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Clogging is ubiquitous. It happens in a wide range of material processing and causes severe performance degradation or process breakdown. In this study, we investigate clogging dynamics in a single micropore by controlling the surface property of the particle and processing condition. Microfluidic observation is conducted to investigate particle deposition in a contraction microchannel where polystyrene suspension is injected as a feed solution. The particle deposition area is quantified using the images taken using a CCD camera in both upstream and downstream of the microchannel. Pressure drop across the microchannel is also measured. When the particle interaction is repulsive, the deposition occurs mostly in downstream, while an opposite tendency is identified when the particle interaction is attractive. More complex deposition characteristics are found as the flow rate is changed. Particle flux density and the ratio of lift force to colloidal force are introduced to explain the clogging dynamics. This study provides a useful insight to alleviate clogging issues by controlling the colloidal interaction and hydrodynamic stress.

Deformation and breakup of a single agglomerate exposed to pure planar extensional flow in a cross-channel were experimentally investigated. Aggregation was generated by applying shear with destabilized polystyrene particles, and the fractal dimension, df, of the agglomerate was 2.25. The aggregation focused on the center of the channel by sheath flow was rotated while approaching stagnant point. Then, the aspect ratio increased as it deformed close to the stagnant point. The probability of the breakup and the fragment distribution were dependent upon the viscosity and flow rate and were superimposed on a master curve as a function of applied stress. With the increase in stress, the projected area of the fragment that was split by the flow decreased with a power-law relationship, and the exponent was in agreement with the model prediction.

A systematic study has been performed to investigate the effect of surface potential of nanoparticles on the rheological behavior of bimodal suspensions, using a model system consisting of polystyrene latex (primary size ∼530 nm) and alumina-coated silica (primary size ∼12 nm) particles. The surface potential of small particles was tuned by varying the solution pH, causing them to be repulsive to each other, attractive to each other, and oppositely charged to the large particles, while the large particles remained electrostatically stabilized. We found that the rheological properties could be dramatically changed from viscous to gel-like depending on the surface potential and concentration of small particles. A colloidal gel was induced by small particles when the small particles had the opposite charge to the large particles and a volume fraction of 10(-4)ϕsmall10(-3), and when the small particles were attractive to each other above a critical threshold, ϕsmall10(-4). Cryo-SEM distinguished the gel structures to be either short bridging gels produced by oppositely charged small particles or long bridging gels or dense gels produced by attractive small particles. On the basis of this rheological behavior and microstructure, we prepared a phase diagram of highly size-asymmetric bimodal colloids with respect to the surface chemistry and concentration of small particles.

Convective assembly is a coating method to fabricate thin films with ordered particle structures that can be used extensively for biochemical sensors, data storage devices, optical devices, and other applications. The fluid flow into or through the close-packed region causes the convective assembly, and it is important to understand the formation mechanism of the close-packed region. In this paper, the length of the close-packed region was predicted, and the dimensionless coating thickness as well as the dimensionless length of the close-packed region was found to be the functions of only three dimensionless variables: two capillary numbers and the initial volume fraction. From the modeling results, coating process regime maps that predict the dimensionless coating thickness in terms of the dimensionless variables were created. In addition, the length of the close-packed region was measured under various coating conditions to validate the model prediction. The experiments firmly supported the model predictions.

We used real-time fluorescence microscopy to investigate the migration of latex particles in drying battery slurries. The time evolution of the fluorescence signals revealed that the migration of the latex particles was suppressed above the entanglement concentration of carboxymethyl cellulose (CMC), while it was significantly enhanced when CMC fully covered the surfaces of the graphite particles. In particular, a two-step migration was observed when the graphite particles flocculated by depletion attraction at high CMC/graphite mass ratios. The transient states of the nonadsorbing CMC and graphite particles in a medium were discussed, and the uses of this novel measurement technique to monitor the complex drying processes of films were demonstrated.

The particle/polymer/solvent suspension system shows complicated microstructure. When the suspension system experiences an industrial process such as coating and drying, the system experiences microstructural change. In this study, we investigated the microstructural change during the drying of a silica/polyvinyl alcohol (PVA) suspension, with an emphasis on suspension stability. We controlled the amount of PVA adsorption on the silica surface by adjusting the pH (1.5, 3.6, and 9) of the silica/PVA suspension. The amount of adsorption was measured to increase with decreasing pH, and the degree of flocculation in the silica/PVA suspension became stronger with decreasing pH. However, through the measurement of stress development during drying and the observation of film microstructure after drying, we found that the more strongly flocculated suspension became a more disperse, close-packed film after drying. By evaluating the potential energy, we could suggest the role of adsorbed polymers in structural change during the drying of the silica/PVA suspension. As pH decreases, the adsorbed polymers could bridge the particles and lead to a flocculated suspension before drying. As the solvent evaporates during drying, the adsorbed polymers introduce steric repulsion between approaching particles, leading to a change from flocculated to dispersed microstructure. This implies that the required silica/PVA film performance and the microstructure of the silica/PVA suspension can be tailored through controlling the polymer adsorption in suspension.

We control the hydrophobicity of submicrometer silica spheres by modifying their surface with -CH3, -CH=CH2, -(CH2)(2)CH3, -CH2(CH2)(4)CH2-, -C(6)H(5), -(CH2)(7)CH3, and -(CH2)(11)CH3 groups through a modified one-step process. The scanning electron microscopy (SEM), quasi-elastic light scattering (QELS), UV-visible spectra, nitrogen sorption, and water vapor adsorption methods are used to characterize the particles. The SEM micrographs of the particles demonstrate that the modified particles are uniformly spherical, monodisperse, and well-shaped with the particle size ranging from 130 to 149 nm depending on the modified organic groups. In aqueous solution, the particles modified with phenyl groups have an obvious UV absorption peak at around 210 nm, whereas the other modified particles and unmodified particles do not have any UV-visible absorption peaks. There exist obvious differences in the amount of water vapor adsorbed depending on the type of surface functional groups of the modified particles. Compared with the unmodified particles, the modified particles have a lower water vapor adsorption because of the improved hydrophobicity of the particle surface. As a potential application, we prepared polystyrene/SiO2 nanocomposites by blending polystyrene with the synthesized particles. Water contact angle measurements show that the surface of the composite prepared with the modified particles are more hydrophobic. Confocal microscopy demonstrates that the particles are less agglomerated in the nanocomposite as the particles become more hydrophobic. These comprehensive experimental results demonstrate that the hydrophobicity of the particles can be easily controlled by surface modification with different organosilanes through a modified one-step process.