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The design of flow processes to build a macroscopic bulk material from rod-shaped colloidal particles has drawn considerable attention from researchers and engineers. Here, we systematically explore and show that the characteristic strain rate of the flow universally determines the orientational ordering of colloidal rods. We employed the fluctuating lattice Boltzmann method by simulating hydrodynamically interacting Brownian rods in a Newtonian liquid moving under various flow types. By modeling a rigid rod as a chain of nonoverlapping solid spheres with constraint forces and torque, we elucidate rigid rod dynamics with an aspect ratio (L / d) either 4.1 or 8.1 under various rotational Péclet number (P e r) conditions. The dynamics of colloidal rods in dilute (n L 3 = 0.05) and semidilute suspensions (n L 3 = 1.1) were simulated for a wide range of P e r (0.01 < P e r < 1000) under shear flows including Couette and Poiseuille flows in a planar channel geometry, and an extensional and mixed-kinematics flow in a periodic four-roll mill geometry, where n is the number density, and d and L are the diameter and length of the rod, respectively. By evaluating the degree of orientational alignment of rods along the flows, we observed that there is no significant difference between flow types, and the flow-induced ordering of rods depends on the variation of P e r up to moderate P e r (P e r < 100). At a high P e r (P e r > --> 100), the degree of orientational ordering is prone to diversify depending on the flow type. The spatial inhomogeneity of the strain-rate distribution leads to a substantial decrease in the orientational alignment at high P e r. [ABSTRACT FROM AUTHOR]

The relationship between the coating bead flow dynamics and the profiles of patch coating films in intermittent slot coating processes was investigated using two-dimensional numerical simulations. In the continuous production of discrete coating patches, the start (rising) and end (falling) edges in the flow direction must be precisely analyzed, considering the liquid’s rheological properties and the residual liquid remaining in the coating bead region between on–off flow control stages. The residual volume, which significantly affects both the start and end edges for Newtonian and non-Newtonian liquids, was found to remain in the coating bead region only above a certain capillary number (Ca) of 0.1, with an increasing trend as Ca increases. The growth of the start edge height of coating patches was greatly influenced by the residual volume. In contrast, the end edge length was inversely proportional to the residual volume concerning Ca. Furthermore, for shear-thinning liquids with various power-law indices (n), a certain Ca value, similar to that for the Newtonian case, was confirmed to induce the occurrence of residual volume.Graphical abstract: The relationship between the coating bead flow dynamics and the profiles of patch coating films in intermittent slot coating processes was investigated using two-dimensional numerical simulations. In the continuous production of discrete coating patches, the start (rising) and end (falling) edges in the flow direction must be precisely analyzed, considering the liquid’s rheological properties and the residual liquid remaining in the coating bead region between on–off flow control stages. The residual volume, which significantly affects both the start and end edges for Newtonian and non-Newtonian liquids, was found to remain in the coating bead region only above a certain capillary number (Ca) of 0.1, with an increasing trend as Ca increases. The growth of the start edge height of coating patches was greatly influenced by the residual volume. In contrast, the end edge length was inversely proportional to the residual volume concerning Ca. Furthermore, for shear-thinning liquids with various power-law indices (n), a certain Ca value, similar to that for the Newtonian case, was confirmed to induce the occurrence of residual volume. [ABSTRACT FROM AUTHOR]

In our previous work [Chun et al., Phys. Fluids 29, 121605 (2017)], we studied the migration behavior of concentrated monodisperse particles in Couette–Poiseuille flows. Here, we extend this study into an investigation of concentrated (ϕ 0 = 0.3) bidisperse suspensions undergoing pressure-driven flows at various combinations of volume fraction ratio (ϕ S 0 / ϕ 0 = 0.13 − 0.67) and size ratio (a L / a S = 1.4 − 2.4) between small (2 a S / H = 0.022) and large (2 a L / H = 0.031 − 0.052) particles in a planar channel of height H. The flow behavior of neutrally buoyant hard spheres suspended in a Newtonian fluid was numerically simulated using the lattice Boltzmann method. The time evolution of the distributions of small and large particles showed segregation under inhomogeneous shear flow, resulting in enrichment of large particles and depletion of small particles near the midplane. Size-dependent particle segregation was more pronounced in these simulations than what was previously observed in the experiment. In bidisperse suspensions, the concentration evolution was markedly slower than in monodisperse situations; the characteristic time to reach a steady state was 5–8 times longer in bidisperse systems. The results of lattice Boltzmann simulation were further analyzed within the framework of a reformulated polydisperse diffusive flux model [Shauly et al., J. Rheol. 42, 1329 (1998)]; four coefficients of the diffusive flux model were adjusted to match steady-state (K L , K S) and transient (R L , R S) predictions. The model coefficients showed strong dependence on the particle number density ratio (ϕ S 0 / ϕ L 0) (a L / a S) 3 ; both K L and R L increased almost linearly with this ratio, whereas K S decreased and R S increased with this ratio. [ABSTRACT FROM AUTHOR]

The coating bead flow and operability window for Newtonian coating liquids are theoretically and experimentally investigated in the slot coating process, with a focus on the shape of the upstream meniscus and contact angles. From the flow visualization in the coating bead region, the contact angles of the upstream meniscus were measured by changing the flow rate and web speed under uniform operating conditions. It was confirmed that the dynamic contact angle is closely related to the capillary number in this process, based on the Hoffman-Voinov-Tanner model. The viscocapillary and two-dimensional Navier-Stokes models using the experimentally observed contact angles accurately predicted the coating bead dynamics and operability windows for two Newtonian liquids. [ABSTRACT FROM AUTHOR]

Critical onsets for draw resonance instability occurring in two-dimensional (2-D) film casting processes with Newtonian and viscoelastic (Upper-Convected Maxwell and Phan-Thien and Tanner) fluids have been newly determined using a transient frequency response method. Under a constant tension condition, which guarantees always stable operation, the trajectory of a transfer function between the output take-up velocity and input tension in a Nyquist plot has been used as an indicator for finding draw resonance onsets. Various stability windows for Newtonian and viscoelastic fluids have been constructed, confirming that the onsets were well-predicted when compared with transient responses in actual velocity-controlled operating systems around the onsets. Interestingly, up-and-down stability patterns along the aspect ratio in the Newtonian cases were found to be closely related to the flow deformation features of the fluid elements within the film width. Dichotomous stability behaviors for extensional-thickening and extensional-thinning fluids were well addressed in the 2-D processes. [ABSTRACT FROM AUTHOR]

• Effect of inlet velocity condition on film formation and process stability was numerically analyzed. • Lowing edge inlet velocity not only mitigated edge-beads but also increased the draw resonance onsets. • Process stability is greatly related to the level of extensional stress and the deformation type at the film edge. The impact of the inlet velocity profile at the die exit on the film dynamics within the air-gap region was investigated using numerical simulations for both Newtonian and viscoelastic Phan-Thien and Tanner (PTT) fluids in isothermal two-dimensional (2-D) film casting processes. In an industrial context, intentional adjustments were made to reduce the inlet velocities at the edge of the casting die, effectively mitigating the edge-beads characterized by a higher edge thickness than the center thickness of the final films. By varying the inlet velocity conditions with decreasing edge velocities, the steady film dynamics were correlated with the onsets of draw resonance instability and frequency responses to a disturbance, which were determined using the transfer function data obtained under tension-controlled conditions. The results revealed that decreasing the inlet velocity at the edge improved not only the formation of films but also the process stability for both Newtonian and viscoelastic fluids. In addition, the sensitivity or frequency response to a disturbance was effectively reduced by decreasing the inlet velocity at the edge. This observed impact of the inlet velocity was closely related to the increased tension levels at take-up and a greater portion of the neck-like deformation type in the air-gap region. [ABSTRACT FROM AUTHOR]

The viscocapillary model for predicting the operability windows in a dual-slot coating process for two Newtonian liquids has been derived by means of a lubrication approximation technique from two-dimensional (2-D) Navier-Stokes equations. The flow rates per width and the pressure differences simplified in several sub-regions within the coating bead regime have been integrated together with approximations of the upstream and downstream menisci and the interlayer between the top and bottom layers. Coating limits or onsets for leaking and bead break-up defects have been determined on the basis of the position of upstream meniscus of the bottom layer as an indicator. Operability coating windows predicted from the viscocapillary model show a good agreement with those by 2-D CFD solver and experiments. In addition, the pressure profiles within the coating bead regime compared between the viscocapillary and 2-D models are qualitatively similar. This simplified model can be applied as a fast tool for usefully manipulating the stable operations in dual-layer slot coating systems. [ABSTRACT FROM AUTHOR]

Abstract: Slot coating, indispensable to the manufacturer of flat panel displays and long-life secondary batteries, can be susceptible to unexpected disturbances at high speeds, leading to many kinds of undesirable defects. Operability coating windows for both Newtonian and non-Newtonian (shear-thinning) liquids have been investigated using a simplified viscocapillary model in a slot coating bead flow regime. Stable coating windows, free from leaking (or dripping) and bead break-up, have been determined by the position of upstream meniscus. They quantitatively coincided with those from two-dimensional calculations by a CFD Fluent solver. The pressure range that allowed a stable bead widened as the viscosity of the coating liquid or the capillary number in downstream die region increased. Also, the sensitivity of the slot coating flow through frequency response method was tested by measuring the amplitude of final wet coating thickness with respect to ongoing sinusoidal disturbances at different frequencies imposed to web speed, flow rate, bead pressure and coating gap. The viscocapillary model was compared with a 2D model and was found to be a fast and efficient tool that could enhance the productivity and processability of coating systems. [Copyright &y& Elsevier]

Abstract: The nonlinear transient dynamics and stability of a two-dimensional (2-D) model of the isothermal film casting process have been investigated using newly devised simulation techniques employing finite element methods (FEM), which can incorporate the viscoelastic nature of polymer melts as well as the Newtonian features of other fluids. To effectively solve the time-dependent free surface problem of this system, the Arbitrary Lagrangian Eulerian (ALE) algorithm together with a spine method well-suited for free surface tracking was incorporated. Adopting robust numerical stabilization techniques for the hyperbolic system of extensional deformation processes in this study, the transient dynamic behavior of viscoelastic fluids in the 2-D film casting process are presented for the first time in the literature. It is also demonstrated that simulation models in the present study can successfully describe the basic flow behavior of the system including the three instability modes therein, i.e., draw resonance, neck-in, and development of edge beads. [Copyright &y& Elsevier]