Microscale Flow Research Articles

Assessment of Measurement Accuracy of a Micro-PIV Technique for Quantitative Visualization of Al2O3 and MWCNT Nanofluid Flows

Nanofluids, which are liquids containing nanoparticles, are used to modify heat transfer performance in various systems. To explain the mechanism of heat transfer modification with nanofluids, many theories have been suggested based on numerical simulations without experimental validation because there is no suitable experimental method for measuring the velocity fields of nanofluid flows. In this study, the measurement accuracy of micro-particle image velocimetry (μ-PIV) is systemically quantified with Al2O3 and multi-walled carbon nanotube (MWCNT) nanofluids. Image quality, cross-correlation signal-to-noise ratio, displacement difference, and spurious vector ratio are investigated with static images obtained at various focal plane positions along the beam pathway. Applicable depth is enough to investigate micro-scale flows when the concentrations of Al2O3 and MWCNT nanofluids are lower than 0.01% and 0.005%, respectively. The velocity fields of Hagen–Poiseuille flow are measured and compared with theoretical velocity profiles. The measured velocity profiles present good agreement with the theoretical profiles throughout. This study provides the criteria for μ-PIV application and demonstrates that μ-PIV is a promising technique for measuring the velocity field information of nanofluids.

Microwheels on Microroads: Enhanced Translation on Topographic Surfaces.

Microbot locomotion is challenging because of the reversible nature of microscale fluid flow, a limitation that can be overcome by breaking flowfield symmetry with a nearby surface. We have used this strategy with rotating wheel-shaped microbots, μwheels, that roll on surfaces leading to enhanced propulsion and fast translation speeds. Despite this, studies to date on flat surfaces show that μwheels roll inefficiently with significant slip. Taking inspiration from the mathematics of roads and wheels, here we demonstrate that μwheel velocities can be significantly enhanced by changing microroad topography. In this, we observe that periodic bumps in the road can be used to enhance the traction between μwheels and nearby walls. While continuous μwheel rotation with slip is observed on flat surfaces, a combination of rotation with slip and non-slip flip occurs when μwheels roll upon surfaces with periodic features, resulting in up to four-fold enhancement in translation velocity. The surprisingly fast rolling speed of μwheels on bumpy roads can be attributed to the hydrodynamic coupling between μwheels and road surface features, allowing non-slip rotation of entire wheels along one of their stationary edges. This road/wheel coupling can also be used to enhance μwheel sorting and separation where the gravitational potential energy barrier induced by topographic surfaces can lead to motion in only one direction and to different rolling speeds between isomeric wheels, allowing one to separate them not based on size but on symmetry.

Pressure Transient Behavior of High-Fracture-Density Reservoirs (Dual-Porosity Models)

Naturally fractured reservoirs (NFRs) contain primary (macro), secondary, and tertiary (minor) fractures. These reservoirs are heterogeneous, where fracture properties such as length, conductivity, aperture, and orientation significantly vary spatially. In general, fractures in these reservoirs are very conductive whether they are in a discrete or continuous fracture network. The conventional dual-porosity-type models (Warren and Root in SPE J 3(3):245–255, 1963) are widely used to model NFRs either in conventional or unconventional formations. In shale, the naturally fractured shale is often considered as a dual-porosity model and is coupled with discrete network of hydraulic fractures. Expressing flow by dual-porosity model in the complex pore structure of NFRs is an oversimplification which has demonstrated several deficiencies and inconsistencies with field transient data and field applications. We show that the dual-porosity models are applicable to specific sets of NFRs and under restrictive conditions. In several NFRs, fractures create a network, communicate hydraulically with each other globally, and provide overall conductivity for the reservoir. The matrix provides the overall storage capacity, but it should be conductive enough to pass the fluid towards the fractures. Given the contrast between the conductivity of the fractures and the matrix, several flow regimes may form at microscale. For low contrasts, the conductivities of the matrix and fractures are close enough that the medium behaves as a homogenous medium. For higher contrasts, matrix is not conductive enough to be a part of the overall flow process in the reservoir, so it behaves as a source of fluid to fractures. For very high contrasts, the matrix is almost not conductive, so it cannot pass the fluid to the fracture. In this case, the overall flow will be governed by the flow in the fracture network. Other intermediate cases may also occur. The pressure behavior of continuously fractured reservoirs is investigated for various contrasts in matrix and fracture properties. The homogenized model is obtained by using the two-scale homogenization technique. The homogenization is performed by starting from the microscale flow equations in the matrix and fracture coupled via a set of no-jump boundary conditions on the interface of fracture and matrix. This approach captures the details of the flow both within and between the porous matrix and fracture. A general equivalent macroscopic model is proposed; the macroscopic homogenized model is the average of the microscale equations of flow in fracture and matrix. The equivalent porosity and the equivalent permeability of the averaged medium are derived. The equivalent permeability depends on both the fracture’s permeability and the geometry of each block of nonhomogeneity. The memory effect in the medium is included in the averaged model through an integro-differential term. Memory effect represents the difference between the response time of matrix and fracture to the same pressure drop. Using the homogenization technique, several averaged models for fractured medium can be obtained, depending on the ratio of the fracture to matrix diffusivity, the fracture-to-matrix volume ratio, and the number of scales of nonhomogeneity in the medium. For certain contrasts in matrix and fracture properties, such as the ratio of matrix to fracture permeability and the fracture aperture, the general model reduces to Warren and Root’s model. In general, Warren and Root’s model does not describe every fractured reservoir. The impact of the presence of additional fracture network, such as secondary and tertiary fractures on the equivalent model, and pressure transient behavior is highlighted.

Tuning crystal structure in a micro-scale reactive flow

In many industrial applications, ranging from chemical reactions to transport of suspensions, colloids can be produced and deposited within the processing equipment leading to significant fouling and clogging issues. Here, a continuous-flow reactor for the production of pharmaceutically relevant arylamines through the Buchwald-Hartwig synthesis is coupled to a microfluidic channel to investigate the complex interplay among fluid dynamics, crystallization, reaction and surface chemistry. The process of wall deposition and crystal formation under flow is studied in situ by fragmenting to the micron-scale the particles coming from the reactor and using glass microchannels. Tuning the flow rate, a dramatic change in crystal morphology and composition is found, from dendritic needle-like potassium tert-pentoxide structures to irregular colloidal clusters mainly made of potassium bromide. The results are characterized in terms of dimensionless parameters and provide novel insights for an improved understanding and control of fouling and crystal growth under flow.

Analysis of the 3D non-linear Stokes problem coupled to transport-diffusion for shear-thinning heterogeneous microscale flows, applications to digital rock physics and mucociliary clearance

This study provides the analysis of the generalized 3D Stokes problem in a time dependent domain, modeling a solid in motion. The fluid viscosity is a non-linear function of the shear-rate and depends on a transported and diffused quantity. This is a natural model of flow at very low Reynolds numbers, typically at the microscale, involving a miscible, heterogeneous and shear-thinning incompressible fluid filling a complex geometry in motion. This one-way coupling is meaningful when the action produced by a solid in motion has a dominant effect on the fluid. Several mathematical aspects are developed. The penalized version of this problem is introduced, involving the penalization of the solid in a deformable motion but defined in a simple geometry (a periodic domain and/or between planes), which is of crucial interest for many numerical methods. All the equations of this partial differential system are analyzed separately, and then the coupled model is shown to be well-posed and to converge toward the solution of the initial problem. In order to illustrate the pertinence of such models, two meaningful micrometer scale real-life problems are presented: on the one hand, the dynamics of a polymer percolating the pores of a real rock and miscible in water; on the other hand, the dynamics of the strongly heterogeneous mucus bio-film, covering the human lungs surface, propelled by the vibrating ciliated cells. For both these examples the mathematical hypothesis are satisfied.

Droplet evaporation: Colloidal interactions vs. evaporation kinetics

The drying of a colloidal droplet has attracted considerable attention because of its inherent complexity and significance in applications such as coating, printing, and layering processes. There has been great interest in controlling the well-known ‘coffee-ring’ effect, in which the contact line of an evaporating drop is pinned to the substrate so that dispersed particles are deposited onto the three-phase contact line. Large micro-scale convection-driven flows typically determine the structure of the deposited film. Interparticle forces which operate over nanometer-separations, at least in aqueous systems, are relatively unimportant. Our aim is to explore if much longer-range colloidal forces, comparable to hydrodynamic flows, could be used to control particle drying. We generate controllable micrometer-range electrostatic interactions in a suspension by using charge control additives; an anionic surfactant and oil-soluble electrolyte in nonpolar solvents. The drying of droplets containing monodisperse, fluorescent poly-(methyl methacrylate) (PMMA) particles was studied by confocal light scanning microscopy (CLSM). The existence and extent of a ‘coffee-ring’ deposit was characterized. We find that the drying patterns are controlled by a competition between the evaporation rate of the solvent and the strength and range of the electrostatic interactions.

A new framework for assessing subject-specific whole brain circulation and perfusion using MRI-based measurements and a multi-scale continuous flow model.

A large variety of severe medical conditions involve alterations in microvascular circulation. Hence, measurements or simulation of circulation and perfusion has considerable clinical value and can be used for diagnostics, evaluation of treatment efficacy, and for surgical planning. However, the accuracy of traditional tracer kinetic one-compartment models is limited due to scale dependency. As a remedy, we propose a scale invariant mathematical framework for simulating whole brain perfusion. The suggested framework is based on a segmentation of anatomical geometry down to imaging voxel resolution. Large vessels in the arterial and venous network are identified from time-of-flight (ToF) and quantitative susceptibility mapping (QSM). Macro-scale flow in the large-vessel-network is accurately modelled using the Hagen-Poiseuille equation, whereas capillary flow is treated as two-compartment porous media flow. Macro-scale flow is coupled with micro-scale flow by a spatially distributing support function in the terminal endings. Perfusion is defined as the transition of fluid from the arterial to the venous compartment. We demonstrate a whole brain simulation of tracer propagation on a realistic geometric model of the human brain, where the model comprises distinct areas of grey and white matter, as well as large vessels in the arterial and venous vascular network. Our proposed framework is an accurate and viable alternative to traditional compartment models, with high relevance for simulation of brain perfusion and also for restoration of field parameters in clinical brain perfusion applications.

The impacts of non-uniform magnetic field on free convection heat transfer of a magnetizable micropolar nanofluid

PurposeThe purpose of this study is to numerically study the heat transfer of free convection of a magnetizable micropolar nanofluid inside a semicircular enclosure.Design/methodology/approachThe flow domain is under simultaneous influences of two non-uniform magnetic fields generated by current carrying wires. The directions of the currents are the same. Although the geometry is symmetric, it is physically asymmetric. The impacts of key parameters, including Rayleigh number Ra = 103-106, Hartman number Ha = 0-50, vortex viscosity parameter Δ = 0-4, nanoparticles volume fraction φ = 0-0.04 and magnetic number Mnf = 0-1000, on the macro- and micro-scales flows, temperature and heat transfer rate are studied.FindingThe outcomes show that dispersing of the nanoparticles in the host fluid increases the strength of macro- and micro-scale flows. When Mnf = 0, the increment of the vortex viscosity parameter increases the strength of the particles micro-rotations, while this characteristic is decreased by growing Δ for Mnf ≠ 0. The increment of Δ and Ha decreases the rate of heat transfer. The increment of Ha decreases the enhancement percentage of heat transfer rate because of dispersing nanoparticles, known as En parameter. In addition, the value of Δ has no effect on En. Moreover, the average Nusselt number Nuavg and En remain constant by increasing the magnetic number Mnf for different volume fraction values.Originality/valueThe authors believe that all of the results, both numerical and asymptotic, are original and have not been published elsewhere yet.

Vortex generation by viscoelastic sheath flow in flow-focusing microchannel

Microfluidics-based technologies have attracted much attention since the fluid flow can be controlled precisely and only small sample volumes are required. Viscoelastic non-Newtonian fluids such as polymer solution and biofluids are frequently used in microfluidic analyses, and it is essential to understand the small-scale flow dynamics of such viscoelastic fluids. In this work, we report on vortex generation at the junction region of a flow-focusing microchannel, where a central flow stream of a Newtonian fluid meets two sheath flows of a non-Newtonian poly (ethylene oxide) aqueous solution. We elucidated the vortex-generation mechanism by the backward-flow component induced by the first normal stress difference in the viscoelastic sheath fluid. We systematically investigated the effects of polymer concentration, total flow rate, and total to central-stream flow-rate ratio, on the vortex generation. In addition, we demonstrated that this phenomenon can be engineered to enhance the mixing in the flow-focusing microchannel. We expect this work to be helpful for the understanding of viscoelastic flow dynamics in microscale flows and also for the development of microfluidic mixers.

Digital Core Reconstruction and Research on Microscale Flow Characteristics of Shale Oil Reservoir: A Case of the Shale Oil in Permian Lucaogou Formation, Jimsar Sag

Digital Core Reconstruction and Research on Microscale Flow Characteristics of Shale Oil Reservoir: A Case of the Shale Oil in Permian Lucaogou Formation, Jimsar Sag

Surface water waves propagating over a submerged forest

This paper reports a semi-analytical theory of water waves obliquely propagating over a submerged vegetated belt area. A mathematical model is developed for small-amplitude periodic waves, scattered by an array of submerged vertical cylinders within an infinitely long strip. Assuming a strong contrast between the cylinder spacing and the typical wavelength, the multi-scale perturbation theory of homogenization is employed to derive the governing equations for the macro-scale wave dynamics and the boundary-value problem of micro-scale flows within a unit cell of the cylinder array. The constitutive coefficients in the macro-scale governing equations are computed using the solutions of the micro-scale boundary-value problem, being driven by the macro-scale pressure gradients. Flow turbulences in the vicinity of cylinders are represented by the eddy viscosity model in which the bulk eddy viscosity is determined by balancing the time-averaged rate of energy dissipation and the rate of work done by wave forces on the cylinders, integrated over the entire submerged forest. The wave forces are calculated by the Morison-type formula, in which a new drag formula as a function of Reynolds number is constructed based on existing and newly conducted experimental data. The potential decomposition method is employed in solving the waves/vegetation interaction problem on the macro-scale, which well captures the effects of wave scattering. The theory was checked with several sets of experimental data for normally incident waves. The agreement between the theory and experiments is very good for cases where the submerged forest heights vary from shallow to near water depth. Results for obliquely incident waves are also presented and discussed.

Liquid film on a hydrophobic radome or roof top in rain

The water film due to rain falling on a radome surface causes severe losses in radio wave transmission. Hydrophobic coatings have been applied as a remedy to reduce the film thickness and to minimize the losses. However, quantitative accounts of the wave scattering are mostly based on empirical estimates of the film thickness. We describe a fluid-mechanical theory for the film under steady rain falling on a textured surface formed by a square array of pillars. Assuming the water surface on top of the pillars to be in the Cassie–Baxter state, the analysis is carried out by making use of the sharp contrast of length scales between the film thickness and the radome radius. The textured surface is viewed as a periodic array of cells around pillars. The macro-scale flow is simple and linear but the micro-scale flow in a typical lattice period is fully nonlinear. These two problems are coupled and are solved iteratively to obtain the slip length and the spatial variation of the film thickness. Numerical results are presented to show the effect of solid fraction on local flow field, the slip length and the non-uniform reduction of the film thickness. To examine the influence of the macro-scale geometry on film formation, the theory is also modified for a hydrophobic roof top formed by two inclined planes.

Hydrodynamic and heat transfer properties of magnetic fluid in porous medium considering nanoparticle shapes and magnetic field-dependent viscosity

Abstract The purpose of this paper is to study the characteristics of the combined convection heat transfer and a micropolar nanofluid flow passing through an impermeable stretching sheet in a porous medium. The nanofluid flow field is affected by a magnetic field perpendicular to the sheet. The dynamic viscosity of the micropolar nanofluid changes under the influence of the magnetic field. The continuity, linear momentum, angular momentum, and energy equations are first simplified using the order of magnitude technique that, along with the applied boundary conditions and the definition of the appropriate parameters, are transferred to the similarity space using the similarity analysis. Then the resulting equations are solved using the Runge–Kutta method. The distinction of the macroscale and microscale flow fields and temperature fields resulting from different nanoparticle shapes was clarified. Increasing the Hartmann number, the vortex viscosity parameter, the magnetic parameter, the nanoparticle volume fraction, and the permeability parameter of the porous media increased the surface friction on the sheet. Increasing the vortex viscosity parameter, the magnetic parameter, and the volume fraction of the nanoparticles increases the Nusselt number.

Dynamic microscale flow patterning using electrical modulation of zeta potential

The ability to move fluids at the microscale is at the core of many scientific and technological advancements. Despite its importance, microscale flow control remains highly limited by the use of discrete channels and mechanical valves, and relies on fixed geometries. Here we present an alternative mechanism that leverages localized field-effect electroosmosis to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an ac voltage in sync with an external electric field, creating nonuniform electroosmotic flow distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. Furthermore, we provide a characterization of the system in terms of time response and dielectric breakdown, as well as engineering guidelines for its robust design and operation. We believe that the ability to create tailored microscale flow using solid-state actuation will open the door to entirely new on-chip functionalities.

Condensation heat transfer in minichannels: a review of available correlations

Heat exchangers with enhanced performance are demanded in various engineering applications. Very often heat transfer devices are requested to guarantee not only high heat transfer coefficients but also small size and weight, thus limiting the charge of the operative fluid inside the heat exchanger. In order to increase the performance of condensers and to properly design new heat exchangers, it is essential to have predictive tools that are validated with experimental data. Sometimes, even well established semi-empirical correlations can be inaccurate in some microscale flow conditions or with new refrigerants. The present paper starts from the experimental database measured during condensation with different fluids (pure fluids and refrigerant blends) inside small channels (hydraulic diameter around 1 mm) at the Department of Industrial Engineering of the University of Padova. A critical review of available correlations for heat transfer during condensation in minichannels is presented. Predictions of heat transfer coefficients obtained applying selected models are compared with the experimental database that covers various refrigerants: hydrofluorocarbons (HFC, i.e. R32 and R134a), new hydrofluoroolefins (HFOs) with low global warming potential (R1234ze(E)), natural refrigerants (hydrocarbons such as propane) and zeotropic refrigerant blends of HFCs and HFOs (R32/R1234ze(E)). Refrigerant mixtures are studied because for some applications they may be a proper solution. For instance in the air-conditioning industry there are not drop-in pure fluids to replace the high global warming potential (GWP) fluids currently employed (e.g. R410A). Unfortunately, the design of condensers working with zeotropic mixtures poses the additional problem to account for the mass transfer resistance that leads to a penalization of the heat transfer coefficient. Experimental data are necessary for the assessment of predicting correlations that can be used with these new refrigerants blends.

The investigation of fluid flow in cartilage contact gap

Synovial fluid flow in articular joint capsule plays an important role during mixed mode lubrication. However, the actual fluid flow behaviour during cartilage contact has not been fully understood so far. This is due to the difficulties in measuring the gap permeability using conventional experimental techniques. The problem becomes further complicated with consideration of the cartilage surface roughness. Here a validated numerical study was developed to quantify the gap permeability of lateral synovial fluid flow. Both macro- and micro-scale gap flow models were created based on Darcy's law at the macro-scale and the Navier-stokes equation at the micro-scale. To generate model inputs, the cartilage topography was numerically synthesised based on the experimental measurements of bovine medial tibia cartilage surface roughness using Dektak Stylus Profilers. The experimental results show that the average roughness height Ra is 1.97 μm and root-mean-square roughness height Rq is 2.44 μm, while the correlation lengths of the secondary and tertiary undulations are round 100 μm and 20 μm, respectively. The numerical results indicate that the contact gap height and fluid pressure gradient are two critical parameters which significantly affect the gap permeability. As the contact gap closes, there is a decrease in gap permeability, and most importantly, the gap permeability is also very sensitive to the fluid pressure gradient. Furthermore, with gap closure, the permeability of the contact gap gradually approaches that of the cartilage tissue, at which point the contact gap is functional closed. This occurs at a contact gap height around 1 μm and fluid pressure gradient below 5 × 105 Pa/m in this study.

Particle aggregation and flow patterns induced by ultrasonic standing wave and acoustic streaming: An experimental study by PIV and PTV

In this study, two flow phenomena induced by the acoustic force, ultrasonic standing wave (USW) and acoustic streaming, were examined by means of flow visualization (FV), particle image velocimetry (PIV) and micro particle tracking velocimetry (μPTV). The control of the aggregation of particles in a suspension solution by ultrasonic standing wave (USW) and acoustic streaming was studied for a PMMA (Poly(methyl methacrylate)) cuvette of length 12.6 mm, width 12.6 mm and height 46 mm. The fluid inside was filled with water and μm-size particles as the suspension solution, and the flow is excited with a piezoelectric plate operating at MHz frequency oscillation of voltage signal. This oscillation generates USW inside the liquid space, while at the same time induce acoustic streaming flows in the confined space. Three particle sizes (5–18 μm) were tested with frequencies vary from 0.847 to 2.032 MHz in the present study. Experimental results show that the ultrasonic standing wave is successfully generated, and the error between the standing wavelength in the theory and in the experiment does not exceed 6%. USW is accompanied by the acoustic streaming flow, and within the performed frequency range, the degree of aggregation for particles in different diameters is 18 µm > 10 µm > 5 µm. With all driving frequencies it is found that the acoustic streaming flow starts as the oscillation is turned on and affecting the flow patterns immediately. At driving frequencies of 0.847, 1.863, 2.032 MHz for 10 µm and 18 µm particles, a stable streaming flow coexists with the USW pattern that cause the flow to move with minimum reduction of the particle aggregation can be observed.The micro-scale flow patterns of acoustic streaming flows induced by a single triangular obstruction were investigated by flow visualization (FV) and μPTV. Different base to altitude ratios from 0.25 to 4 of the isosceles triangular obstructions were tested, corresponding to the top angles varied from 127° to 18°, the heights from 0.3 mm to 0.6 mm, and the base widths from 1.2 mm to 0.1 mm. By using the piezoelectric plates, the steady acoustic streaming flow patterns were successfully created and observed at a range of driving voltages from 5 to 35 V and frequencies from 0.5 to 3 kHz. The vector field calculated using μPTV results show that the streaming flow is more concentrated at the vertex of the triangular obstructions, and flow acceleration at the vertex is more significant in the case of small base to altitude ratio. These experimental results suggest the probabilities to utilize both acoustic streaming and USW for applications such as particle sorting and manipulation.

Mass transfer from a Taylor bubble to the surrounding flowing liquid at the micro-scale: a numerical approach

Gas–liquid slug flow is characterized by complex and intermittent hydrodynamic features that offer an efficient alternative to promote biofilm control. In the present work, the mechanism of transferring a gaseous solute into a co-current liquid in a micro-scale slug flow system was inspected in detail. Specifically, the gas–liquid mass transfer from an individual Taylor bubble filled with oxygen was numerically studied using CFD techniques. To accurately describe the referred phenomenon, the hydrodynamic and concentration fields were simultaneously solved. Furthermore, the interface capturing based on the VOF methodology was also coupled to this solution approach. Three sub-categories within slug flow pattern were identified based on the flow behavior in the liquid phase: no liquid in recirculation (Case A); closed wake below the bubble tail (Case B); and recirculation ahead and below bubble (Case C). Regarding the solute distribution, in Case A the solute is dispersed only backwards, it accumulates in the closed wake structure in Case B, and it reaches the wall within the film region in Case C. Local and average mass transfer coefficients were also estimated for the different cases. The influence of the two most relevant dimensionless groups (Reynolds and Capillary numbers) was also briefly analyzed. Global mass transfer coefficients results confirmed that the penetration theory can provide reasonable estimations for systems like Case C.

Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS)

We report the spontaneous formation of multilamellar vesicles (MLVs) from low concentration (<30 wt%) aqueous micellar solutions of sodium linear alkylbenezene sulfonate (NaLAS) upon cooling, employing a combination of optical microscopy (OM), Small Angle Neutron Scattering (SANS), and Cryo-TEM. Upon cooling, MLVs grow from, and coexist with, the surfactant micelles, attaining diameters ranging from hundreds of nanometers to a few micrometers depending on the cooling rate, whilst the d-spacing of internal lamellae remains unchanged, at ≃ 3 nm. While microscale fluid and flow properties of the mixed MLVs and micellar phase depend on rate of cooling, the corresponding nanoscale structure of the surfactant aggregates, resolved by time-resolved SANS, remains unchanged. Our data indicate that the mixed MLV and micellar phases are in thermodynamic equilibrium with a fixed relative volume fraction determined by temperature and total surfactant concentration. Under flow, MLVs aggregate and consequently migrate away from the channel walls, thus reduce the overall hydrodynamic resistance. Our findings demonstrate that the molecular and mesoscopic structure of ubiquitous, low concentration NaLAS solutions, and in turn their flow properties, are dramatically influenced by temperature variation about ambient conditions.

Effect of interface wettability on the flow characteristics of liquid in smooth microchannels

Rapid advances in microfluidic devices have induced interest in the study of the microscale flow mechanism. However, the experimental results of microscale flow often deviate from the classical theory, and we attribute this deviation to the changing liquid viscosity in the microchannels. Because of the effect of the solid–liquid intermolecular force, the viscosity of the liquid near the walls is different from the bulk viscosity. Based on molecular theory and wetting theory, we propose a modified apparent viscosity model. The apparent viscosity of the liquid in microchannels increases with the increase in wettability and decreases with the increase in distance from the wall and the increase in drive pressure. The apparent viscosity near the hydrophilic wall is higher than the bulk viscosity, which increases the flow friction in the microchannels. To validate this model, we experimentally investigate the frictional characteristic of a deionized water flow in smooth parallel-plate microchannels with different wettabilities and heights of approximately 20 and 50 $$\upmu \hbox {m}$$ . The results indicate that the friction factor is higher than that predicted by the classical theory. Such a difference increases with increasing wettability and decreases with increasing hydraulic diameter and pressure drop, which is consistent with the results of theoretical analysis. The apparent viscosity calculated by the apparent viscosity model notably fit the experimental results, with a relative difference of less than ± 2.1%.