Demulsification of Pickering Emulsions by Chemical Dissolution of Stabilizers.

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.

Drying-Induced Flash Nanoprecipitation in a Sessile Drop: A Route to Synthesize Polymeric Nanoparticles.

Flash nanoprecipitation is a simple and scalable method to produce nanoparticles by rapid mixing of a polymer solution with an antisolvent. High-speed mixing devices for the continuous synthesis of polymeric nanoparticles and drug-encapsulated nanoparticles have been designed. In this work, we demonstrate a different approach to induce flash nanoprecipitation using the differential evaporation of solvents in a sessile drop. To show proof of concept, we use polymethyl-methacrylate (PMMA) dissolved in a tetrahydrofuran (THF)-water mixture as a model system. A sessile drop of the polymer solution is allowed to dry under controlled conditions. The sessile drops of the PMMA-THF-water ternary mixture are observed to dry in the constant radius mode. As THF in the drop evaporates faster than water, PMMA supersaturates and precipitates as nanoparticles. Although coffee-ring formation is well-studied in the drying of colloidal suspensions, this work demonstrates the formation of nanoparticles in situ due to a change of solvent quality and subsequent deposition of particles at the pinned contact line. Using the theory of drying of binary solutions, we calculate the temporal variation of composition. The drying paths passing through the low-concentration branch of the binodal give rise to nanoparticles, whereas those passing through the high-concentration branch yield porous films. Spherical polymeric nanoparticles in the size range of 250-700 nm were synthesized using this technique starting from drops with different initial polymer concentration. The method is a cost-effective (no high-speed mixing is required) and scalable alternative to conventional flash nanoprecipitation for synthesizing polymeric nanoparticles for potential applications in drug delivery, diagnostics, and polymer recycling.

Contact Angle Modulation: In Situ Polymer Deposition during Sessile Drop Evaporation.

The drying kinetics of a sessile drop on a solid surface are a widely studied phenomenon because of their relevance to various fields such as coating, printing, medical diagnostics, sensing, and microfluidic technology. Typically, the drop undergoes drying either at a constant contact radius (R) with a decrease in the three-phase contact angle or at a constant contact angle (θ) with a reduction in the radius with time. These two drying modes are referred to as CCR and CCA, respectively. It is not uncommon where both R and θ may decrease during drying, especially in the penultimate stage of drying. In this work, we report a scenario wherein the θ increases while R decreases during the drying process of an aqueous polymer solution on a high surface energy substrate. This behavior is observed across different polymer systems (such as poly(ethylene oxide) and polyvinyl pyrrolidine), varying molecular weights, and polymer concentrations. As the drop dries, the polymer gets deposited at the three-phase contact line, thus reducing the surface energy of the substrate and leading to an increase in the contact angle. The drop responds by attempting to reach a new equilibrium contact angle through slipping. The temporal increase in contact angle follows a power law scaling behavior. This study demonstrates an in situ modulation of contact angle facilitated by evaporation and polymer deposition, showcasing unconventional drying dynamics.

Electrostatic Heteroaggregation: Fundamentals and Applications in Interfacial Engineering.

The aggregation of oppositely charged soft materials (particles, surfactants, polyelectrolytes, etc.) that differ in one or more physical or chemical attributes, broadly referred to as electrostatic heteroaggregation, has been an active area of research for several decades now. While electrostatic heteroaggregation (EHA) is relevant to diverse fields such as environmental engineering, food technology, and pharmaceutical formulations, more recently there has been a resurgence to explore various aspects of this phenomenon in the context of interface stabilization and the development of functional materials. In this Feature Article, we provide an overview of the recent contributions of our group to this exciting field with particular emphasis on fundamental studies of electrostatic heteroaggregation between oppositely charged systems in the bulk, at interfaces, and across the bulk/interface. The influence of the size and shape of particles and the surface charge of heteroaggregates on the formation of Pickering emulsions and their utilization in the development of porous ceramics is discussed.

Oil-in-Water Emulsions Stabilized by Hydrophilic Homopolymers.

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.

Collective behavior of passive and active circle swimming particle mixtures.

We present a numerical study on a binary mixture of passive and circle swimming, self-propelling particles which interact via the Lennard-Jones (LJ) potential in two dimensions. Using Brownian Dynamics (BD) simulations, we present state diagrams using the control parameters such as attraction strength, angular velocity, self-propulsion velocity and composition. In a symmetric mixture, the system undergoes a transition from a mixed gel to a rotating passive cluster state and finally to a homogeneous fluid state as translational activity increases. The formation of the rotating cluster of passive particles surrounded by active and passive monomers is attributed to the combined effect of composition, activity and strength of attraction of the active particles. Different phases are characterized using radial distribution functions, bond order parameters, cluster fraction and probability distribution of local volume fractions. The present study addresses comprehensively the intricate role of activity, angular velocity, inter-particle interaction and compositional variation on the phase behavior. The predictions presented in the study can be experimentally realized in synthetic colloidal swimmers and motile bacterial suspensions.

Effect of substrate charge density on the adsorption of intrinsically disordered protein amyloid ß40: a molecular dynamics study.

The inhibitory effect of negatively charged gold nanoparticles (AuNPs) on amyloidogenic protein fibrillation has been established from experiments and computer simulations. Here, we investigate the effect of the charge density (σ) of gold (Au) surfaces on the adsorption of the intrinsically disordered amyloid ß40 (Aß40) monomer using molecular dynamics (MD) simulations. On the basis of the binding free energy, some key residues (ARG5, LYS16, LYS28, LEU17-ALA21, ILE31-VAL38) were found to be responsible for preventing the ß-sheet formation, which is known to be a precursor for fibrillation. Until a critical charge density (σc) of -0.167 e nm-2, the key residues remained adsorbed on the Au slab. A saturation in the number of condensed counterions (Na+) on Aß40 was also observed at σc. Beyond σc, the condensation of Na+ occurs only on the Au slab, leading to competition between positively charged key residues and condensed ions. This competition was found to be responsible for the lack of adsorption of the key residues, leading to ß-sheet formation for σ > -0.167 e nm-2. This study suggests that if the key residues are not adsorbed, then ß-sheet formation is observed, which can then lead to the development of proto-fibrils and subsequently fibrillation. Therefore the surface should have an optimal charge density to be an effective inhibitor of fibrillation.

Light-induced destabilisation of oil-in-water emulsions using light-active bolaform surfactants.

External stimuli-induced destabilisation of oil-in-water emulsions is of both fundamental and technological importance. In this work we synthesize light-active bolaform-type surfactants (LABSs) and show the preparation of decane-in-water emulsions over a range of surfactant and salt concentrations. Under ultraviolet (UV) illumination, LABSs undergo trans to cis isomerization affecting their interfacial activity. Therefore when stable emulsions stabilized by LABSs are exposed to UV light, they undergo partial destabilization. To induce interfacial flow, a small amount of volatile solvent (methanol, ethanol, tetrahydrofuran, etc.) is added at the emulsification stage and in this case complete phase separation is observed. This study demonstrates a facile route to induce destabilization of surfactant-stabilized emulsions using benign solvents and minimal use of energy (UV light) and this method could be of importance in wastewater treatment, enhanced oil recovery, protein separation, etc. where emulsion destabilization is desired.

Brownian dynamics simulations of telechelic polymer - latex suspensions under steady shear.

We carry out coarse-grained Brownian dynamics simulations of shearing flow of a colloidal suspension bridged by telechelic polymers with "sticky" end groups and vary sticker strength ε over a range from 3 to 12 in units of kBT, motivated by an interest in simulating the rheology of latex paints. The most extensive results are obtained for dumbbells, but the trends are confirmed for 3-bead trumbbells and chains of up to 11 beads. The numbers of colloids and of polymers are also varied over a wide range to confirm trends established for smaller, more computationally affordable, systems. The dynamics are the result of an interplay of the shear rate and three different times scales: the time τBridge for a sticker on a bridging chain to be released from a particle surface, which scales as exp(0.77ε), the time for the polymer chain to relax, τR, which scales as the square of polymer chain length, and the time τD for a colloid to diffuse a distance comparable to its own radius, R, which scales as R3. The scalings of the bridge-to-loop and loop-to-bridge times namely τBL ∝ exp (0.75ε) and τLB ∝ exp (0.71ε), are similar to those of τBridge, for ε values above around 5 kBT, because of the relatively short chains considered here (i.e., 60 Kuhn steps). However, τR becomes more dominant for longer chains, as shown by Travitz and Larson. The zero-shear viscosity η0 is estimated from the Green-Kubo relation, and found to scale as exp (0.69ε), similar to that of τBridge. A weak influence of η0 on τD is observed, with the influence expected to become stronger when τD becomes larger, as shown previously by Wang and Larson. At shear rates in the nonlinear regime, shear-thinning is found with exponents ≈ -0.10 to -0.60, and the first normal stress difference is positive, consistent with some of the experimental data of Chatterjee et al. on model latex paint formulations. The weakness of the shear thinning, relative to that of hydrophobically modified ethoxylated urethane (HEUR) solutions without colloids, is likely due to the observed insensitivity of the loop-to-bridge and bridge-to-loop transition times to the imposed shear rate. This preliminary study provides the first mesoscale simulations of these suspensions, useful for assessing and improving both more accurate multi-scale models and eventually constitutive equations for these complex suspensions.

Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces.

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.

Collective dynamics of active circle-swimming Lennard-Jones particles.

We report a numerical study on the collective dynamics of self-propelling and circle-swimming Lennard-Jones (LJ) particles in two dimensions using Brownian dynamics simulations. We investigate the combined role of attraction, self-propulsion and rotation in their phase behavior. At a low rotational speed, the system shows re-entrant phase behavior as a function of self-propulsion similar to active Brownian particles (ABPs). Increasing the rotational speed shifts the point of re-entrance or makes it disappear depending on the attractive strength. Although active rotation is known to suppress motility induced phase separation, the presence of attractive interactions reduces this effect.

Self-assembly of a CTAB surfactant on gold nanoparticles: a united-atom molecular dynamics study.

Self-assembly of a cetyltrimethyl ammonium bromide (CTAB) surfactant on gold nanoparticles (AuNPs) is studied using united-atom molecular dynamics (MD) simulations. For AuNPs in the size range of 1-3 nm, CTAB self-assembles such that the tail groups adsorb on the AuNP surface while the ionic head group is exposed to water, giving a net negative charge to the AuNPs. Near the AuNP surface, water molecules are depleted. The fraction of adsorbed CTAB molecules increased with AuNP size, while packing density decreased with size. Binding free energy also increased with AuNP size. The microscopic structural aspects of CTAB on AuNP and water-AuNP correlations are obtained from radial distribution functions. Contrary to the bilayer model proposed in the literature, the present simulations show the formation of a monolayer at CTAB concentrations equivalent to AuNP synthesis conditions. Even immobilizing bromide ions on the AuNP surface did not facilitate bilayer formation. Our simulation studies show that for very small nanoparticles, bilayer formation is unfavorable and instead a single monolayer of CTAB is formed around AuNPs.

Adsorption of the amyloid ß40 monomer on charged gold nanoparticles and slabs: a molecular dynamics study.

Negatively charged nanoparticles are known to inhibit the fibrillation of amyloidogenic protein amyloid ß (Aß40), though the overall charge on the protein is negative. In this work a molecular dynamics study is reported to investigate the interaction of Aß40 on negatively charged gold nanoparticles (3-5 nm) and charged (positive and negative) and neutral gold slabs. The equilibrium structures of Aß40 on gold surfaces are characterized using residue-specific contacts on the gold surface, secondary structure analysis and binding free energy calculations. The simulation results reveal that the Aß40 protein in water interconverts into ß-sheets, which are building blocks of the mature fibrils, whereas on gold nanoparticles Aß40 unfolds and adsorbs. Both the negatively charged gold nanoparticles and gold slabs arrest the formation of ß-sheets in Aß40, whereas the positively charged gold slab does not inhibit the formation of ß-sheets. The residue-specific interactions between Aß40 and the gold surfaces are important in governing the adsorption of Aß40 on charged surfaces.

Self assembly of nanorods on microspheres at fluid-fluid interfaces.

The potential applications of metal nanoparticles require their assembly/deposition on different solid matrices. In this work, an experimental method is demonstrated to assemble gold nanorods (AuNRs) as a ring-like structure on polystyrene (PS) microspheres at the fluid-fluid interface via dip-coating followed by solvent evaporation. The effects of AuNR concentration, size and surface charge of PS particles and size of AuNRs on the formation of AuNR ring-like structures on templated PS particles are investigated. A mechanism based on the evaporative drying of a liquid capillary bridge hinged between two PS microspheres is proposed for the formation of the ring-like structure on the PS microspheres. As the liquid evaporates from the pinning line on the PS microsphere surface, the ring-like structure is formed by the convective deposition of AuNRs. The decane-water interfacial tension dictates the position of the pinning line and thus controls the position/diameter of the ring on the PS microspheres. The ring diameter is found to be strongly affected by the template particle diameter. The generality of the experimental scheme is demonstrated by making a ring-like deposit of hematite ellipsoids on PS particles and their position is varied by changing the oil-water interfacial tension via the addition of a surfactant. The work demonstrates a simple, scalable and interface-based method of depositing both spherical and non-spherical nanoparticles on microspheres, which allows the manipulation of nanoparticles as functional components in fabricating devices.

Rapid Dissolution of Amyloid ß Fibrils by Silver Nanoplates.

Plaques of amyloid beta (Aß) protein are associated with neurodegenerative diseases, and preventing their formation and dissolution of plaques are essential to the development of therapeutics. In this study, silver triangular nanoplates (AgTNPs) are shown to dissolve mature Aß fibrils because of their plasmonic photothermal property. Mature Aß fibrils treated with AgTNPs under near-infrared (NIR)-illuminated conditions are dissolved in less than 1 h, while an equal concentration of silver spherical nanoparticles took about 70 h. The concentration of the fibrils decreased from 10 to 0.3 µM upon treating the amyloid fibrils with AgTNPs under NIR. AgTNPs are also shown to prevent the formation of Aß fibrils by selective binding to the positively charged amyloidogenic sequence of the Aß monomer. The kinetics of inhibition by AgTNPs follows the predictions of the detailed kinetic model (Ramesh et al., Langmuir 2018, 34, 4004-4012). The kinetics of dissolution and inhibition are characterized by Congo red/ThT assay, transmission electronic microscopy, atomic force microscopy, and attenuated total reflectance Fourier transform-infrared spectroscopy. Cell viability studies on SH-SY5Y and BE-(2)-C cells using 3-[4,5-dimethy-lthi-azol-2-yl]-2,5-diphenyl-tetrazdium bromide and lactate dehydrogenase assay show that the viability of the cells increased from 33 to 70% on treating the cells with AgTNP-incubated Aß fibrils compared to the mature Aß fibrils. The study provides new insights to design plasmonic nanoparticle-based therapeutics to cure neurodegenerative diseases.

Transition from Linear to Circular Motion in Active Spherical-Cap Colloids.

Nonspherical self-propelling colloidal particles offer many possibilities for creating a variety of active motions. In this work, we report on the transition from linear to circular motion of active spherical-cap particles near a substrate. Self-propulsion is induced by self-diffusiophoresis by catalytic decomposition of hydrogen peroxide (H2O2) on one side of the particle. Asymmetric distribution of reaction products combined with the asymmetric shape of the particle gives rise to two types of motions depending upon the relative orientation of the particle with respect to the underlying substrate. At a low concentration of H2O2, linear active motion is observed, whereas increasing the H2O2 concentration leads to persistent circular motion. However, the speed of self-propulsion is nearly independent of the size of the particle. The study demonstrates the use of nonspherical particles to create linear and circular motion by varying the fuel concentration.

Modeling of the Inhibitory Effect of Nanoparticles on Amyloid ß Fibrillation.

Experiments have shown that charged nanoparticles (NP) inhibit, partially or completely, the aggregation of Aß protein monomers into fibrils. The equilibrium fibril content is found to be inversely proportional to the concentration of NP. In this work, we report a kinetic model for the fibrillation of Aß protein in the presence of NP. In the model, apart from nucleation, elongation and fragmentation processes, the effect of NP is considered to cause a conformational change to the protein monomer, making the latter incompatible for aggregation. The simulated results explain the growth kinetics of pure Aß (1-40) protein, and the kinetics in the presence of NP. The NP-monomer interaction considered in the model captures the significant effect of NP on the fibrillation process at a very molar ratio (NP to Aß monomer) as low as 10-4. The model predictions are compared with two different NP systems, namely, gold and silica NP. The model can be applied to explain the inhibitory effect of other additives such as small molecules, NP, lipids, and surfactants that show a similar inhibition trend for fibril formation of Aß and other proteins.

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, SS-H) are varied to tune the colloidal stability/instability of the suspension. At pH 6.5 and low SS-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 SS-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 SS-H. Calculation of Derjaguin, Landau, Verwey, and Overbeek (DLVO) interaction between particles in the binary mixture, as a function of pH and SS-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 ( W) calculated from DLVO interactions demark the unstable-stable dispersion regions, which is found to be in agreement with the experimental results.

Mechanism and modeling of poly[vinylpyrrolidone] (PVP) facilitated synthesis of silver nanoplates.

Silver triangular nanoplates (AgTNP) present unique surface plasmonic and catalytic properties depending upon the thickness and edge length. AgTNP are synthesized in a kinetically controlled growth process, by and large, using the polymer poly-vinylpyrrolidone (PVP) as a reductant. In this work, we present a systematic study to uncover the effect of the molecular weight (MW) of PVP and the PVP to silver salt (AgNO3) molar ratio ([P : S]) on the physical dimensions of AgTNP. The edge length of AgTNP shows a non-monotonic variation with respect to [P : S] for all MWs. Based on several control experiments, a kinetic mechanism is proposed and a mathematical model is developed to explain the formation of AgTNP. The elementary processes of the model include the reduction of Ag+ by the -OH group in PVP, followed by instantaneous nucleation. This phase is then followed by a slow reduction of Ag+ and growth of the nuclei to AgTNP. The model shows a reasonable agreement with experiments on the non-monotonic variation of edge length of AgTNP with respect to [P : S], as well as on the temporal evolution of the edge length.

Staggered Linear Assembly of Spherical-Cap Colloids.

Linear assembly of colloidal particles is of fundamental interest in visualizing polymer dynamics and living organisms. We have developed a fluid-fluid interface-based method to synthesize spherical-cap polymeric latex particles. These particles are shown to spontaneously self-assemble in zigzag arrangement. The linear assembly is induced due to the shape anisotropy (one side is curved and the other side is nearly flat) and heterogeneous charge distribution on the particle surfaces. The necessities of these conditions are justified within the framework of DLVO theory. Spherical-cap particles of various size and aspect ratio reproduced the observed linear assembly, thus demonstrating the robustness of the self-assembly mechanism. While these types of assemblies are observed in spherical particles using microfluidic devices or electric field, the proposed approach is rather facile and does not require any external field. These novel assemblies could be potentially useful to understand kinetics of nucleation and growth of amyloidogenic proteins and to prepare artificial swimming microorganisms.