Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as “particle ordering,” is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.     

Effect of shear history on the steady shear flow behavior of a thermotropic liquid-crystalline polymer

The steady shear stress (σ) and first normal stress difference (N₁) of a thermotropic liquid-crystalline polyester, poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylene-bis(4-oxybenzoate)] (PSHQ10), in both the isotropic and nematic regions were measured as a function of shear rate (γ), using a cone-and-plate rheometer. For the study, PSHQ10 was synthesized via solution polymerization in our laboratory. The PSHQ10 was found to have (a) the weight-average molecular weight of 45,000 relative to polystyrene standards and a polydispersity index of 2, (b) a glass transition temperature of 88°C, (c) a melting point of 115°C, and (d) a nematic-to-isotropic transition temperature of 175°C. For the measurements of σ and N₁ in the nematic region of PSHQ10, its initial conditions for the startup of shear flow was controlled by (a) first heating an as-cast specimen to 190°C, (b) shearing there at γ = 0.085 s^?1 for about 5 min, and then (c) cooling slowly down to a predetermined temperature (130, 140, 150, 160, or 171°C) in the nematic region. For each γ chosen, after start-up of shear flow, we waited for a sufficiently long time until both the shear stress and first normal stress difference leveled off, giving rise to steady-state values of σ and N₁. Emphasis was placed on investigating the effect of shear history on σ and N₁ of PSHQ10 in the nematic region. For this, the following experiments were conducted: (a) a fresh specimen was sheared continuously by increasing the γ stepwise, and (b) a presheared specimen was further sheared continuously by increasing the γ stepwise. We have found that fresh specimens exhibited ‘shear-thinning’ behavior over the entire range of γ (0.008?0.27 s^?1) tested, whereas the presheared specimens exhibited both zero-shear viscosities and shear-thinning behavior. When using fresh specimens, we found that N₁ was positive over the entire range of γ (0.008–0.27 s^?1) tested. However, when using presheared specimens we found that (a) at very low γ, N₁ initially was negative and then became positive as shearing continued, and (b) at higher γ, N₁ was positive over the entire duration of shearing. © 1994 John Wiley & Sons, Inc.     

On the wall slip of polymer blends

The slip flow of the polypropylene (PP)/poly[ethylene‐co‐(vinyl acetate)] (EVA) system was studied in a capillary rheometer for shear rates of 40–1000 s^?1 at four temperatures. Three dies made of aluminum with a length/diameter (L/D) ratio of 15 and diameters of 1.59, 1.19, and 0.79 mm provided the flow data. Calculations of the slip velocity by the Mooney method showed power‐law behavior with the stress. Blends were prepared at various proportions of PP and EVA for observation of the variation of the slip velocity for different compositions and temperatures. Direct microscopic observations of the slip layer on extruded samples showed domains of the dispersed phase unevenly distributed between the slip layer and the core and provided estimates of the thickness of the layer adjacent to the capillary wall. Results showed that the viscosity in the slip layer was 10–100 times lower than that in the bulk for the same value of the shear stress. In terms of the extrapolation length, the development of the slip layer was the result of different disentanglement dynamics of the molecules in the slip layer in comparison with those in the bulk. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 303–316, 2002     

Microvascular blood flow with heat transfer in a wavy channel having electroosmotic effects

The present paper addresses microvascular blood flow with heat and mass transfer in complex wavy microchannel modulated by electroosmosis. Investigation is carried out with joule heating and chemical reaction effects. Further, viscous dissipation is also considered. Using Debye–Huckel, lubrication theory, and long wavelength approximations, analytical solutions of dimensionless boundary value problems are obtained. The impacts of different parameters are examined for temperature and concentration profile. Furthermore, nature of pressure rise is also investigated to analyze the pumping characteristics. Important results of flow phenomena are explored by means of graphs.     

On the wall slip phenomenon of elastomers in oscillatory shear measurements using parallel-plate rotational rheometry: I. Detecting wall slip

A systematic study is presented in order to reveal the occurrence of wall slip of pre-prepared elastomeric samples characterized with the use of rotational rheometry. To exclude effects that could be attributed to additional functional fillers, both an unfilled (primarily used) and lightly silica reinforced (complementary system) silicone rubber are evaluated. Cylindrical samples are prepared by casting using a standardized methodology and examined by means of a stress-controlled parallel-plate rotational rheometer. As a control test, samples are also cured within the rheometer (in situ), thereby fixing them to the measuring plates and firmly establishing their response in “no-slip” conditions. The experiments suggest that wall slip, postulated to be caused by an adhesive failure at the sample-plate interface, may occur if the deformation is sufficiently large and no cohesive failure is present. It is detected by an increase in the loss modulus that is related to the adhesive failure associated with local dynamic friction, resulting in increased dissipated energy. Direct (via raw waveform data and normalized Lissajous figures) and indirect (via fast-Fourier-transformation) analysis of the overall system response for a single steady state deformation cycle provided further insights into the mechanism of wall slip.     

Nanoparticles phenomenon for the thermal management of wavy flow of a Carreau fluid through a three-dimensional channel

Journal of Thermal Analysis and Calorimetry - A hybrid nanofluid phenomenon is considered involving nanoparticles since such particles are potential medication transportation devices in biomedical...     

Effect of nanometric-scale roughness on slip at the wall of simple fluids

It is commonly acknowledged that roughness decreases the aptitude of simple liquids to exhibit flow with slip at solid interfaces. Most available studies have, however, been conducted on substrates for which both the surface chemistry and the roughness were varied simultaneously, making it difficult to identify their respective role on wall slip. To overcome this difficulty, we have developed a series of surfaces formed by grafting hyperbranched polymeric nanoparticles on a smooth, dense, self-assembled monolayer of SiH-terminated short poly(dimethylsiloxane) oligomers, allowing us to vary independently the surface density, the height, and the width of the grafted nanoparticles, and thereby the roughness parameters, while keeping similar surface chemistry. On such substrates, the boundary condition for the flow velocity of hexadecane has been characterized through near-field laser velocimetry. We demonstrate that decreasing the wavelength of the roughness at a fixed height strongly decreases slip, while increasing the height of the nanoparticles at a fixed aspect ratio of the roughness also dramatically affects slippage.     

Electrokinetically-driven flow mixing in microchannels with wavy surface

This paper investigates the mixing characteristics of electrokinetically-driven flow in microchannels with different wavy surface configurations. Numerical simulations are performed to analyze the influence of the wave amplitude and the length of the wavy section on the mixing efficiency within the microchannel. Typically, straight channels have a poor mixing performance because the fluid flow is restricted to the low Reynolds number regime, and hence mixing takes place primarily as a result of diffusion effects. However, the wavy surfaces employed in the current microchannels increase the interfacial contact area between the two species in the microchannel and therefore improve the mixing efficiency. The mixing performance is further enhanced by the application of a heterogeneous charge pattern on the wavy surfaces. The numerical results show that the heterogeneous charge pattern generates flow circulations near the microchannel walls. These circulations are shown to provide an effective enhancement in the mixing performance. Overall, the present results show that the mixing performance is improved by increasing the magnitude of the heterogeneous surface zeta potential upon the wavy surface or by increasing the wave amplitude or the length of the wavy section in the microchannel.     

Tumbling dynamics of individual absorbed polymer chains in shear flow

The tumbling dynamics of individual absorbed polymer chains in shear flow is studied by employing multi-particle collision dynamics simulation techniques combined with molecular dynamics simulations.We find that the dependence of tumbling frequencies on shear rate is independent of both adsorption strength and surface corrugate.     

Determination of molecular parameters from polymer solutions in shear flow

A light scattering photometer with a double cylinder shear cell has been developed which allows detection not only in one plane as usual, but in a half-spherical range around the scattering volume. The anisotropic scattering behavior of oriented and deformed polymer molecules in dilute solution was investigated. All measurements were performed on polystyrene in several viscous solvents. From measurements in the plane of flow the average orientation angle was determined. By variation of detector position and wavelength of the primary beam, the determination of all three axes of the molecular gyration space of polymer coils was possible. Compared to predictions of the well-known dynamic theories by Kuhn, Rouse and Zimm, corresponding orientation data were found while the molecular deformation ratio shows much lower values than expected.     

Rheo-optical characterization of dilute polymer mixtures under shear flow

Optical properties can estimate morphological changes of polymer chains under flow. This work proposes a rheo-optical procedure to determine turbidity and both flow and form birefringence of diluted polymer mixtures of polystyrene (PS) and polypropylene (PP) during a controlled shear flow, by measuring the transmitted light intensity with and without crossed polarizers via an own built optical sensor. The turbidity in these dilute mixtures decreased with the increase of the shear rate due to deformation of the dispersed phase droplets, which reduces their cross-sections. The presence of PP as the dispersed phase in the PS matrix caused a decrease in the total birefringence measured, whereas PS as the dispersed phase in the PP matrix caused an increase in it. Both effects are associated to the positive contribution of the form birefringence, produced by the shear-induced elongated morphology of the dispersed phase.     

Effect of liquid slip in electrokinetic parallel-plate microchannel flow

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in parallel-plate microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. Our parametric results suggest that electrokinetic transport phenomena and liquid slip at channel walls are both important and should be considered simultaneously. Their significance depends on channel wall material, electrolyte concentration, and pH. For pressure-driven-flow, liquid slip counteracts the effect by the electrical double layer and induces a larger flow rate. A higher apparent viscosity would be predicted if slip is neglected. For electroosmotic flow, liquid slip alters the flow rate by about 20% for a thick electrical double layer. Our results provide design guidelines to precisely control time-dependent microflow in hydrophobic microfluidic microelectromechanical system devices.     

Structure and dynamics of concentration fluctuations in a polymer blend solution under shear flow

The technique of small-angle light scattering (SALS) has been employed to investigate the time-dependent behavior of a single-phase, semidilute solution of polystyrene and polybutadiene in dioctyl phthalate under shear flow. Concentration fluctuations in the polymer blend solution are found to grow with time in the direction of flow, and their orientation angles evolve from 45° from the flow direction toward 0°, with the steady-state value being dependent on shear rate. SALS patterns are simulated using a modified Cahn-Hilliard-Cook model, with an additional collective restoring force to account for polymer elasticity. Predictions from this modified model for the orientation angles of the concentration fluctuations are in excellent agreement with the experimental results. Our model also predicts that the quiescent structure factor has a Gaussian form and that the steady-state orientation of the scattering patterns is dependent on shear rate. These predictions are also in good agreement with our experimental observations. © 1994 John Wiley & Sons, Inc.     

Formation of polymer microrods in shear flow by emulsification--solvent attrition mechanism

Rodlike polymer particles could have interesting properties and could find many practical applications; however, few methods for the production of such particles are available. We report a systematic study of a droplet shearing process for the formation of polymer rods with micrometer or submicrometer diameter and a length of up to tens of micrometers. The process is based on emulsification of a polymer solution under shear, combined with solvent attrition in the surrounding organic medium. The droplets deform and elongate into cylinders, which solidify when the solvent transfers to the dispersion medium. Stopped flow experiments allow distinguishing all stages of the mechanism. The results are interpreted on the basis of the theory of droplet elongation and breakup under shear. The effects of the viscosity ratio and shear stress are matched against the theoretical expectations. The method is simple, efficient, and scalable, and we demonstrate how it can be controlled and modified. The experimental parameters that allow varying the rod size and aspect ratio include shear rate, medium viscosity, and polymer concentration. Examples of the specific properties of the polymer rods, including self-organization, alignment in external fields and in fluid flows, and stabilization of bubbles, droplets, and capsules, are presented.     

Electroosmotic flow in a rectangular channel with variable wall zeta-potential: comparison of numerical simulation with asymptotic theory

Electroosmotic flow in a straight micro-channel of rectangular cross-section is computed numerically for several situations where the wall zeta-potential is not constant but has a specified spatial variation. The results of the computation are compared with an earlier published asymptotic theory based on the lubrication approximation: the assumption that any axial variations take place on a long length scale compared to a characteristic channel width. The computational results are found to be in excellent agreement with the theory even when the scale of axial variations is comparable to the channel width. In the opposite limit when the wavelength of fluctuations is much shorter than the channel width, the lubrication theory fails to describe the solution either qualitatively or quantitatively. In this short wave limit the solution is well described by Ajdari's theory for electroosmotic flow between infinite parallel plates (Ajdari, A., Phys. Rev. E 1996, 53, 4996-5005.) The infinitely thin electric double layer limit is assumed in the theory as well as in the simulation.     

Thermal analysis of peristaltic flow of nanosized particles within a curved channel with second-order partial slip and porous medium

Journal of Thermal Analysis and Calorimetry - In a thermo-dynamical system, maximum transfer of energy takes the center of attention. Industrial advancement in recent years augmented the need for...     

Effects of slip on the viscosity of polymer melts

We use existing scaling theories by de Gennes, Brochard, and Ajdari to calculate the apparent viscosity of multilayer blends with weakly entangled interfaces. The lowering of the apparent viscosity with respect to the bulk is a manifestation of interfacial slip. The theoretical predictions are compared with the recent experimental data of Zhao and Macosko. The theory is able to describe a continuous transition from a low-slip regime to a high-slip regime when the bulk rheology is still Newtonian, in agreement with experiments. However, the dependence of the apparent viscosity on the shear rate and layer thicknesses is much stronger than what is observed experimentally. The apparent viscosity is also calculated for dilute polymer emulsions. We modify a theory of Palierne, which is valid in the linear viscoelastic regime for the bulk, to include the effects of interfacial slip. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1888–1904, 2004