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.

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.

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.

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.

Liquid-Liquid Phase Separation (LLPS)-Driven Fibrilization of Amyloid-ß Protein.

Amyloid-ß [Aß(1-40)] aggregation into a fibrillar network is one of the major hallmarks of Alzheimer's disease (AD). Recently, a few studies reported that polyphosphate (polyP), an anionic biopolymer that participates in various cellular physiological processes in humans, induces fibrilization in many amyloidogenic proteins [ 2020 Alzheimer's Disease Facts and Figures; John Wiley and Sons Inc., 2020; Tanzi, R. E.; Bertram, L. Cell 2005, 120, 545-555; Selkoe, D. J. Proc. Natl. Acad. Sci. U.S.A. 1995, 275, 630-631; and Rambaran, R. N.; Serpell, L. C. Prion 2008, 2, 112-117]. However, the role of polyP in Aß(1-40) fibrilization and the underlying mechanism are unclear. In this study, we report experimental investigations on the role of polyP in the fibrilization kinetics of Aß(1-40). It is found that polyP exhibits a dual effect depending upon the pH value. At pH = 7 (neutral), polyP inhibits amyloid fibrilization in a dose-dependent manner similar to negatively charged nanoparticles. On the contrary, at pH = 3 (acidic), polyP accelerates amyloid fibrilization kinetics via liquid-liquid phase separation (LLPS), wherein the protein-rich droplets contain mature fibrils. In the parameter space spanned by concentrations of Aß(1-40) and polyP, a phase diagram is constructed to demark the domain where LLPS is observed at pH = 3. Characterization of the protein aggregates, secondary structure content in the aggregates, and cell viability studies in the presence of aggregates are discussed at both pH values. This study reveals that anionic biopolymers can modulate amyloid fibrilization kinetics, linked to neurodegenerative diseases, depending upon their local concentrations and pH.

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.

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.

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.

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.

Destabilization of Pickering emulsions by interfacial transport of mutually soluble solute.

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 propose 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 generality of the method is demonstrated for different kind of solutes and stabilizers including particles of different sizes (nm to µm), shapes (sphere, spheroids, spherocylinders) and types (polystyrene, metal oxides). 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.

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.

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.

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.

Competing effects of rotational diffusivity and activity on finite-sized clusters.

Colloidal particles interacting via short-range attraction and long-range repulsion are known to stabilize finite-sized clusters under equilibrium conditions. In this work, the effect of self-propulsion speed (activity) and rotational diffusivity (Dr) on the phase behavior of such particles is investigated using Brownian dynamics simulations. The system exhibits rich phase behavior consisting of clusters of different kinds. The cluster size varies non-monotonically with activity: increasing first and decreasing at higher activity, thus driving cluster-to-fluid phase transition. Rotational diffusivity also facilitates the formation of clusters. Larger clusters could be stabilized at lowDrvalues while at highDrvalues, clusters are stable even at higher activities. The analysis of the static structure factor of the system confirms that rotational diffusivity delays the cluster-to-fluid transition driven by activity.

Computer simulations of self-assembly of anisotropic colloids.

Computer simulations have played a significant role in understanding the physics of colloidal self-assembly, interpreting experimental observations, and predicting novel mesoscopic and crystalline structures. Recent advances in computer simulations of colloidal self-assembly driven by anisotropic or orientation-dependent inter-particle interactions are highlighted in this review. These interactions are broadly classified into two classes: entropic and enthalpic interactions. They mainly arise due to shape anisotropy, surface heterogeneity, compositional heterogeneity, external field, interfaces, and confinements. Key challenges and opportunities in the field are discussed.

Kinetics of aggregation of amyloid ß under different shearing conditions: Experimental and modelling analyses.

Amyloid ß (Aß40) is a class of amyloidogenic proteins known to aggregate into a fibrillar network. The rate of aggregation and fibril yield is sensitive to external energy input, such as shear. In this work, simple shear and shaking experiments are performed on Aß40 solution using a Couette cell and an orbital shaker, respectively. Experiments show that, under uniform shear, both the mass of fibrils and aggregation rate increase with the shear rate. In the case of orbital shaking, the lag time decreases with the rotational speed of the shaker, but the final fibril mass is the same for all agitation speeds. To explain this contrasting behavior of aggregation kinetics, a population balance model is developed to account for the effect of shear on the aggregation of Aß. The kinetic model includes primary nucleation, secondary nucleation, elongation, fragmentation, and depolymerization steps. The effect of steady uniform shear is encoded in the depolymerization rate constant (kd), and it is shown that kd decreases with shear rate initially and saturates at high shear rates. A competition between elongation and depolymerization rates yields different equilibrium masses of fibril at different shear rates. The model results agree quantitatively well with experimental data on the rate of aggregation and mass of fibrils as a function of shear rate. The modeling framework can be used to explain the shear rate-dependent aggregation of other amyloidogenic proteins.

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.

Stabilizing ordered structures with single patch inverse patchy colloids in two dimensions.

Oppositely charged bipolar colloids or colloids decorated with complementary deoxyribonucleic acid (DNA) on their surfaces are special kinds of patchy particles where only patch and non-patch parts are attractive. These are classified as inverse patchy colloids (IPCs). In this work, equilibrium self-assembly of IPC in two-dimensions is reported using Monte Carlo simulations. Square (SCs) and triangular crystals (TCs) are found to be stable at 0.5 patch coverage. Upon decreasing the patch coverage to 0.33, the regular SC is destabilized; instead rhombic and TCs are found to be stable. At low patch coverages such as 0.22 and 0.12, only TC is stabilized at high density. Particles of all the patch coverages show kinetically stable cluster phases of different shapes and sizes at low densities, and the average cluster size depends on the patch coverage and particle density. State-diagrams showing all the stable phases for each patch coverage are presented. Ordered phases are characterized by bond order parametersψ4,ψ6and radial distribution function. The effect of polydispersity in patch coverage on the polarization of the stable structures are also studied. The study demonstrates that IPCs can stabilize various ordered two-dimensional structures by tuning the size of the patch, density and interaction strengths.

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.