Shear Flow Research Articles

Decoupling shear stress and pressure effects in the biomechanics of autosomal dominant polycystic kidney disease using a perfused kidney-on-chip.

Kidney tubular cells are submitted to two distinct mechanical forces generated by the urine flow: shear stress and hydrostatic pressure. In addition, the mechanical properties of the surrounding extracellular matrix modulate tubule deformation under constraints. These mechanical factors likely play a role in the pathophysiology of kidney diseases as exemplified by autosomal dominant polycystic kidney disease, in which pressure, flow and matrix stiffness have been proposed to modulate the cystic dilation of tubules with PKD1 mutations. The lack of in vitro systems recapitulating the mechanical environment of kidney tubules impedes our ability to dissect the role of these mechanical factors. Here we describe a perfused kidney-on-chip with tunable extracellular matrix mechanical properties and hydrodynamic constraints, that allows a decoupling of shear stress and flow. We used this system to dissect how these mechanical cues affect Pkd1 -/- tubule dilation. Our results show two distinct mechanisms leading to tubular dilation. For PCT cells (proximal tubule), overproliferation mechanically leads to tubular dilation, regardless of the mechanical context. For mIMCD-3 cells (collecting duct), tube dilation is associated with a squamous cell morphology but not with overproliferation and is highly sensitive to extracellular matrix properties and hydrodynamic constraints. Surprisingly, flow alone suppressed Pkd1 -/- mIMCD-3 tubule dilation observed in static conditions, while the addition of luminal pressure restored it. Our in vitro model emulating nephron geometrical and mechanical organization sheds light on the roles of mechanical constraints in ADPKD and demonstrates the importance of controlling intraluminal pressure in kidney tubule models.

Development of microstructure-rheological and electrical properties relationship in PS/POE/HNTs blend nanocomposites using machine learning

Halloysite nanotubes (HNTs) and polypropylene-grafted maleic anhydride (PP-g-MA) were studied for their effects in blends of polystyrene (PS) and polyolefin elastomer (POE). The method used to prepare PS/POE blends (90/10 and 80/20 wt/wt) containing 1, 3, and 5 phr HNTs with or without PP-g-MA (a compatibilizer) was melt blending. Structural and morphological studies using X-ray diffraction analysis (XRD), scanning electron microscopy assisted energy dispersive X-ray spectroscopy (SEM-EDS), and transmission electron microscopy (TEM) confirmed a matrix-droplet morphology and the sample containing compatibilizer has better microstructure than the other formulations. The presence of both PP-g-MA and HNTs together has been discovered to improve the viscoelastic properties of the solid, as evidenced by increased storage modulus and complex viscosity. A notable change occurred in the rheological behavior of the PS/POE blends containing 5 phr of PP-g-MA and HNTs. The dependence of zero-shear viscosity on HNTs loading (0–5 phr) was approximated as a polynomial curve by fitting the experimental data with the Carreau-Yasuda model. Computational fluid dynamics (CFD) simulations were also used to study the effects of HNTs loading on changes in fluid flow patterns and shear rates. The calculated effective viscosities at a given shear rate (0.05 1/s) were in qualitative agreement with the experimental results. Moreover, we utilized various machine-learning techniques to predict the complex viscosity of PS/POE blends and their nanocomposites. The results showed that Extreme Gradient Boosting (XGBoost) outperformed other predictive models based on evaluation metrics. Four-point probe measurements found that the samples containing 5 phr HNT had the lowest conductivities due to the presence of aggregated structures. However, the homogeneous distribution of HNT led to a sudden rise in conductivity in the samples containing 5 phr of PP-g-MA. Computer modeling results of samples with uniform and non-uniform HNT distributions showed that the conductivity decreased with HNT loading and decreased considerably compared to the uniform distribution.

Acoustic radiation force-induced longitudinal shear wave for ultrasound-based viscoelastic evaluation

Acoustic radiation force (ARF) is widely used to induce shear waves for evaluating the mechanical properties of biological tissues. Two shear waves can be generated when exciting with ARF: a transverse shear wave, also simply called shear wave (SW), and a longitudinal shear wave (LSW). Shear waves (SWs) have been broadly used to assess the mechanical properties. Some articles have reported that the LSW can be used to evaluate mechanical properties locally. However, existing LSW studies are mainly focused on the group velocity evaluation using optical coherence tomography (OCT). Here, we report that a LSW generated with ARF can be used to probe viscoelastic properties, including shear modulus and viscosity, using ultrasound. We took advantage of the surface boundary effect to reflect the LSW, named RLSW, to address the energy deficiency of LSW induced by ARF. We systematically evaluated the experiments with tissue-mimicking viscoelastic phantoms and validated by numerical simulations. Phase velocity and dispersion comparison between the results induced by a RLSW and a SW exhibit good agreement in both the numerical simulations and experimental results. The Kelvin-Voigt (KV) model was used to determine the shear modulus and viscosity. RLSW shows great potential to evaluate localized viscoelastic properties, which could benefit various biomedical applications such as evaluating the viscoelasticity of heterogeneous materials or microscopic lesions of tissues.

Heat transfer in shear flow over a still nanofluid: A direct integration approach

Heat transfer in a two-layer fluid system, including shear-driven viscous fluid in the upper layer and still nanofluid in the lower layer, is investigated in this work. Following single phase model for nanofluid, three different types of nanofluids are considered. Different from existing literatures on two-layer fluid flow problem, this work introduces the heat transfer of shear-driven viscous fluid over a still nanofluid which makes this investigation new. The main equations which are partial in nature are reduced to a set of coupled nonlinear ordinary differential equation (ODE) by employing suitable similarity transformations. These ODEs together with the coupled interfacial boundary conditions are supplied to MATLAB software to get the solution numerically. Comparing the results obtained in this study with the previous authoritative research results, the accuracy and correctness of the present research methodology are ensured. The results show that the temperature of the upper fluid and the lower fluid are controlled by the thermophysical properties of different types of nanofluids. Also, opposite behavior of temperature distribution for the upper and the lower fluid are observed.

Global flux-driven gyrofluid simulations of internal transport barrier formation by external torque in weak magnetic shear configuration

Using a global gyrofluid code, we conduct comprehensive flux-driven simulations incorporating external heat and momentum sources/sinks. They involve an external torque ramp while keeping a heating power constant. Simulations show the formation of internal transport barriers (ITBs) in ion heat and parallel momentum in weak magnetic shear configuration. The ITB formation is attributed to the reduction of turbulent transport accompanied by strong E×B flow shear generation, where the majority of E×B shear arises from plasma rotation shear. In contrast, only a minor confinement improvement is observed in strong magnetic shear configuration. There exists a substantial difference in the rotation shear between the weak and strong magnetic shear cases at the same amount of external torque. Transport analysis implies that the difference comes from intrinsic rotation generated by residual stress. Global linear simulations demonstrate that the effect of E×B shear on k∥ symmetry breaking is enhanced in weak magnetic shear configuration, explaining the origin of the difference in intrinsic rotation. The ITB is not achieved when the external torque is injected in counter-direction to intrinsic rotation. It is also shown that threshold torque for the ITB formation depends on power and the ratio of external torque to heating power should exceed a critical value to attain an ITB.

Forces acting on near-wall spherical particles in shear flows of diluted gases

In the present paper, we studied the forces on a spherical particle of radius R moving in the vicinity of the plane wall in a shear flow of free molecular regime. We consider that the distance ratio between the plane wall and the particle (L) and the particle radius (R) is large (e.g., L/R > 5), and the gas molecular mean free path (λ) is much higher than the particle size (λ/R ≫1). An analytical formula for the forces is obtained based on gas kinetic theory and certain simplifying assumptions, and is verified by using Direct Simulation Monte Carlo Method. It is found that the forces acting on the particle can be affected by the momentum accommodation coefficients (σ) of the wall and particle surfaces, the wall/gas temperature ratios (Tw/T), and the velocity gradient (G) of the gas flow. In the cases of specular reflections (σ = 0), the near-wall effect can be neglected. With the increase of the momentum accommodation coefficients, the near-wall effect can be enhanced. When near-wall particles move in the direction parallel to the plane wall, there is a lift force which is perpendicular to the wall due to the near-wall effect and the shear flow. For Tw/T < 1, the lift force for the near-wall particles is in the direction against the wall. While for Tw/T > 1, the force is in the direction away from the plane wall. The findings presented in this paper can provide theoretical guidance for the application of near-wall particles in shear flows.

Interfacial Polymer Rheology of Entangled Short-Chain Branched Ring Melts in Shear Flow

Interfacial Polymer Rheology of Entangled Short-Chain Branched Ring Melts in Shear Flow

Fabrication of electrically conductive microparts by constructing carbon black-rich network under high shear conditions in microinjection molding

Microinjection molding (μIM) is an important technique to fabricate microparts for applications in the fields of automotive and microelectromechanical systems. However, the prevailing high shear conditions in μIM are unfavorable for constructing intact electrically conductive networks because the added fillers tend to be preferentially aligned along the melt flow direction. In this work, a series of polypropylene/polyamide 6/carbon black (PP/PA6/CB) composites with a selective localization of CB in the PA6 phase were used as the model system to prepare electrically conductive microparts. The prevailing high shearing and extensional flow effects in μIM were utilized to deform CB-rich phase with an aim to in situ construct electrically conductive network, thereby improving the electrical conductivity (σ) of subsequent moldings. The results indicated that a higher σ was achieved for PP/PA6/CB microparts when compared with their PP/CB and PA6/CB counterparts, at a lower filler content (&amp;lt;10 wt%). The influence of blending sequence of various components (i.e., PP, PA6, and CB) and annealing treatment on the σ of microparts was also studied. This work provided an approach to the design and preparation of electrically conductive microparts that can be potentially used in high-tech sectors.

Experimental study on the continuous preparation of high‐performance thermal conductivity CNTs/EG/SSBR composite based on the shear and elongation flow fields with the assistance of the silanization reaction

AbstractA self‐developed Serial Rubber Continuous Mixing Device (SRCMD) based on the shear and elongation flow fields was adopted to continuously prepare the carbon nanotubes (CNTs)/expanded graphite (EG)/Solution‐polymerized styrene butadiene rubber (SSBR) composite. Meanwhile, the silanization reaction between the Silica and Si69 was introduced to improve the performance of the CNTs/EG/SSBR composite. When comparing the sample prepared using the two‐stage mixing process based on the shear flow field in the internal mixer, the continuously prepared CNTs/EG/SSBR composite with 3 wt% CNTs and 15 wt% EG demonstrates a higher thermal conductivity of 1.32 W/m·K. At the same time, it exhibits significant advantages in terms of mechanical properties, processability and dispersion rating. In addition, the mechanism for the continuous preparation of CNTs/EG/SSBR composite was revealed through visual experiments. This study aims to achieve the real continuous production of high‐performance thermally conductive polymer composites to meet the urgent demand for thermal conductivity materials in 5G technology.Highlights Real continuous preparation of CNTs/EG/SSBR composite was realized. The silanization index of 92.26% between Silica and Si69 was achieved. Good silanization promotes the construction of thermal conductivity networks. High‐performance CNTs/EG/SSBR composite with low Payne effect was obtained.

Investigation of the mechanical behavior of rock-like material with two flaws subjected to biaxial compression

The biaxial compression experiments of rock-like materials with two flaws are carried out under different flaw inclination angle, rock bridge ligament angle, lateral stress. The experimental studies show that crack propagation modes of rock-like material are as follows: wing crack through mode (Y mode), shear crack through mode (J mode), mixed crack through mode (wing shear JY mode), longitudinal extension of crack and transverse shear splitting. prefabricated fractured rock specimens have experienced the closing stage of prefabricated fractures, the elastic deformation stage, the generation and expansion of cracks (or plastic strengthening), and the residual loading stage. The peak strength of the specimen is increases with the increase of flaw inclination angle and lateral stress. With the increase of the rock bridge ligament angle, the failure of the rock bridge region changes from the shear crack failure to composite failure of shear crack and the wing type tensile crack failure, and then to the wing crack failure. With the increase of the lateral pressure, the failure of the specimen changes from the wing type tensile crack failure to the wing type and shear crack failure, and then to shear crack failure. The flaw inclination angle mainly changes the form of crack growth but does not effect on the failure modes. The counting number of acoustic emission events at the center of the sample is relative large, indicating that the cacks appear in the part of the rock bridge firstly. With the increasing of loads, the cracks of the rock bridge expanding constantly and connecting finally. The changes of acoustic emission event counts is consistent with the macroscopic damage form obtained from the experiments.

Experimental research on creep characteristics and failure mechanism of mining roadway in nearly vertical coal seams

Introduction: Nearly vertical coal seams present a significant challenge for the coal mining industry due to their difficult accessibility. However, these seams account for a substantial proportion of the world’s coal reserves. Therefore, it is vital to conduct research on disaster control techniques for safe mining of these seams.Method: The research team used experimental research, theoretical analysis, and numerical calculation methods to investigate the creep characteristics and failure mechanisms of layered sandstone roadway in nearly vertical coal seam.Results and discussion: These findings revealed that the maximum moment and concentrated stress of the sandstone located on the side of the roadway roof was positioned in the middle of the nearly vertical structure, making it more susceptible to transverse shear failure. On the other hand, the nearly vertical structure on the floor side was prone to shear slip failure initiated from the bottom of the structure. This led to the asymmetric instability of the roadway. The practical implications of this research are significant for the safe mining of nearly vertical coal seams. The results could help inform the development of disaster control techniques.

Durability of concrete-embedded GFRP bars after 20 years of tidal zone exposure: Correlation with accelerated aging tests

The application of non-metallic glass fiber-reinforced polymer (GFRP) bars as embedded reinforcement in reinforced concrete (RC) has been of great interest among researchers and practicing engineers alike, owing to its properties such as high strength-to-weight ratio and corrosion resistance. This study investigated the long-term performance of ribbed (RB) and sand-coated (SC) GFRP bars embedded in concrete columns exposed to harsh seawater conditions for approximately 20 years. The bars extracted from the concrete elements were tested to evaluate strength retention based on ASTM standard transverse and interlaminar shear strength tests. After the long-term field exposure, the ribbed and sand-coated GFRP bars exhibited 82.5 % and 79.4 % transverse shear strength retention, and 84.4 % and 78.1 % interlaminar shear strength retention, respectively, which corresponds to approximately 5 months of exposure in the accelerated solution specified by ACI 440.3R-12 guide. Scanning electron microscopy analyses revealed the extent of damage in the fibers, the matrix, and their interface, which validated the degradation in the mechanical strength of the exposed GFRP bars. Finally, correlations between long-term and accelerated exposure test conditions were developed to predict transverse and interlaminar shear strength in the field based on laboratory conditioning investigations.

Hygroscopic damage of fiber–matrix interface in unidirectional composites: A computational approach

The mechanical properties of fiber-reinforced composites are significantly affected by hygroscopic exposure. Recent experimental findings reveal that the diffusion of water molecules results in void formation at the fiber–matrix interface, contributing to ∼1% increase in porosity. This paper presents a computational micromechanics framework to analyze the damage behavior of unidirectional composites under hygroscopic aging and transverse loads. First, a mathematical relation for the porosity evolution with moisture content in the composite is obtained. Leveraging this correlation, the aged composite is modeled as a porous microstructure consisting of fiber, matrix, and voids at the fiber–matrix interface in finite element simulations. The fiber is considered elastic, and the matrix is modeled as an elastoplastic material using a pressure-dependent yield model to capture tension–compression asymmetry. A cohesive interface is considered between the fiber and matrix. Simulation results indicate that a void at the interface significantly influences plastic strain evolution and stress distribution, leading to early damage initiation and reduced strength. Previous works have indicated that matrix and inter-fiber voids do not have a significant impact on the strength of composites when their combined porosity is below 1%. On the contrary, this study demonstrates that a porosity of as low as 0.1% at the fiber–matrix interface can result in an 11% decrease in the transverse tensile strength of the composite. A parametric study is conducted to understand the fundamental effects of pore size, location, and relative position on the transverse tension, compression, and shear loading responses of a unidirectional composite. For a given moisture content, the predicted strength of the hygroscopic aged composite exhibits a lower and upper bound. The strengths observed in the experiments fall within the model-predicted range. Additionally, a computational method is proposed that incorporates the effects of porosity at the fiber–matrix interface into a modified cohesive strength. This method allows efficient computations and failure predictions for water or moisture-aged strength degradation of unidirectional composites.

Experimental investigations on the blast response of reinforced NSC, FRC and UHPC I-shape girders under contact explosions

To investigate the blast response and evaluate the structural resilience of reinforced I-shape girders, seven reinforced normal strength concrete (NSC), fiber reinforced concrete (FRC), and ultra-high performance concrete (UHPC) specimens are subjected to multiple contact explosions in this study. With the test results, the concrete damage regions and plastic deformation of steel bars are used to recognize the damage mechanism of the test specimens. The experimental results show that the damage modes of I-shape girders are mainly dominated by vertical punctures and transverse shear failures. Due to fragile characteristics, nearly 35 % of the punching crater in concrete webs, the entire shear failure of top plates, and severe plastic deformation of steel bars are discovered in NSC specimens. Polyethylene fibers improve ductility, allowing for smaller steel bar deflection and a reduction in punching depth to 21.6 % in FPC specimens. However, due to the oversized impact energies and the insufficient material strength, the transverse shear failure cannot be significantly mitigated. By contrast, not only significantly reduced punching depth but also greatly mitigated transverse shear failures are observed in UHPC specimens. Under 50 g∼200 g contact explosions, the punching depth ratios are reduced to 3.3 %, 16.6 %, and 25.8 %, merely. Apart from that, after explosions, the transverse shear failure and plastic deformation of steel bars can be effectively reduced. Due to the remarkably decreased damage regions, the UHPC specimens exhibit better structural resilience after blast loadings.

Generalized scaling laws for the irrotational motions bordering a turbulent region.

In turbulent free shear flows such as jets and wakes, and also in turbulent boundary layers, the turbulent region is bounded by a region of irrotational flow where the magnitude of the potential velocity fluctuations can be very high. This is particularly true close to the turbulent-nonturbulent interface layer (TNTI) that separates the regions of turbulent (rotational) and nonturbulent (irrotational) fluid motion in these flows. Previous works have shown that for distances from the TNTI x_{2} much bigger than the integral scale L in the nearby turbulent region (x_{2}≫L), the variance of the velocity fluctuations 〈u_{i}^{2}〉 (i=1,2,3) depends on the shape of the kinetic energy spectrum in the infrared region E(k)∼k^{n} [O. M. Phillips, Proc. Camb. Phil. Soc. 51, 220 (1955)10.1017/S0305004100030073; Xavier etal., J. Fluid Mech. 918, A3 (2021)10.1017/jfm.2021.296]. Using rapid distortion theory, we derive the generalized scaling laws for the potential velocity fluctuations, at distances sufficiently far from the TNTI layer, for any value of n. While the cases n=4 (Batchelor turbulence) and n=2 (Saffman turbulence) have been previously derived, with 〈u_{i}^{2}〉∼x_{2}^{-4} and 〈u_{i}^{2}〉∼x_{2}^{-3}, for n=4 and n=2, respectively [O. M. Phillips, Proc. Camb. Phil. Soc. 51, 220 (1955)10.1017/S0305004100030073; Xavier etal., J. Fluid Mech. 918, A3 (2021)10.1017/jfm.2021.296.], we extend these results by including any other value of n. In particular, we obtain 〈u_{i}^{2}〉∼x_{2}^{-2} and 〈u_{i}^{2}〉∼x_{2}^{-4}, for n=1 and n≥5, respectively, while n=3 yields 〈u_{i}^{2}〉∼x_{2}^{-4}ln(x_{2}). These theoretical results are confirmed by direct numerical simulations of turbulent fronts evolving into an irrotational flow region in the absence of mean shear.

Shear response and deformation mechanism of boron nitride nanosheets reinforced aluminum matrix composites

Atomistic simulations are performed to study the shear response and deformation mechanism of boron nitride nanosheet (BNNS) reinforced aluminum (Al) matrix composites. In this work, stacked BNNS/Al composites and dispersed BNNS/Al composites with various BNNS layers are constructed, respectively. Our computations show that the wrinkle deformation of BNNS under shear loading does not lead to the loss of load-bearing capacity. However, the yield of the Al matrix causes it to lose load-bearing capacity. The occurrence of wrinkles in BNNS aggravates the plastic failure of the Al matrix. High BNNS volume fraction increases the shear modulus and flow stress of stacked BNNS/Al composites but decreases the shear strength. Compared with stacked BNNS/Al composites, the mechanical properties of dispersed BNNS/Al composites are remarkably improved with the increase of BNNS interface in both elastic and plastic stages. In addition, the BNNS interface is able to obstruct the propagation of dislocations and stacking faults in the Al matrix. These findings may deepen our understanding of the mechanical properties and deformation mechanisms of BNNS/Al composites.

Sound propagation in the presence of non-isothermal sheared flow refraction and metasurface impedance reflection

Sheared flow can profoundly influence the aeroacoustic performance of near-wall structures like metasurfaces on moving vehicles. In this work, a refractive generalized Snell’s law is developed by combining Snell’s law with the generalized Snell’s law to manipulate anomalous reflections in an idealized sheared flow, comprising a uniform flow region and a stationary air region with velocity and temperature gradients. Theoretical analysis and numerical simulations of planar wave reflection by metasurface impedance under non-isothermal sheared flows are conducted, yielding favorable predictions of the zone of silence, as well as anomalous and total reflection behaviors. The total reflection between the shear layer and the metasurface is analyzed with the objective of confining incident wave below the shear layer. Remarkably, a specific configuration of hot-sheared flow and metasurface length can achieve total absorption over a range of incident wave angles from 0° to almost 90°. Moreover, the refractive generalized Snell’s law, extended to account for directional deflection induced by flow convection, is utilized to regulate sound beam reflection, resulting in satisfactory outcomes under sheared conditions. This study offers a theoretical framework for wave control under non-isothermal and sheared flow conditions, which cannot be resolved by the traditional generalized Snell’s law, and can be implemented in the design of metasurfaces for moving applications.

Flow Analysis and Shear Rate Comparison of Counter-rotating and Co-rotating Intermeshing Twin-screw Extruders for Filament Extrusion of Polypropylene-based Biocomposites

This study investigates and compares the performance of counter-rotating and co-rotating intermeshing twin-screw designs in filament extruder machines. The research sought to determine whether the counter-rotating intermeshing design with its opposite flow direction offers advantages over the co-rotating intermeshing design in terms of flow analysis and shear rates. Flow analysis was conducted to examine the velocity of the polypropylene-based biocomposite material inside the barrel. Shear rate data was obtained by evaluating the relationship between shear rate and screw speed to assess the stability and maximum shear rate of the twin-screw extruders. The results revealed that the counter-rotating intermeshing twin-screw extruders exhibited higher shear rates and more consistent pressure compared to the co-rotating intermeshing design. The superiority of the counter-rotating extruder was attributed to its opposite flow direction and distinct thread shapes, facilitating efficient material compression and improved dispersion of polymer-based biocomposite materials. The study suggested the potential for further exploration and refinement of the counter-rotating intermeshing twin-screw extruder design, particularly in producing polypropylene-based biocomposite filaments for Fused Deposition Modeling (FDM) machines.

Thermal buckling analysis of nano composite laminated and sandwich beams based on a refined nonlocal zigzag model

A refined nonlocal zigzag model for thermal buckling analysis of nano composite laminated and sandwich beams is proposed in this study based on a refined zigzag theory and Eringen’s nonlocal theory. Firstly, present model satisfies the stress-free and continuity conditions a priori by introducing the piecewise-linear zigzag functions and a preprocessing, such that the transverse shear correction factors are not needed. In the preprocessing, accurate and continuous transverse shear stresses are obtained with the aid of the general mixed variational principle, which can be solved simultaneously with other stresses in the governing equations. This is quite different from the previous post-processing. Subsequently, thermal buckling problems of nano composite laminated and sandwich beams are analytically solved in simply supported boundary conditions. The degenerated results without small effect indicate that the non-dimensional critical loads and critical temperatures have a good agreement with the 3D elasticity solutions and previous results, which demonstrate the accuracy and reliability of present model. Moreover, it is observed that the small effect of the critical temperatures can be effectively captured by Eringen’s differential constitutive law (EDCL), which shows the small effect decreases the critical temperature by weakening the stiffness of the beam. Finally, the effects of different thermal expansion coefficients, laminations, geometric sizes and beam theories are discussed. The results show that present model is robust in the arbitrary layouts for both of composite and sandwich structures, which may have some referential significance to Micro-Electro-Mechanical Systems (MEMS) sensors and actuators.

Stability analysis of viscous multi-layer shear flows with interfacial slip

Abstract One of the most fundamental interfacial instabilities in ideal, immiscible, incompressible multifluid flows is the celebrated Kelvin–Helmholtz (KH) instability. It predicts short-wave instabilities that, in the absence of other mollifying physical mechanisms (e.g. surface tension, viscosity), render the nonlinear problem ill-posed and lead to finite-time singularities. The crucial driving mechanism is the jump in tangential velocity across the liquid–liquid interface, i.e. interfacial slip, that can occur since viscosity is absent. The purpose of the present work is to analyse analogous instabilities for viscous flows at small or moderate Reynolds numbers as opposed to the infinite Reynolds numbers that underpin KH instabilities. The problem is physically motivated by both experiments and simulations. The fundamental model considered consists of two superposed viscous, incompressible, immiscible fluid layers sheared in a plane Couette flow configuration, with slip present at the deforming liquid–liquid interface. The origin of slip in viscous flows has been observed in experiments and molecular dynamics simulations, and can be modelled by employing a Navier-slip boundary condition at the liquid–liquid interface. The emerging novel instabilities are studied in detail here. The linear stability of the system is addressed asymptotically for long- and short-waves, and for arbitrary wavenumbers using a combination of analytical and numerical calculations. Slip is found to be capable of destabilising perturbations of all wavelengths. In regimes where the flow is stable to perturbations of all wavelengths in the absence of slip, its presence can induce a Turing-type instability by destabilization of a small band of finite wavenumber perturbations. In the case where the underlying layer is asymptotically thin, the results are found to agree with the linear properties of a weakly non-linear asymptotic model that is also derived here. The weakly nonlinear model extends previous work by the authors that had a thin overlying layer that produces a different evolution equation.