Measurement of electroosmotic and electrophoretic velocities using pulsed and sinusoidal electric fields.

In this work, we explore two methods to simultaneously measure the electroosmotic mobility in microchannels and the electrophoretic mobility of micron-sized tracer particles. The first method is based on imposing a pulsed electric field, which allows to isolate electrophoresis and electroosmosis at the startup and shutdown of the pulse, respectively. In the second method, a sinusoidal electric field is generated and the mobilities are found by minimizing the difference between the measured velocity of tracer particles and the velocity computed from an analytical expression. Both methods produced consistent results using polydimethylsiloxane microchannels and polystyrene micro-particles, provided that the temporal resolution of the particle tracking velocimetry technique used to compute the velocity of the tracer particles is fast enough to resolve the diffusion time-scale based on the characteristic channel length scale. Additionally, we present results with the pulse method for viscoelastic fluids, which show a more complex transient response with significant velocity overshoots and undershoots after the start and the end of the applied electric pulse, respectively.

Measurement of relaxation times in extensional flow of weakly viscoelastic polymer solutions.

The characterization of the extensional rheology of polymeric solutions is important in several applications and industrial processes. Filament stretching and capillary breakup rheometers have been developed to characterize the extensional properties of polymeric solutions, mostly for high-viscosity fluids. However, for low concentration polymer solutions, the measurements are difficult using available devices, in terms of the minimum viscosity and relaxation times that can be measured accurately. In addition, when the slow retraction method is used, solvent evaporation can affect the measurements for volatile solvents. In this work, a new setup was tested for filament breakup experiments using the slow retraction method, high-speed imaging techniques, and an immiscible oil bath to reduce solvent evaporation and facilitate particle tracking in the thinning filament. Extensional relaxation times above around 100 µs were measured with the device for dilute and semi-dilute polymer solutions. Particle tracking velocimetry was also used to measure the velocity in the filament and the corresponding elongation rate, and to compare with the values obtained from the measured exponential decay of the filament diameter.

The stabilizing effect of shear thinning on the onset of purely elastic instabilities in serpentine microflows.

We determine both experimentally and numerically the onset of elastic flow instabilities in viscoelastic polymer solutions with different levels of shear thinning. Previous experiments realized in microfluidic serpentine channels using dilute polymeric solutions showed that the onset of elastic instabilities strongly depends on the channel curvature. The scaling dependence is well captured by the general instability scaling criterion proposed by Pakdel and McKinley [Phys. Rev. Lett., 1996, 76, 2459:1-4]. We determine here the influence of fluid shear thinning on the onset of such purely-elastic flow instabilities. By testing a set of polyethylene oxide solutions of high molecular weight at different polymer concentrations in microfluidic serpentine channels we observe that shear thinning has a stabilizing effect on the microfluidic flow. Three-dimensional numerical simulations performed using the White-Metzner model predict similar trends, which are not captured by a simple scaling analysis using the Pakdel-McKinley criterion.

Flow cells as quasi-ideal systems for biofouling simulation of industrial piping systems.

Semi-circular flow cells are often used to simulate the formation of biofilms in industrial pipes with circular section because their planar surface allows easy sampling using coupons. Computational fluid dynamics was used to assess whether the flow in pipe systems can be emulated by the semi-circular flow cells that are used to study biofilm formation. The results show that this is the case for Reynolds numbers (Re) ranging from 10 to 1000 and 3500 to 10,000. A correspondence involving the friction factor was obtained in order to correlate any semi-circular flow cell to any circular pipe for Re between 10 and 100,000. The semi-circular flow cell was then used to assess experimentally the effect of Reynolds number (Re = 4350 and 6720) on planktonic cell concentration and biofilm formation using Escherichia coli JM109 (DE3). Lower planktonic cell concentrations and thicker biofilms (>1.2 mm) were obtained with the lower Re.

Optimized cross-slot flow geometry for microfluidic extensional rheometry.

A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number Wi(crit) ≈ 0.5. Birefringence and bulk pressure drop measurements provide self-consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates (26 s(-1) ≤ ε ≤ 435 s(-1)) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model.

Setup and validation of flow cell systems for biofouling simulation in industrial settings.

A biofouling simulation system consisting of a flow cell and a recirculation tank was used. The fluid circulates at a flow rate of 350 L· h⁻¹ in a semicircular flow cell with hydraulic diameter of 18.3 mm, corresponding to an average velocity of 0.275 m· s⁻¹. Using computational fluid dynamics for flow simulation, an average wall shear stress of 0.4 Pa was predicted. The validity of the numerical simulations was visually confirmed by inorganic deposit formation (using kaolin particles) and also by direct observation of pathlines of tracer PVC particles using streak photography. Furthermore, the validity of chemostat assumptions was verified by residence time analysis. The system was used to assess the influence of the dilution rate on biofilm formation by Escherichia coli JM109(DE3). Two dilution rates of 0.013 and 0.0043 h⁻¹ were tested and the results show that the planktonic cell concentration is increased at the lower dilution rate and that no significant changes were detected on the amount of biofilm formed in both conditions.

Flow dynamics of vitreous humour during saccadic eye movements.

In this work, we reveal the flow dynamics of Vitreous Humour (VH) gel and liquid phases during saccadic movements of the eye, considering the biofluids viscoelastic character as well as realistic eye chamber geometry and taking into account the saccade profile. We quantify the differences in the flow dynamics of VH gel and liquid phases using viscoelastic rheological models that are able to model the VH shear rheology, considering different amplitudes of saccadic movements (10∘, 20∘, 30∘ and 40∘). For this purpose, the computational fluid dynamics (CFD) open source software OpenFOAM® was used. The results portray a distinct flow behaviour for the VH gel and liquid phases, with inertial effects being more significant for the VH liquid phase. Moreover, the Wall Shear Stress (WSS) values produced by the VH gel phase are more than twice of those generated by the VH liquid phase. Results also show that for different amplitudes of eye movement both the velocity magnitude in the vitreous cavity and the shear stresses on the cavity walls rise with increasing saccadic movement displacement.

Optimization of flow-focusing devices for homogeneous extensional flow.

We present a methodology for the shape optimization of flow-focusing devices with the purpose of creating a wide region of homogeneous extensional flow, characterized by a uniform strain-rate along the centerline of the devices. The numerical routines employed include an optimizer, a finite-volume solver, and a mesh generator operating on geometries with the walls parameterized by Bézier curves. The optimizations are carried out for devices with different geometric characteristics (channel aspect ratio and length). The performance of the optimized devices is assessed for varying Reynolds numbers, velocity ratio between streams, and fluid rheology. Brownian dynamics simulations are also performed to evaluate the stretching and relaxation of λ-DNA molecules in the devices. Overall, the optimized flow-focusing devices generate a homogeneous extensional flow over a range of conditions typically found in microfluidics. At high Weissenberg numbers, the extension of λ-DNA molecules in the optimized flow-focusing devices is close to that obtained in an ideal planar extensional flow with an equivalent Hencky strain. The devices presented in this study can be useful in microfluidic applications taking advantage of homogeneous extensional flows and easy control of the Hencky strain and strain-rate.

Tricritical spiral vortex instability in cross-slot flow.

We examine fluid flow through cross-slot devices with various depth to width ratios α. At low Reynolds number, Re, flow is symmetric and a sharp boundary exists between the two incoming fluid streams. Above an α-dependent critical value, Re(c)(α), a steady symmetry-breaking bifurcation occurs and a spiral vortex structure develops. Order parameters characterizing the instability grow according to a sixth-order Landau potential, and show a progression from second- to first-order transitions as α increases beyond a tricritical value of α ≈ 0.55. Flow simulations indicate the instability is driven by vortex stretching at the stagnation point.

Serpentine channels: micro-rheometers for fluid relaxation times.

We propose a novel device capable of measuring relaxation times of viscoelastic fluids as small as 1 ms. In contrast to most rheometers, which by their very nature are concerned with producing viscometric or nearly-viscometric flows, here we make use of an elastic instability that occurs in the flow of viscoelastic fluids with curved streamlines. To calibrate the rheometer we combine simple scaling arguments with relaxation times obtained from first normal-stress difference data measured in a classical shear rheometer. As an additional check we also compare these relaxation times to those obtained from Zimm theory and good agreement is observed. Once calibrated, we show how the serpentine rheometer can be used to access smaller polymer concentrations and lower solvent viscosities where classical measurements become difficult or impossible to use due to inertial and/or resolution limitations. In the absence of calibration, the serpentine channel can still be a very useful comparative or index device.