Charged Polystyrene Particles Research Articles

Do Surface Charges on Polymeric Filters and Airborne Particles Control the Removal of Nanoscale Aerosols by Polymeric Facial Masks?

The emergence of facial masks as a critical health intervention to prevent the spread of airborne disease and protect from occupational nanomaterial exposure highlights the need for fundamental insights into the interaction of nanoparticles (<200 nm) with modern polymeric mask filter materials. While most research focuses on the filtration efficiency of airborne particles by facial masks based on pore sizes, pressure drop, or humidity, only a few studies focus on the importance of aerosol surface charge versus filter surface charge and their role in the net particle filtration efficiency of mask filters. In this study, experiments were conducted to assess mask filter filtration efficiency using positively and negatively charged polystyrene particles (150 nm) as challenge aerosols at varying humidity levels. Commercial masks with surface potential (Ψf) in the range of -10 V to -800 V were measured by an electrostatic voltmeter and used for testing. Results show that the mask filtration efficiency is highly dependent on the mask surface potential as well as the charge on the challenge aerosol, ranging from 60% to 98%. Eliminating the surface charge results in a maximum 43% decrease in filtration efficiency, emphasizing the importance of electrostatic charge interactions during the particle capture process. Moreover, increased humidity can decrease the surface charge on filters, thereby decreasing the mask filtration efficiency. The knowledge gained from this study provides insight into the critical role of electrostatic attraction in nanoparticle capture mechanisms and benefits future occupational and environmental health studies.

Synchronous oscillatory electro-inertial focusing of microparticles

Here, results are presented on the focusing of 1μm polystyrene particle suspensions using a synchronous oscillatory pressure-driven flow and oscillatory electric field in a microfluidic device. The effect of the phase difference between the oscillatory fields on the focusing position and focusing efficiency was investigated. The focusing position of negatively charged polystyrene particles could be tuned anywhere between the channel centerline to the channel walls. Similarly, the focusing efficiency could range from 20% up to 90%, depending on the phase difference, for particle Reynolds numbers of order O(10−4). The migration velocity profile was measured and the peak velocity was found to scale linearly with both the oscillatory pressure-driven flow amplitude and the oscillatory electric field amplitude. Furthermore, the average migration velocity was observed to scale with the cosine of the phase difference between the fields, indicating the coupled non-linear nature of the phenomenon. Last, the peak migration velocity was measured for different particle radii and found to have an inverse relation, where the velocity increased with decreasing particle radius for identical conditions.

Particle Adsorption Using a Quartz Crystal Microbalance with Dissipation by Applying a Kelvin-Voigt-Based Viscoelastic Model and the Gauss-Newton Method.

The use of a quartz crystal microbalance with dissipation (QCM-D) to study the adsorption of particles larger than 100 nm, such as liposomes, viruses, and nano/micro-plastics, remains challenging owing to the lack of appropriate models for data evaluation. This study presents a method for quantifying the adsorption of negatively charged polystyrene latex (100 nm-1 μm) at the solid-liquid interface. The validity of a viscoelastic model based on Kelvin-Voigt theory was assessed, and the model was used to evaluate particle adsorption data obtained from QCM-D measurements. The Gauss-Newton method was used to fit the data; the values obtained were larger than results from atomic force microscopy, indicating that the viscoelastic model combined with the Gauss-Newton method can quantify the adsorption of large polystyrene particles and the surrounding water around them. We suggested that QCM-D, in combination with an appropriate viscoelastic model, is applicable to estimate adsorption at the solid-liquid interface even for soft particles larger than 1 μm, which are out of the range of applications to the hydrodynamics model. Furthermore, we successfully showed that the recorded dissipation reflects the viscoelastic properties of the layer. The viscoelastic model allowed quantification of the rheological properties of the layer. The ratio of the viscous and elastic contributions was characterized by using loss tangent (tan δ) values that were extracted from the experimental data by applying the viscoelastic model. These values were lower for the adsorption of the negatively charged polystyrene particles on a positive surface than on a negative surface. This suggests that tan δ reflects the strength of the contact between the particle and substrate.

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.

Temperature dependence of diffusiophoresis via a novel microfluidic approach.

The temperature dependence of the diffusiophoretic mobility (DDP) is investigated experimentally and compared with theoretical predictions. These systematic measurements were made possible by a new microfluidic approach that enables truly steady state gradients to be imposed, and direct and repeatable measurements of diffusiophoretic migration to be made over hours-long time scales. Diffusiophoretic mobilities were measured for fluorescent, negatively charged polystyrene particles under NaCl gradients, at temperatures ranging from 20 °C to 70 °C. Measured DDP values were found to increase monotonically with temperature, and to agree, both qualitatively and relatively quantitatively, with theoretical predictions based on electrophoretically-measured zeta potentials. These results provide confidence that existing diffusiophoresis theories can accurately predict DP mobilities over a range of temperatures. More broadly, we anticipate our new microfluidic approach will facilitate and enable new tests of diffusiophoretic phenomena under a wide range of physical and chemical conditions.

Patchy Colloidal Clusters with Broken Symmetry.

Colloidal clusters are prepared by assembling positively charged cross-linked polystyrene (PS) particles onto negatively charged liquid cores of swollen polymer particles. PS particles at the interface of the liquid core are closely packed around the core due to interfacial wetting. Then, by evaporating solvent in the liquid cores, polymers in the cores are solidified and the clusters are cemented. As the swelling ratio of PS cores increases, cores at the center of colloidal clusters are exposed, forming patchy colloidal clusters. Finally, by density gradient centrifugation, high-purity symmetric colloidal clusters are obtained. When silica-PS core-shell particles are swollen and serve as the liquid cores, hybrid colloidal clusters are obtained in which each silica nanoparticle is relocated to the liquid core interface during the swelling-deswelling process breaking symmetry in colloidal clusters as the silica nanoparticle in the core is comparable in size with the PS particle in the shell. The configuration of colloidal clusters is determined once the number of particles around the liquid core is given, which depends on the size ratio of the liquid core and shell particle. Since hybrid clusters are heavier than PS particles, they can be purified using centrifugation.

Experimental investigation of dry powder coating processing parameters on the polystyrene particle's distribution on the surface of carbon fibers

Powder technology has been used to coat fibers with polymeric particles to manufacture composites. The material parameters such as particle size and fiber diameter and process parameters such as the particle charge and the particle velocity will influence the powder coating process. In this work, we present an experimental setup in which we characterize the coating of micron size particles on a fiber and examine the role of process parameters on the coating density. Monodispersed Polystyrene (PS) particles (2.8 ± 0.4 μm in diameter) were used to coat a carbon fiber (CF) 7 μm in diameter. A model setup was designed and fabricated in which the PS particles were charged with an ozone charging system, while the CF was maintained at the grounded state at all times. The setup consisted of two parts namely, a cyclone and a coating chamber. The distance between the PS particles was measured and the density of coating was examined using a microscope as a function of the processing parameters used in the setup. The measured average distance between the PS particles on the CF was used in our developed numerical model to estimate the friction coefficient between the charged PS particle and the grounded CF and the fiber volume fraction. The proposed methodology can be extended to any particle size, particle charge, and fiber diameter to estimate the friction coefficient and the fiber volume fraction at the microscale level.

Polymer-Like Self-Assembled Structures from Particles with Isotropic Interactions: Dependence upon the Range of the Attraction.

We conduct Metropolis Monte Carlo simulations on models of dilute colloidal dispersions, where the particles interact via isotropic potentials of mean force (PMFs) that display a long-ranged repulsion, combined with a short-ranged and narrow attraction. Such systems are known to form anisotropic clusters. There are two main conclusions from this work. First, we demonstrate that the width of the attractive region has a significant impact on the type of structures that are formed. A narrow attractive well tends to produce clusters in which particles possess fewer neighbors than in systems where the attraction is wider. Second, metastable clusters appear to persist in the absence of specific simulation moves designed to overcome large energy barriers to particle accumulation. The so-called “Aggregation-Volume Bias Monte Carlo” moves were previously developed by Chen and Siepmann, and they facilitate particle exchanges between clusters via unphysical moves that bypass high energy intermediate states. These facilitate the progression of metastable clusters to equilibrium clusters. Metastable clusters are generally large with significant branching of thin filaments of aggregated particles, while stable clusters have thicker backbones and tend to be more compact with significantly fewer particles. This general behavior is observed in both two- and three-dimensional systems. In two dimensions, less anisotropic clusters with backbones possessing lattice structures will occur, particularly for systems where the particles interact with a PMF that has a relatively wide attractive region. We compare our results with PMF calculations established from a more specific model, namely weakly charged polystyrene particles, which carry a thin surface layer of grafted polyethylene oxide polymers in aqueous solution. We hope that our investigations can serve as crude guidelines for experimental research, aiming to construct linear or branched polymers in aqueous solution built up by colloidal monomers that are large enough to be studied by confocal microscopy. We suggest that metastable clusters are more relevant to experimental scenarios where the energetic barriers are too large to be surmounted over typical timescales.

Tailoring Disorder and Quality of Photonic Glass Templates for Structural Coloration by Particle Charge Interactions

To obtain high-quality homogeneous photonic glass-based structural color films over large areas, it is essential to precisely control the degree of disorder of the spherical particles used and reduce the crack density within the films as much as possible. To tailor the disorder and quality of photonic glasses, a heteroaggregation-based process was developed by employing two oppositely charged equal-sized polystyrene (PS) particle types. The influence of the particle size ratio on the extent of heteroaggregation in the suspension mixes is investigated and correlated with both the morphology and the resultant optical properties of the films. The results show that the oppositely charged particle size ratio within the mix greatly influences the assembled structure in the films, affecting their roughness, crack density, and the coffee-ring formation. To better differentiate the morphology of the films, scanning electron microscopy images of the microstructures were classified by a supervised training of a deep convolutional neural network model to find distinctions that are inaccessible by conventional image analysis methods. Selected compositions were then infiltrated with TiO2 via atomic layer deposition, and after removal of the PS spheres, surface-templated inverse photonic glasses were obtained. Different color impressions and optical properties were obtained depending on the heteroaggregation level and thus the quality of the resultant films. The best results regarding the stability of the films and suppression of coffee-ring formation are obtained with a 35 wt % positively charged over negatively charged particle mix, which yielded enhanced structural coloration associated with improved film quality, tailored by the heteroaggregation fabrication process.

Diffusiophoresis in Multivalent Electrolytes.

Diffusiophoresis is the spontaneous movement of colloidal particles in a concentration gradient of solutes. As a small-scale phenomenon that harnesses energy from concentration gradients, diffusiophoresis may prove useful for passively manipulating particles in lab-on-a-chip applications as well as configurations involving interfaces. Though naturally occurring ions are often multivalent, experimental studies of diffusiophoresis have been mostly limited to monovalent electrolytes. In this work, we investigate the motion of negatively charged polystyrene particles in one-dimensional salt gradients for a variety of multivalent electrolytes. We develop a one-dimensional model and obtain good agreement between our experimental and modeling results with no fitting parameters. Our results indicate that the ambipolar diffusivity, which is dependent on the valence combination of cations and anions, dictates the speed of the diffusiophoretic motion of the particles by controlling the time scale at which the electrolyte concentration evolves. In addition, the ion valences also modify the electrophoretic and chemiphoretic contributions to the diffusiophoretic mobility of the particles. Our results are applicable to systems where the chemical concentration gradient is comprised of multivalent ions, and motivate future research to manipulate particles by exploiting ion valence.

Multistable interaction between a spherical Brownian particle and an air-water interface.

We report the measurement of the interaction energy between a charged Brownian polystyrene particle and an air-water interface. The interaction potential is obtained from the Boltzmann equation by tracking particle interface distance with a specifically designed Dual-Wave Reflection Interference Microscopy (DW-RIM) setup. The particle has two equilibrium positions located at few hundreds of nanometers from the interface. The farthest position is well accounted by a DLVO model complemented by gravity. The closest one, not predicted by current models, more frequently appears in water solutions at relatively high ions concentrations, when electrostatic interaction is screened out. It is accompanied by a frozen rotational diffusion dynamics that suggests an interacting potential dependent on particle orientation and stresses the decisive role played by particle surface heterogeneities. Building up on both such experimental results, the important role of air nanobubbles pinned on the particle interface is discussed.

Two-Dimensional Nonclose-Packed Colloidal Crystals by the Electrostatic Adsorption of Three-Dimensional Charged Colloidal Crystals.

We demonstrate that nonclose-packed two-dimensional (2D) colloidal crystals fixed on flat solid surfaces can be obtained by the electrostatic adsorption of three-dimensional (3D) charged colloidal crystals onto oppositely charged substrates. 3D colloidal crystals of negatively charged polystyrene (diameter d = 500 nm) and silica (d = 510 and 550 nm) particles were formed in their aqueous dispersions. Then, a single layer of the 3D crystals (the particle volume fraction = ∼0.07-0.3) was adsorbed onto a glass surface, which was earlier modified with 3-aminopropyltriethoxysilane (APTES), a cationic silane coupling reagent. Under salt-free conditions, the lowermost layer of the 3D crystals, which was oriented parallel to the substrate, was adsorbed onto the substrate surface, forming 2D crystals. Centimeter-sized, large 2D silica crystals were produced by combining a unidirectional 3D crystallization of the silica colloid under a base concentration gradient and a unidirectional adsorption under an acidic concentration gradient, which allowed tuning of the charge number on the APTES-modified substrate. The interparticle separations of the resulted 2D crystals did not vary greatly (within 5%) over a large area (length: 2 cm); however, the separations were smaller than the initial value because of gravitational sedimentation. We also produced 2D crystals of gold particles (d = 250 nm), which we expect to be applicable as plasmonic materials. The present study will provide a facile strategy to produce nonclose-packed 2D colloidal crystals of various types of particles, including large and high-density particles.

Mucus penetrating properties of soft, distensible lipid nanocapsules

Designing nanomaterials to release their drug pay-load upon exposure to an exogenous trigger can help to direct drug delivery, but how the triggered release, which often modifies the nanomaterial properties, influences the biological fate of these systems is currently unknown. The aim of this study was to investigate how the triggered drug release from PEG coated, soft, 50 nm distensible lipid nanocapsules (LNC) influenced their diffusion across a mucus barrier. The translocation speed of the non-triggered LNC across a 35 µm thick purified gastric mucin (PGM) barrier was 3 times faster (30.08 ± 2.49 × 10−10 cm2 s−1) compared to equivalent-sized negatively charged polystyrene particles (9.87 ± 0.61 × 10−10 cm2 s−1, p < 0.05). In cystic fibrosis mucus (CFM), harvested from patient primary cells, the non-triggered LNC translocation speed was similar to the PGM, but the polystyrene particle diffusion was so slow it could not be measured. The trigger induced LNC distension process had no effect on the particle diffusion rate in both PGM and CFM (p > 0.05) in a static mucus barrier, but when shear was applied to the barrier the distended LNCs diffused more slowly (3.97 ± 1.38 × 10−8 cm2 s−1, p < 0.05) compared to the non-distended materials (4.94 ± 0.04 × 10−8 cm2 s−1). This data suggested the rapid mucus penetration of the distended LNCs, despite their increased size, was a consequence of their capacity to take a less tortuous path through the barrier, i.e., they experienced less steric hinderance, compared to the non-distended LNC.

An experimental/theoretical method to measure the capacitive compactness of an aqueous electrolyte surrounding a spherical charged colloid.

The capacitive compactness has been introduced very recently [G. I. Guerrero-García et al., Phys. Chem. Chem. Phys. 20, 262-275 (2018)] as a robust and accurate measure to quantify the thickness, or spatial extension, of the electrical double layer next to either an infinite charged electrode or a spherical macroion. We propose here an experimental/theoretical scheme to determine the capacitive compactness of a spherical electrical double layer that relies on the calculation of the electrokinetic charge and the associated mean electrostatic potential at the macroparticle's surface. This is achieved by numerically solving the non-linear Poisson-Boltzmann equation of point ions around a colloidal sphere and matching the corresponding theoretical mobility, predicted by the O'Brien and White theory [J. Chem. Soc., Faraday Trans. 2 74, 1607-1626 (1978)], with experimental measurements of the electrophoretic mobility under the same conditions. This novel method is used to calculate the capacitive compactness of NaCl and CaCl2 electrolytes surrounding a negatively charged polystyrene particle as a function of the salt concentration.

The adsorption of polystyrene nanoparticles on selected commercially available fibers: a streaming potential study

The article describes the analysis of the adsorption properties of commercially available fibers. Negatively charged polystyrene particles were used for the adsorption on the selected fibers at different pH ranges of the used electrolyte. The streaming potential method proved useful in the assessment of the electrokinetic properties of the fibers. As expected, good adsorption was observed for most fibers that were positively charged in the conditions of the experiments due to electrostatic attraction. Interestingly, for some fibres an increase in zeta potential was observed in the conditions in which both the adsorbent and the adsorbate were negatively charged. An explanation was proposed for this phenomenon, but could not be proved. The adsorption in the conditions of the isoelectric points of the fibers resulted in little to no changes in their zeta potential in most cases. Moreover, in some cases, the adsorption study produced scattered and irreproducible results. This behavior may have its origin in the nature of the selected fibers, which were mostly technical fibers for applications in fields other than adsorption.

Fabrication of a single sub-micron pore spanning a single crystal (100) diamond membrane and impact on particle translocation

The fabrication of sub-micron pores in single crystal diamond membranes, which span the entirety of the membrane, is described for the first time, and the translocation properties of polymeric particles through the pore investigated. The pores are produced using a combination of laser micromachining to form the membrane and electron beam induced etching to form the pore. Single crystal diamond as the membrane material, has the advantages of chemical stability and durability, does not hydrate and swell, has outstanding electrical properties that facilitate fast, low noise current-time measurements and is optically transparent for combined optical-conductance sensing. The resulting pores are characterized individually using both conductance measurements, employing a microcapillary electrochemical setup, and electron microscopy. Proof-of-concept experiments to sense charged polystyrene particles as they are electrophoretically driven through a single diamond pore are performed, and the impact of this new pore material on particle translocation is explored. These findings reveal the potential of diamond as a platform for pore-based sensing technologies and pave the way for the fabrication of single nanopores which span the entirety of a diamond membrane.

Colloidal Crystals with Low Crack Densities Formed on Polymer Hydrogel Surfaces

We report that close-packed colloidal crystals with low crack densities could be formed by evaporation method using polymer hydrogels as substrates. Aqueous dispersions of charged polystyrene particles (diameter = 600 nm) were dried on polyacrylamide gels. No distinct crack was observed in the resulting crystals, though many cracks (an area fraction ≈ 8%) were formed when glass substrates were used. This difference appears to be attributable to much longer crystallization time on the gel surface, due to continuous supply of water from inside the gel.

The Investigation of Dynamic Changes of the Particle Surface Charge with Resistive-Pulse Technique

In the resistive-pulse technique, individual particles passing through a pore cause transient changes of the transmembrane current, and the amplitude of the current change corresponds to the object size. This method has been applied to detect cells, viruses, molecules (e.g. DNA and proteins) and particles. With the special structure of track-etched polycarbonate pores, i.e. narrower entrances and cylindrical interior, the events of highly carboxylated charged polystyrene particles can show a current decrease when a particle enters the pore, a flat region in the center part, and a current increase when the particle exits the pore. The current increase results from concentration polarization as well as ionic sourcing effects, which depend on the particle surface charge density. Thus, the characteristics of the obtained resistive-pulse signal carry information not only on the size of the objects, but also on dynamic changes of the particle surface charge via protonation/deprotonation of surface carboxyl groups. Experiments were performed with a pH gradient set across the pore, so that the particles while translocating would change their effective surface charge due to the dependence of protonation of carboxyl groups on the local proton concentration. Analysis of the current increase at the end of the event revealed that the process of deprotonation/protonation required longer time than the transit time. This finding was unexpected, because the transit times were tens of milliseconds thus significantly longer then the chemical reaction time estimated based on proton diffusion. Our experiments also indicated that a longer pore and a higher pH gradient led to a more complete protonation/deprotonation in the pore during transit. Using our finding, we designed a protocol to trap individual particles in a pore for as long as 3 seconds. The experiments are important for developing new methods which can be used to in situ characterize and precisely control single molecules and particles.

Dynamic surface properties of mixed monolayers of polystyrene micro- and nanoparticles with DPPC

The dynamic surface properties of mixed monolayers of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and negatively charged polystyrene particles (PS) of different size were investigated with the aim to model the interactions of particles with biointerfaces. Special attention was paid to the aggregation of the particles at the water/air interface. The particle aggregates have a weaker influence on the dynamic properties of DPPC monolayers as compared with non-aggregated particles because of a shorter three phase contact line. The incorporation of particles into a DPPC monolayer leads to disordering of the monolayer structure, decreases the changes of surface properties in the course of two-dimensional phase transitions and the dynamic surface elasticity of the condensed lipid film. The influence of particles on the properties of mixed monolayers depends strongly on the ratio of lipid molecules and particles, and decreases with the decrease of particle surface concentration. The Brewster angle microscopy gives direct evidence of the PS particle aggregation in the course of compression of mixed monolayers.

Osmotic Bulk Modulus of Charged Colloids Measured by Ensemble Optical Trapping.

The optical-bottle technique is used to measure osmotic bulk moduli of colloid suspensions. The bulk modulus is determined by optically trapping an ensemble of nanoparticles and invoking a steady-state force balance between confining optical-gradient forces and repulsive osmotic-pressure forces. Osmotic bulk moduli are reported for aqueous suspensions of charged polystyrene particles in NaCl solutions as a function of particle concentration and ionic strength, and are compared to those determined by turbidity measurements under the same conditions. Effective particle charges are calculated from the bulk moduli and are found to increase as a function of ionic strength, consistent with previously reported results.