Multi-component molecular assembly involves self-assembly of two or more types of molecules on surfaces. Increased complexity of the inter-molecular interactions among various types of molecules and also with the substrates leads to a rich assembly on surfaces. The co-assembly of two types of molecules of dissimilar symmetries and sizes such as three-fold symmetric trimesic acid (TMA) and two-fold symmetric 4,4 ',4 '' ,4 '''- ([1,1 '-biphenyl]-4,4 '-diylbis(azanetriyl)) tetrabenzoic acid (H4BPTA) molecules on Highly Oriented Pyrolytic Graphite (HOPG) surface at the solid-liquid interface exhibits two kind of phases. At a comparable concentration of TMA with respect to H4BPTA, the assembly results into a phase where one or two TMA molecules occupy a single pore of H4BPTA network. On the other hand, at a relatively higher concentration of TMA molecules, a lateral supramolecular heterojunction of mono-component assemblies of TMA and H4BPTA with an atomically sharp interface is observed. While the earlier phase may be attributed to small size of TMA molecules compared to the pore size of H4BPTA network, the latter phase may be attributed to compatible lattice type of the TMA and H4BPTA assemblies where the strain due to large lattice mismatch is mitigated by the presence of flexible hydrogen bonding at the interface of the two mono-component self-assemblies. These results provide insight into two-component self-assembly of molecules with dissimilar symmetries and sizes and may have implications in the development of novel molecular materials.

Janus particles provide a powerful platform for studying asymmetric surface functionality and active colloidal matter, yet their broader utilization is hindered by the lack of fabrication strategies that offer quantitative and independent control over patch geometry. Here, we report an interfacial confinement-assisted electroless deposition process that exploits irreversible particle adsorption at the liquid-liquid interface for directed metal deposition. This process enables systematic control over metal patch thickness (5-50 nm) and surface coverage while remaining robust to the deposition of various metals (Au, Ag, Pt, Ni, Cu) and scalable across centimeter-scale interfacial areas (≈ 16 cm2). The generality of the approach is demonstrated by the synthesis of Janus particles with varied shapes (sphere, ellipsoid, peanut, sphero-cylinder, cube) and surface chemistries (hematite, silica, polystyrene, p-NIPAM) at liquid-liquid interfaces formed with different oils. To characterize the activity of particles, we perform experiments in hydrogen peroxide solutions of varying concentrations and demonstrate the influence of the catalytic patch thickness on the self-propulsion of Janus particles. Because the proposed process relies on general principles of colloids and interfacial chemistry, it is readily extendable to diverse systems and provides a versatile chemical route to architected Janus colloids.

The deposit patterns obtained from the evaporation of drops containing insoluble solute particles are vital for several technologies, including inkjet printing and optical and electronic device manufacturing. In this work, we consider the evaporation of an aqueous reaction mixture typically used for gold nanoparticle (AuNP) synthesis. The patterns obtained from the evaporation-driven assembly of in situ generated AuNPs are studied using optical microscopy and SEM analyses. The evaporation of drops withdrawn at different reaction times is found to significantly influence the distribution of AuNPs in the dried patterns. The evolution of the deposit patterns is also explored by drying multiple drops on the solid substrate, wherein a drop of a fresh reaction mixture is introduced over the deposit pattern left by the evaporation of the drop dispensed at an earlier time. Using quantitative image analysis, we show that the interparticle separation between the AuNPs in the dried patterns left on the solid substrate decreases when the number of drops is increased. We find optimal conditions to achieve solid-supported AuNP films, wherein the particles are in close physical contact, leading to a conducting deposit. The current through the AuNP deposit is found to increase with increase in the number of drops due to evaporation-driven self-assembly of AuNPs into branch-like structures with reduced interparticle separation. In addition, we also show that it is possible to produce conducting AuNP deposits by drying multiple drops withdrawn from the same reaction mixture. The evaporation-driven assembly of the in situ grown nanoparticles from a reaction mixture presented in this work can be further exploited in optical and electronic device fabrication.

Demulsification of particle-stabilized oil-in-water emulsions is crucial in diverse fields such as treatment of produce water, recovery of valuable products of Pickering emulsion catalysis, and so on. In this work, we investigated a facile method for destabilizing emulsions by dissolving stabilizer particles by the introduction of acid or base. Nanoellipsoidal hematite-stabilized decane-in-water emulsions are destabilized by dissolving hematite with oxalic or hydrochloric acid in situ. Time required for complete demulsification decreased as the acid concentration is increased. The demulsification time is typically on the order of a few hours for the chosen protocol. Similarly, the silica-stabilized decane-water emulsion is demulsified by the addition of aqueous sodium hydroxide. Demulsification kinetics is presented as the temporal change of the emulsion volume with time. Emulsion volume decreases in two stages: an initial slow decrease followed by an exponential decrease. Scanning electron microscopy analysis shows that the stabilizing particles are completely dissolved and recrystallized as salts of respective kinds. An estimate of the desorption free energy suggests that particle size should be reduced to a few nanometers for inducing destabilization. This work describes a facile method to destabilize oil-in-water emulsion, and it can be generalized to any other particle-stabilized emulsions by choosing appropriate chemical reagent for dissolution.

The ability to synthesize highly stable monodisperse iron oxide particles of different sizes and shapes, in particular, hematite (α-Fe2O3), has enabled fundamental investigation of particle shape effects in colloids and interface science. In this chapter, an overview of wet chemical approaches for the synthesis of hematite particles of different shapes and their use in the fundamental study of interfacial behavior of particles is discussed.

We present a modular single-step strategy for the formation of single and Pickering double emulsions (DEs). To this end, we consider the role of surface modification of particles and their dispersibility in different phases in the context of the design of Pickering emulsions by varying the volume fraction of oil in the oil-water mixture (ϕoil) used for emulsification. In particular, the experiments are performed by considering (a) model spherical and nonspherical colloids of different wettabilities which are tailored by oleic acid treatment, (b) immiscible liquids with or without particles, and (c) varying ϕoil from 0.1 to 0.9. We show that it is possible to affect a transition from (i) oil-in-water (O/W) emulsion to water-in-oil (W/O) emulsion and (ii) oil-in-water (O/W) to oil-in-water-in-oil (O/W/O) to water-in-oil (W/O) as ϕoil is systematically varied. We elucidate that the range of ϕoil at which particle stabilized DEs of the O/W/O type form can be tuned by engineering surface modification of particles to different extents. Furthermore, the arrangement of particles on the surface of droplets in the Pickering DEs is discussed. Our results conclusively establish that the differential wettability of particles is the key for the design of Pickering DEs. The versatility of the proposed strategy is established by developing DEs using a number of model colloidal systems.

Hypothesis: Pickering emulsions (PEs) once formed are highly stable because of very high desorption energies (-107 kBT) associated with particles adsorbed to the interfaces. The destabilization of PEs is required in many instances for recovery of valuable chemicals, products and active compounds. We pro-pose to exploit interfacial instabilities develop by the addition of different types of solutes to PEs as a route to engineer their destabilization.Experiments: PEs stabilized by (i) spherical particles, (ii) non-spherical particles, (iii) oppositely charged particle-particle mixtures, and (iv) oppositely charged particle-polyelectrolyte mixtures are formulated. Different types of solutes are added to these highly stable PEs and the macroscopic as well as microscopic changes induced in the PEs is recorded by visual observation and bright field optical microscopy.Findings: Our results point to a simple yet robust method to induce destabilization of PEs by transiently perturbing the oil-water interface by transport of a mutually soluble solute across the interface. The gen-erality of the method is demonstrated for different kind of solutes and stabilizers including particles of different sizes (nm to lm), shapes (sphere, spheroids, spherocylinders) and types (polystyrene, metal oxi-des). The method works for both oil-in-water (o/w) and water-in-oil (w/o) PEs with different kinds of non-polar solvents as oil-phase. However, the method fails when the solute is insoluble in one of the phases of PEs. The study opens up a new approach to destabilization of particle stabilized emulsions.(c) 2022 Elsevier Inc. All rights reserved.

Most of the polymeric emulsifiers have diblock and triblock copolymer architecture containing hydrophilic and hydrophobic domains. In this work, we show that hydrophilic homopolymers can be effective stabilizers of oil-in-water emulsions. Using polyethelyne oxide and poly(vinylpyrrolidone) as model hydrophilic homopolymers and n-decane and n-hexane as model nonpolar phases, we show that high-molecular weight polymers can stabilize emulsions over 24 h beyond a threshold concentration. We highlight the role of the molecular weight and concentration of the polymer in the stability of emulsions through kinetic measurements of emulsion volume, microscopic analysis, interfacial tension, and dilational rheology. We explain the mechanism of stabilization to stem from buoyancy-driven creaming of emulsion drops and film drainage and dilational elasticity of the interface in relation to the molecular weights and concentrations of polymers. This study demonstrates that water-soluble homopolymers can stabilize oil-in-water emulsions and open avenues for the use of eco-friendly biopolymers, which are inherently hydrophilic, as an alternative to synthetic emulsifiers.

We exploit the aggregation between oppositely charged particles to visualize and quantify the equilibrium position of charged colloidal particles at the fluid-water interface. A dispersion of commercially available charge-stabilized nanoparticles was used as the aqueous phase to create oil-water and air-water interfaces. The colloidal particles whose charge was opposite that of the nanoparticles in the aqueous phase were deposited at the chosen fluid-water interface. Heteroaggregation, i.e., aggregation between oppositely charged particles, leads to the deposition of nanoparticles onto the larger particle located at the interface; however, this only occurs on the surface of the particle in contact with the aqueous phase. This selective deposition of nanoparticles on the surfaces of the particles exposed to water enables the distinct visualization of the circular three-phase contact line around the particles positioned at the fluid-water interface. Since the electrostatic association between the nanoparticles and the colloids at interfaces is strong, the nanoparticle assembly on the larger particles is preserved even after being transferred to solid substrates via dip-coating. This facilitates the easy visualization of the contact line by electron microscopy and the determination of the equilibrium contact angle of colloidal particles (θ) at the fluid-water interface. The suitability of the method is demonstrated by the measurement of the three-phase contact angle of positively and negatively charged polystyrene particles located at fluid-water interfaces by considering particles with sizes varying from 220 nm to 8.71 μm. The study highlights the effect of the size ratio between the nanoparticles in the aqueous phase and the colloidal particles on the accuracy of the measurement of θ.

Emulsions are a class of high-surface-energy materials typically stabilized by surfactants, polymers, particles, or a combination of these. There has been considerable effort to develop new emulsifiers by exploiting developments in synthetic chemistry; however, synthetic surface-active species may assist in the stabilization of a specific type of immiscible liquid-liquid systems. That is, one stabilizer does not provide a solution for all interface stabilization problems. Moreover, the synthesis of surface-active systems involves high production costs and complex synthesis routes and generates a substantial amount of chemical waste. In this work, we show that plant latex, an aqueous dispersion of colloidal-scale particles in which small as well as large bioactive species are also present, can be used as a versatile and sustainable source for interface stabilization. The constituents of the latex are found to reduce the oil-water interfacial tension due to the spontaneous adsorption of surface-active species present in the latex. The surface-active nature of latex is further exploited to obtain very stable single emulsions, double emulsions (DEs), and multiple emulsions (MEs). Our results conclusively show that plan latex is a potential versatile source for the stabilization of emulsions created by considering different types of immiscible liquid systems.

Primary and secondary cracks often coexist in a dried colloidal deposit. While, primary cracks have been the focus of investigation in most of the theoretical and experimental studies, understanding the formation and time evolution of secondary cracks is scarce. In addition, secondary cracks at times are undesirable especially when used as a template in the nano-lithography. Therefore, a control over their formation and suppression is of high importance. We consider the drying of sessile drops containing colloidal ellipsoids on a solid substrate. The resulting coffee-ring-like deposit of ellipsoids is found to accompany primary cracks in the circumferential direction and secondary cracks in the radial direction. A systematic study is performed to understand the nucleation and propagation of cracks. Our experiments explicitly showed that the primary cracks always nucleate at the top surface of the deposit and then penetrate the interior of the deposit. However, in contrast, the secondary cracks always form at the bottom surface of the deposit, which is in contact with the solid substrate. The secondary cracks are found to form only when the deposit thickness exceeds a critical value. In addition, we discuss methods to completely suppress the formation of secondary cracks by (i) tuning the aspect ratio of the ellipsoidal particles, and (ii) incorporating a small fraction of spherical particles in the drying drop of ellipsoids prior to the evaporation experiments.

The efficacy of anisotropic particles in Pickering emulsion stabilization, attributed to shape-induced capillary interactions, is well-documented in the literature. In this contribution, we show that the surface of hematite ellipsoids can be modified in situ by the addition of oleic acid to effect transitional phase inversion of Pickering emulsions. Interestingly, incorporation of oleic acid results in the formation of nonspherical emulsion drops. The phase inversion of oil-in-water to water-in-oil and the transition in shape of emulsion drops from spherical to nonspherical is observed in two different particle systems, namely, nanoellipsoids and microellipsoids. The surface of spherical emulsion drops stabilized by particles or particles along with high concentration of oleic acid is found to consist of ellipsoids arranged in a close-packed configuration with their major axis parallel to the interface. In contrast, at intermediate oleic acid concentration, the surface of nonspherical emulsion drops is observed to be covered with loosely packed particle monolayer, with the ellipsoids at the oil/water interface taking up many different orientations. Using contact angle goniometry, the change in the wettability of hematite particles due to adsorption of oleic acid is established to be the mechanism responsible for the phase inversion of Pickering emulsions.