A new paradigm of materials processing—heterogeneity control

Effect of Colloidal Interactions and Hydrodynamic Stress on Particle Deposition in a Single Micropore

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.

Stratification Mechanism in the Bidisperse Colloidal Film Drying Process: Evolution and Decomposition of Normal Stress Correlated with Microstructure

The evolution of the normal stress and microstructure in the drying process of bidisperse colloidal films is studied using the Brownian dynamics simulation. Here, we show that the formation process of small-on-top stratification can be explained by normal stress development. At high Pe_L's, a stratified layer with small particles is formed near the interface. The accumulated particles near the interface induce the localization of normal stress so that the normal stress at the interface increases from the beginning of drying. We analyze this stress development from two points of view, on the global length scale and particle length scale. On the global length scale, the localization of normal stress is quantified by the scaled normal stress difference between the interface and substrate. For all Pe_L's tested in this study, the scaled normal stress difference increases until the accumulation region reaches the substrate. After the maximum, the stress difference remains at the maximum at lower Pe_L's, while it decreases at higher Pe_L's. The microstructural analysis shows that this stress development is explained through the evolution of the particle contact number distribution at the interface and substrate. On the particle length scale, we derive the scaled local force applied to each type of particle by decomposing the local normal stress. At high Pe_L's, the scaled local force for the large particle is large compared to that for the small particle near the interface, indicating that the large particles are strongly pushed away from the interface. Associating the volume fraction profile with the local force field, we suggest that the strong scaled force for the large particle is attributed to the significant increase in the average number of small particles in contact with large ones. This study has significance in probing the drying mechanism of bidisperse colloidal films and the stratification mechanism.

Role of PVDF in Rheology and Microstructure of NCM Cathode Slurries for Lithium-Ion Battery

A binder plays a critical role in dispersion of coating liquids and the quality of coating. Poly(vinylidene fluoride) (PVDF) is widely used as a binder in cathode slurries; however, its role as a binder is still under debate. In this paper, we study the role of PVDF on the rheology of cathode battery slurries consisting of Li(Ni1/3Mn1/3Co1/3)O2 (NCM), carbon black (CB) and N-methyl-2-pyrrolidone (NMP). Rheology and microstructure of cathode slurries are systemically investigated with three model suspensions: CB/PVDF/NMP, NCM/PVDF/NMP and NCM/CB/PVDF/NMP. To highlight the role of PVDF in cathode slurries, we prepare the same model suspensions by replacing PVDF with PVP, and we compare the role of PVDF to PVP in the suspension rheology. We find that PVDF adsorbs neither onto NCM nor CB surface, which can be attributed to its poor affinity to NCM and CB. Rheological measurements suggest that PVDF mainly increases matrix viscosity in the suspension without affecting the microstructure formed by CB and NCM particles. In contrast to PVDF, PVP stabilizes the structure of CB and NCM in the model suspensions, as it is adsorbed on the CB surface. This study will provide a useful insight to fundamentally understand the rheology of cathode slurries.

Orthogonal superposition rheometry of colloidal gels: time-shear rate superposition

We explore the relaxation behavior of model colloidal gels under steady shear flow by means of orthogonal superposition rheometry. Fumed silica and carbon black dispersions in Newtonian matrices are used as a model system. As shear rate increases, the frequency dependent orthogonal moduli of the gels shift along the frequency axis without changing their shape, which finally can be superimposed to yield a single master curve. This indicates that the shear rate tunes a master clock for overall relaxation modes in the sheared colloidal gels to produce a "time-shear rate superposition (TSS)", as temperature does in polymeric liquids to produce a time-temperature superposition (TTS). The horizontal shift factor required at each shear rate to obtain the master curve is found to be directly proportional to the suspension viscosity for all the cases. From this result, we suggest that the suspension viscosity determines the overall relaxation time of the particles in the flowing colloidal gel.

Agglomerate Breakup of Destabilized Polystyrene Particles under a Cross-Channel Planar Extensional Flow

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, d_f, 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 critical and quantitative review of the stratification of particles during the drying of colloidal films

For a wide range of applications, films are deposited from colloidal particles suspended in a volatile liquid. There is burgeoning interest in stratifying colloidal particles into separate layers within the final dry film to impart properties at the surface different to the interior. Here, we outline the mechanisms by which colloidal mixtures can stratify during the drying process. The problem is considered here as a three-way competition between evaporation of the continuous liquid, sedimentation of particles, and their Brownian diffusion. In particle mixtures, the sedimentation of larger or denser particles offers one means of stratification. When the rate of evaporation is fast relative to diffusion, binary mixtures of large and small particles can stratify with small particles on the top, according to physical models and computer simulations. We compare experimental results found in the scientific literature to the predictions of several recent models in a quantitative way. Although there is not perfect agreement between them, some general trends emerge in the experiments, simulations and models. The stratification of small particles on the top of a film is favoured when the colloidal suspension is dilute but when both the concentration of the small particles and the solvent evaporation rate are sufficiently high. A higher particle size ratio also favours stratification by size. This review points to ways that microstructures can be designed and controlled in colloidal materials to achieve desired properties.