Applied Shear Flow Research Articles

Drop Retention and Departure in Adiabatic Shear Flow on Structured Superhydrophobic Surfaces.

Drops are retained or held on surfaces due to a retention force exerted on the drop by the surface. This retention force is a function of the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When external or body forces exceed the retention force, the drop begins to move. This work explores the conditions for which drop departure occurs on structured superhydrophobic surfaces in the presence of an applied shear flow. Drop departure is explored for five microstructured superhydrophobic surfaces, one nanostructured carbon nanotube surface and one smooth hydrophobic surface. Surface solid fractions range from 0.05 to 1.00, and measured static contact angles range from 121° to 161°. Droplet volumes of 5, 10, 20, 30, 40, and 50 μL are tested on each surface. For each experiment, increasing air velocity is applied to a droplet placed on a surface until the droplet departs. High-speed imaging is used to track droplet base length, height, cross-section area (as viewed from the side) and advancing/receding contact angles. Measurements of drop advancing and receding contact angles are reported at the point of departure, with increasing contact angle hysteresis observed prior to departure. Contact angle hysteresis is observed to be a good indicator of droplet mobility. Measurements of the average air velocity over the height of the droplet are determined at the point of departure for all conditions. The measured air velocity shows strong dependence on the surface solid fraction, and the required shear flow velocity decreases as the surface solid fraction decreases. This is most pronounced at very low solid fractions. A coefficient of drag for the departing drops in shear flow is calculated and is shown to decrease with increasing Reynolds number.

Analysis of the number of topological defects in active nematic fluids under applied shear flow.

The number of topological defects in the shear flow of active nematic fluids is numerically investigated in this study. The evolution of the flow state of extensile active nematic fluids is explored by increasing the activity of active nematic fluids. Evidently, medium-activity active nematic fluids exhibit a highly ordered vortex lattice fluid state. However, high-activity active nematic fluids exhibit a meso-scale turbulent flow accompanied by topological defects. The number of topological defects (Ndef) increases with increasing shear Reynolds number (Res). Fluid viscosity strongly influences Ndef, while the influence of fluid density is relatively weak. Ndef decreases with increasing activity length scale (lζ) value. A small Res value strongly influences Ndef, whereas a large lζ value only weakly influences Ndef. As the activity increases, Ndef in contractile active nematic fluids becomes larger than that of extensile active nematic fluids.

Bacteria Tune a Trade-off between Adhesion and Migration to Colonize Surfaces under Flow

The bacterial colonization of surfaces is a ubiquitous process that shapes nature and profoundly affects human health. While much is known about the biology of this process, the pivotal interplay between physical environment and active bacterial micromechanics remains poorly understood. In fact, strong adhesion and high motility, both of which are essential for surface colonization, are two apparently contradictory goals, as they mutually obstruct each other. Here, we investigate how the human pathogen optimizes its behavior for colonization of surfaces under flow. From the analysis of the dynamics of fluorescently labeled type-IV pili, we construct a mathematical model that quantitatively connects individual motor dynamics with whole-cell motility and migration. The data analysis also reveals that cells upregulate the number of visible pili on surface contact, although individual pili do not display a measurable sensory response to surfaces. When applying shear flow, we unexpectedly find that robust sticking to a surface requires passive surface adhesion rather than pilus activity. Instead, pilus activity actually promotes cell detachment while enabling migration. Using genetic perturbations of the pilus apparatus, it is shown that wild-type cells achieve a trade-off between adhesion and migration by limiting the number of pili. Simulations reveal a generic underlying trait space, where, depending on the interplay of active and passive forces, adhesion and migration are either compatible or a trade-off is required for efficient bacterial surface colonization. The discovered adhesion-migration problem is paradigmatic of a broad class of piliated bacteria and may also have implications for other cells. Published by the American Physical Society 2024

Hyperreflective photonic crystals created by shearing colloidal dispersions at ultrahigh volume fraction

Colloidal crystallization serves as one of the most economic and scalable production methods for photonic crystals. However, insufficient optical performance, nonuniformity and low reproducibility remain challenges for advanced high-value applications. In this study, we optimally formulate a photocurable dispersion of silica particles and apply shear flow to unify the orientation of the colloidal crystals, ensuring high optical performance and uniformity. The silica particles experience strong repulsion at ultrahigh volume fractions of 50% but demonstrate low mobility, leading to polycrystalline structures. Applying shear flow to the dispersions allows the silica particles to rearrange into larger crystalline domains with a unidirectional orientation along the flow. This shear-induced structural change produces absolute reflectivity at the stopband as high as 90% and a high transparency of 90% at off-resonant wavelengths with minimal diffusive scattering. Furthermore, the strong interparticle repulsion ensures a uniform volume fraction of particles throughout the dispersion, reducing deviations in the optical properties. We intricately micropattern the photocurable dispersions using photolithography. Additionally, the photonic films and patterns can be stacked to form multiple layers, displaying mixed structural colors and multiple reflectance peaks without sacrificing reflectivity. These superior photonic materials hold promise for various optical applications, including optical components and anticounterfeiting patches.

Shear-induced vorticity aligned flocs in a temperature responsive colloid-polymer mixture

Shear driven patterning is seen in many soft matter systems. We use rheology and optical microscopy to probe the structures formed when we shear a colloid-polymer mixture containing temperature-sensitive microgel particles. By increasing the temperature, we can increase the particle attraction and transition from liquid-like to gel-like behavior. And by applying shear flow to the sample as the temperature and, hence, state of the system changes, we can affect the morphology of mesoscopic colloidal clusters. We can produce gels comprised of fibrous, elongated colloid-dense clusters, or we can form more isotropic clusters. The rheology is measured and shear-induced flocculation observed for colloid-polymer systems with different cluster morphologies. At shear rates high enough to produce elongated clusters but low enough to not break clusters apart, we observe log-like flocs that are aligned with the vorticity direction and roll between the parallel plates of our rheometer.

Hollow Filaments Synthesized by Dry-Jet Wet Spinning of Cellulose Nanofibrils: Structural Properties and Thermoregulation with Phase-Change Infills.

We use dry-jet wet spinning in a coaxial configuration by extruding an aqueous colloidal suspension of oxidized nanocellulose (hydrogel shell) combined with airflow in the core. The coagulation of the hydrogel in a water bath results in hollow filaments (HF) that are drawn continuously at relatively high rates. Small-angle and wide-angle X-ray scattering (SAXS/WAXS) reveals the orientation and order of the cellulose sheath, depending on the applied shear flow and drying method (free-drying and drying under tension). The obtained dry HF show Young’s modulus and tensile strength of up to 9 GPa and 66 MPa, respectively. Two types of phase-change materials (PCM), polyethylene glycol (PEG) and paraffin (PA), are used as infills to enable filaments for energy regulation. An increased strain (9%) is observed in the PCM-filled filaments (HF-PEG and HF-PA). The filaments display similar thermal behavior (dynamic scanning calorimetry) compared to the neat infill, PEG, or paraffin, reaching a maximum latent heat capacity of 170 J·g–1 (48–55 °C) and 169 J·g–1 (52–54 °C), respectively. Overall, this study demonstrates the facile and scalable production of two-component core-shell filaments that combine structural integrity, heat storage, and thermoregulation properties.

Production of Plant-Based Seafood: Scallop Analogs Formed by Enzymatic Gelation of Pea Protein-Pectin Mixtures.

This study investigated the possibility of using a phase separation, mixing, and enzymatic gelation approach to construct seafood analogs from plant protein-polysaccharide mixtures with properties mimicking real seafood. Heat-denatured pea protein (10%, w/w) and pectin (0–1%, w/w) were mixed to produce phase separated biopolymer blends. These blends were then subjected to mild shearing (350 rpm) to obtain fiber-like structures, which were then placed in molds and set by gelling the pea proteins using transglutaminase (2%, w/w). The appearance, texture, and cooking properties of the resulting scallop analogs were characterized and compared to those of real scallop. The presence of the pectin promoted the formation of a honeycomb structure in the scallop analogs, and microscopic orientation of the proteins was observed in the plane parallel to the applied shear flow. Lower pectin concentrations (0.5%, w/w) led to stronger gels with better water holding capacity than higher ones (1.0%, w/w). The appearance and texture of the plant-based scallop analogs were like those of real scallop after grilling, indicating the potential of using this soft matter physics approach to create plant-based seafood analogs. One of the main advantages of this method is that it does not require any expensive dedicated equipment, such as an extruder or shear cell technology, which may increase its commercial viability.

Polymer Stabilized Paraboloid Liquid Crystal Microlenses with Integrated Pancharatnam–Berry Phase

AbstractAspheric lenses reduce aberration and provide sharper images with improved spot size compared to spherical lenses. This paper demonstrates that applying shear flow can produce plano‐concave liquid crystal (LC) lens arrays with paraboloid aspheric profiles. The focal length of individual lenses, with a 0.2 mm aperture, decreases from 0.67 to 0.45 mm as the chiral dopant increases from 0 to 6 wt%. The focal length is also sensitive to the polarization state of the incoming light. The lenses are stabilized by photopolymerizing with 6 wt% of reactive monomer added to the LC. A qualitative explanation for the flow‐induced lens formation and the optical properties of the lenses is provided. The potential tunability of the lenses in various fields and their use as paraboloid reflectors are discussed.

Microfluidic system for applying shear flow to endothelial cells on culture insert with collagen vitrigel membrane

This study proposes a microfluidic system comprising a microfluidic channel and a cylindrical hole for holding a commercially available culture insert with a collagen vitrigel membrane. Because the bottom surface of the culture insert and top surface of the channel are in the same plane, the flow of the culture medium in the channel can be directly applied to cells on the bottom surface. Additionally, the culture insert is easily removable from the microfluidic system for various biological analyses in a standard protocol, making this system accessible to users who are not experts in engineering. To demonstrate the potential applications of the system, we confirmed that by applying the flow of the culture medium, the endothelial cells cultured on the bottom surface of the insert and myotubes cultured in the insert aligned depending on the flow direction. We believe that the microfluidic system will be a useful tool for morphological analyses of cells based on fluidic shear stress.

Orientation of Cellulose Nanocrystals Controlled in Perpendicular Directions by Combined Shear Flow and Ultrasound Waves Studied by Small-Angle X-ray Scattering

The combined effect of shear flow and ultrasound (US) waves on the dynamical structural orientations of cellulosic cholesteric liquid crystals was investigated by time-resolved, in situ small-angle X-ray scattering (SAXS). A dedicated channel-type shear flow/ultrasound cell for SAXS characterization was developed to simultaneously generate a shear flow-induced horizontal stress force and a US-induced vertical acoustic radiation force. The control of the alignment of anisometric cellulose nanocrystals (CNCs), with their director parallel to the ultrasonic wave direction of propagation, was revealed at an unprecedented nanometer scale. Concurrently, the application of shear flow induced a horizontal orientation of CNCs with their directors aligned along the velocity direction. By adjusting the level of simultaneously applied shear flow and US intensity to the CNC suspensions, it was possible to tune the direction and the level of orientation of the CNCs during a period of time. For a specific ratio of the applied shear rate to acoustic power, some transient orientation of the CNCs at an intermediate angle between horizontal and vertical directions was evidenced. Relaxation of these orientations’ phenomena upon cessation of flow and/or US was also highlighted.

Adsorption and self-assembly properties of the plant based biosurfactant, Glycyrrhizic acid

There is an increased interest in the use of natural surfactant as replacements for synthetic surfactants due to their biosustainable and biocompatible properties. A category of natural surfactants which are attracting much current interest is the triterpenoid saponins; surface active components found extensively in a wide range of plant species. A wide range of different saponin structures exist, depending upon the plant species they are extracted from; but regardless of the variation in structural details they are all highly surface active glycosides. Greater exploitation and application requires a characterisation and understanding of their basic adsorption and self-assembly properties. HypothesisGlycyrrhizic acid, extracted from Licorice root, is a monodesmosidic triterpenoid saponin. It is widely used in cosmetic and pharmaceutical applications due to its anti-inflammatory properties, and is an ingredient in foods as a sweetener additive. It has an additional attraction due to its gel forming properties at relatively low concentrations. Although it has attracted much recent attention, many of its basic surface active characteristics, adsorption and self-assembly, remain relatively unexplored. How the structure of the Glycyrrhizic acid saponin affects its surface active properties and the impact of gelation on these properties are important considerations, and to investigate these are the focus of the study. ExperimentsIn this paper the adsorption properties at the air–water interface and the self-assembly in solution have been investigated using by neutron reflectivity and small angle neutron scattering; in non-gelling and gelling conditions. FindingsThe adsorption isotherm is determined in water and in the presence of gelling additives, and compared with the adsorption behaviour of other saponins. Gelation has minimal impact on the adsorption; apart from producing a rougher surface with a surface texture on a macroscopic length scale. Globular micelles are formed in aqueous solution with modest anisotropy, and are compared with the structure of other saponin micelles. The addition of gelling agents results in only minimal micelle growth, and the solutions remain isotropic under applied shear flow.

Microstructural characterization of a star-linear polymer blend under shear flow by using rheo-SANS

We present an investigation into the dynamic relaxation mechanisms of a polybutadiene blend composed of a four-arm star (10 wt. %) and a linear polymer matrix in the presence of an applied shear flow. Our focus was the response of the star polymer, which cannot be unambiguously assessed via linear viscoelastic measurements since the signature of the star polymer can barely be detected due to the dominant contribution of the linear matrix. By utilizing small-angle neutron scattering (SANS) coupled with a Couette shear device and a deuterated matrix polymer, we investigated the dynamics of the minority star component of the blend. Our results confirm that the stars deform anisotropically with increasing shear rate. We have compared the SANS data with predictions from the well-established scattering adaptation of the state-of-the-art tube model for entangled linear polymer melts undergoing shear, i.e., Graham, Likhtman, Milner, and McLeish (GLaMM) approach, appropriately modified following earlier studies in order to apply to the star. This modified model, GLaMM-R, includes the physics necessary to understand stress relaxation in both the linear and nonlinear flow regimes, i.e., contour length fluctuations, constraint release, convective constraint release, and chain retraction. The full scattering signal is due to the minority star component and, although the contribution of the linear chains is hidden from the neutron scattering, they still influence the star polymer molecular dynamics, with the applied shear rate ranging from approximately 8 to 24 s−1, below the inverse relaxation time of the linear component. This study provides another confirmation that the combination of rheology and neutron scattering is an indispensable tool for investigating the nonlinear dynamics of complex polymeric systems.

Dynamics of phase separation of sheared inertialess binary mixtures

When a viscous binary mixture subject to an applied shear flow is rapidly quenched into the unstable region of its phase diagram, the resulting phase separation is influenced by two competing effects. On one hand, nuclei of the minority phase tend to grow with a constant growth rate, while, on the other hand, they are stretched along the flow direction, forming thinner and thinner layered domains that eventually break. We simulate the dynamics of this system with a thermodynamics-based diffuse interface model, accounting for the full interplay between hydrodynamics (i.e., the Navier–Stokes equations) and species conservation (i.e., the Cahn–Hilliard equation) coupled via the Korteweg body-force. We show that periodic steady-state configurations with stable droplets are obtained for low capillary numbers while phase separation takes place along bands oriented in the direction of the flow in the case of strong shear because, in the long term, diffusion in the cross-flow direction prevails on the convective flow field. The dynamics of phase separation is highly non-linear and diverse even for inertialess flow, featuring multiple coalescence and breakups: although some typical time scaling for the characteristic droplet size in the flow and cross-flow directions can be obtained, the full evolution cannot be characterized only by the capillary number. The wide range of droplet morphologies predicted by the model, from round and elongated shapes to bands and hollow droplets, suggests interesting applications for manufacturing of polymers and soft materials.

Visualizing protein motion in Couette flow by all-atom molecular dynamics

In living cells, biomacromolecules are exposed to a highly crowded environment. The cytoplasm, the nucleus, and other organelles are highly viscous fluids that differ from dilute in vitro conditions. Viscosity, a measure of fluid internal friction, directly affects the forces that act on immersed macromolecules. Although active motion of this viscous fluid – cytoplasmic streaming – occurs in many plant and animal cells, the effect of fluid motion (flow) on biomolecules is rarely discussed. Recently NMR experiments that apply a shearing flow in situ have been used for protein studies. While these NMR experiments have succeeded in spectroscopically tracking protein aggregation in real time, they do not provide a visual picture of protein motion under shear. To fill this gap, here we have used molecular dynamics simulations to study the motion of three proteins of different size and shape in a simple shearing flow. The proteins exhibit a superposition of random diffusion and shear-flow-induced rotational motion. Random rotational diffusion dominates at lower shear stresses, whereas an active “rolling motion” along the axis of the applied flow occurs at higher shear stress. Even larger shear stresses perturb protein secondary structure elements resulting in local and global unfolding. Apart from shear-induced unfolding, our results imply that, in an ideal Couette flow field biomolecules undergo correlated motion, which should enhance the probability of inter-molecular interaction and aggregation. Connecting biomolecular simulation with experiments applying shear flow in situ appears to be a promising strategy to study protein alignment, deformation, and dynamics under shear.

Emergence of Goldstone excitations in stress correlations of glass-forming colloidal dispersions

We compute the dynamic correlations of the nonlocal shear stress at the liquid to glass transition in colloidal dispersions. Whereas Eshelby's elastic pattern is recovered independently of the dynamics, the Goldstone modes of the colloidal glass are diffusive, , because the transverse momentum is not conserved —excluding propagating transverse sound. Precursors of both, the Goldstone modes and the long-ranged, anisotropic stress correlations, can be observed in the colloidal fluid. The slow diffusive mode dominates the dynamics in the supercooled liquid for wave numbers with a correlation length which grows like the square root of the relaxation time τ. Time- and space-dependent stress correlations are anisotropic, decaying like . These results are derived within a hydrodynamic theory, generalising Maxwell‘s model to finite wave numbers, after the dynamics has been decomposed into potentially slow modes, associated with the order parameter, and fast microscopic degrees of freedom. Alternatively, hydrodynamic equations, closed by constitutive laws, can be used to predict the linear response of stress to an applied shear flow.

Orthogonal co-cultivation of smooth muscle cell and endothelial cell layers to construct in vivo-like vasculature.

Development of a three-dimensional (3D) vascular co-cultivation system is one of the major challenges to provide an advanced analytical platform for studying blood vessel related diseases. To date, however, the in vivo-like vessel system has not been fully realized due to the difficulty of co-cultivation of the cells with orthogonal alignment. In this study, we report the utilization of microfabrication technology to construct biomimetic 3D co-cultured vasculature. First, microwrinkle patterns whose direction was perpendicular to the axis of a circular microfluidic channel were fabricated, and vascular smooth muscle cells (VSMCs) were cultured inside the microchannel, leading to an in vivo-like circumferential VSMC layer. Then, human umbilical vein endothelial cells (HUVECs) were co-cultured on the circumferentially aligned VSMC, and the success of double layer formation of HUVEC-VSMC in the circular microchannel could be monitored. After HUVEC cultivation, we applied shear flow in order to induce the orientation of HUVEC parallel to the axis, and the analysis of orientation angle and spreading area of HUVECs indicated that they were changed by shear stress to be aligned to the direction of flow. Thus, the HUVEC and VSMC layer could be aligned with a distinct direction. The expression level of VE-Cadherin located at the boundary of HUVECs implies in vivo-like vascular behavior. The proposed in vitro microfluidic vascular assay platform would be valuable for studying vascular diseases with high reliability due to in vivo-likeness.

Stochastic thermodynamics of a confined colloidal suspension under shear flow

Based on Brownian dynamics simulations, we investigate the thermodynamic signatures of non-equilibrium steady states in a confined colloidal suspensions under shear flow. Specifically, we consider a thin film consisting of charged particles in narrow slit-pore confinement, forming two layers with quadratic in-plane structure in equilibrium. This many-body system displays three distinct steady states, characterized by unique dynamical and rheological response to the applied shear flow. Calculating the work and heat, we find that both quantities indicate the different states by their mean and by their distributions. A particularly interesting situation occurs at large shear rates, where the particles perform a collective zig-zag motion. Here, we find a bistability regarding the degree of phase synchronization of the particle motion. It turns out that this bistability is key to understanding the resulting ensemble-averaged work distributions. For all states, we compare the work and heat distributions to those of effective single-particle systems. By this, we aim at identifying the many-body character of the stochastic thermodynamic quantities.

Critical Insights into the Effect of Shear, Shear History, and the Concentration of a Diluent on the Polymorphism in Poly(vinylidene fluoride)

The effect of shear, shear history, and addition of poly(methyl methacrylate) (PMMA) in inducing the β-polymorph in poly(vinylidene fluoride) (PVDF) was investigated systematically for melt-mixed PVDF/PMMA blends. Various techniques such as polarized optical microscopy (POM) coupled to a hot stage and a shear cell, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis, melt-rheology, and dielectric relaxation spectroscopy were used to gain a mechanistic insight for the observed polymorphism in PVDF. Different PVDF rich blends were prepared by melt-mixing and were subjected to different shear histories on a rheometer, and the induction time was monitored with respect to PMMA content and the applied shear flow in the blends. The rheological measurements revealed that the induction time was significantly lower for blends with a higher PVDF content (≥80 wt %), which was ascribed to the diluent effect of PMMA that restricts the chain mobility of PVDF and a longer time is required to start the crystallization process. The crystalline morphology observed from POM demonstrates that the growth rate of spherulites was greatly reduced with increasing PMMA content in the blends. FTIR results were used to determine the amount of β phase in the blends before and after the shear history. The blends that were sheared at high temperature (220 °C) showed more β phase than the blends that were sheared around the crystallization temperature. This study clearly demonstrates the fact that shear, shear history, and the content of PMMA significantly influence the conformational change that results in phase transformation in PVDF. Further, this study will help guide researchers working on various aspects of polymer processing where the effect of blending and shear on polymorphism is very important.

Harnessing Cooperative Interactions between Thermoresponsive Aptamers and Gels To Trap and Release Nanoparticles.

We use computational modeling to design a device that can controllably trap and release particles in solution in response to variations in temperature. The system exploits the thermoresponsive properties of end-grafted fibers and the underlying gel substrate. The fibers mimic the temperature-dependent behavior of biological aptamers, which form a hairpin structure at low temperatures (T) and unfold at higher T, consequently losing their binding affinity. The gel substrate exhibits a lower critical solution temperature and thus, expands at low temperatures and contracts at higher T. By developing a new dissipative particle dynamics simulation, we examine the behavior of this hybrid system in a flowing fluid that contains buoyant nanoparticles. At low T, the expansion of the gel causes the hairpin-shaped fibers to extend into the path of the fluid-driven particle. Exhibiting a high binding affinity for these particles at low temperature, the fibers effectively trap and extract the particles from the surrounding solution. When the temperature is increased, the unfolding of the fiber and collapse of the supporting gel layer cause the particles to be released and transported away from the layer by the applied shear flow. Since the temperature-induced conformational changes of the fiber and polymer gel are reversible, the system can be used repeatedly to "catch and release" particles in solution. Our findings provide guidelines for creating fluidic devices that are effective at purifying contaminated solutions or trapping cells for biological assays.

Triggers and signatures of shear banding in steady and time-dependent flows

This précis is aimed as a practical field guide to situations in which shear banding might be expected in complex fluids subject to an applied shear flow. Separately for several of the most common flow protocols, it summarizes the characteristic signatures in the measured bulk rheological signals that suggest the presence of banding in the underlying flow field. It does so both for a steady applied shear flow and for the time-dependent protocols of shear startup, step stress, finite strain ramp, and large amplitude oscillatory shear. An important message is that banding might arise rather widely in flows with a strong enough time dependence, even in fluids that do not support banding in a steadily applied shear flow. This suggests caution in comparing experimental data with theoretical calculations that assume a homogeneous shear flow. In a brief postlude, we also summarize criteria in similar spirit for the onset of necking in extensional filament stretching.