medium. The direct surface modification (which involves the addition of a known quantity of oleic acid to the oil–water mixtures at once) of hematite particles in both binary and ternary systems shows transitional phase inversion from oil-in-water (o/w) to water-in-oil (w/o) emulsions. However, sequential surface modification results in the transition of a single emulsion to double emulsions. In the case of the binary system, the sequential surface modification of the hematite-particle-stabilized o/w emulsion can be converted into double emulsions of o/w/o type. However, in the case of the ternary system, i.e., in the presence of silica particles, sequential surface modification of hematite particles stabilizes both single (o/w) and double (w/o/w and o/w/o) emulsions. The critical concentration of oleic acid required to form a double emulsion is observed to be dependent on the ratio of the surface area of the silica particle to the total surface area of particles (S) and mixing protocols. A study of the size distribution of oil and water droplets of double emulsions shows that droplet size can be controlled by oleic acid concentration and magnitude of S. The arrangements of the particles at interfaces are visualized by SEM imaging. In this way, we developed an easy and novel single-step method of double emulsion preparation and provide a strategy to tailor the formation of different types of emulsions with a single/binary particle system by sequential in situ surface modification of the particles

Synthesis of Single and Multipatch Particles by Dip-Coating Method and Self-Assembly Thereof

We report a simple strategy to produce single and multipatch particles via the conventional dip-coating process. In this method, a close-packed monolayer of micron-sized silica particles is first formed at air–polymer solution interface, followed by dip coating of particles on a glass substrate. The simultaneous deposition of both polymer and particles on the substrate gives rise to a thin polymer layer and a monolayer of silica particles. Sonication of the substrate leads to the formation of a polymeric patch on one side of the particles. The patch shape depends on the aging of the polymer film prior to sonication. With aging time the patch evolves from ring-like to disk-like. This technique allows easy control of patch width by varying the concentration of polymer in the solution. We further show that the number of patches on the particle can be increased by controlling the concentration of silica particles at the interface such that surface coverage is less than that required for the formation of a close-packed monolayer. The single and multipatch particles are characterized by scanning electron and optical microscopy for the patch size, shape, and number distribution. The as-synthesized particles are used as a model to study self-assembly of colloids with electrostatic repulsion and patchy hydrophobic attractions due to polymeric patches. We find the formation of doublets and finite-sized clusters due to patchy interactions. Dip coating can be automated to produce large quantities of patchy particles, which is one of the major limitations of other methods of producing patchy particles

Spontaneous Thermoreversible Formation of Cationic Vesicles in a Protic Ionic Liquid

The search for stable vesicular structures is a long-standing topic of research because of the usefulness of these structures and the scarcity of surfactant systems that spontaneously form vesicles in true thermodynamic equilibrium. We report the first experimental evidence of spontaneous formation of vesicles for a pure cationic double tail surfactant (didodecyldimethylammonium bromide, DDAB) in a protic ionic liquid (ethylammonium nitrate, EAN). Using small and ultra-small angle neutron scattering, rheology and bright field microscopy, we identify the coexistence of two vesicle containing phases in compositions ranging from 2 to 68 wt %. A low density highly viscous solution containing giant vesicles (<i>D</i> ∼ 30 μm) and a sponge (L₃) phase coexists with a dilute high density phase containing large vesicles (<i>D</i> ∼ 2.5 μm). Vesicles form spontaneously via different thermodynamic routes, with the same size distribution, which strongly supports that they exist in a true thermodynamic equilibrium. The formation of equilibrium vesicles and the L₃ phase is facilitated by ion exchange between the cationic surfactant and the ionic liquid, as well as the strength of the solvophobic effect in the protic ionic liquid