Index of content:
Volume 10, Issue 4, July 2016
- SPECIAL TOPIC: INVITED ARTICLES ON MICROFLUIDIC RHEOLOGY (GUEST EDITORS ANKE LINDNER AND PAULO ARRATIA)
- Review Articles
10(2016); http://dx.doi.org/10.1063/1.4945604View Description Hide Description
Characterization of the extensional rheometry of fluids with complex microstructures is of great relevance to the optimization of a wide range of industrial applications and for understanding various natural processes, biological functions, and diseases. However, quantitative measurement of the extensional properties of complex fluids has proven elusive to researchers, particularly in the case of low viscosity, weakly elastic fluids. For some time, microfluidic platforms have been recognized as having the potential to fill this gap and various approaches have been proposed. This review begins with a general discussion of extensional viscosity and the requirements of an extensional rheometer, before various types of extensional rheometers (particularly those of microfluidic design) are critically discussed. A specific focus is placed on microfluidic stagnation point extensional flows generated by cross-slot type devices, for which some important developments have been reported during the last 10 years. Additional emphasis is placed on measurements made on relevant biological fluids. Finally, the operating limits of the cross-slot extensional rheometer (chiefly imposed by the onset of elastic and inertial flow instabilities) are discussed.
- Regular Articles
10(2016); http://dx.doi.org/10.1063/1.4945603View Description Hide Description
Microrheometry is very important for exploring rheological behaviours of several systems when conventional techniques fail. Microrheometrical measurements are usually carried out in microfluidic devices made of Poly(dimethylsiloxane) (PDMS). Although PDMS is a very cheap material, it is also very easy to deform. In particular, a liquid flowing in a PDMSdevice, in some circumstances, can effectively deform the microchannel, thus altering the flow conditions. The measure of the fluid relaxation time might be performed through viscoelasticity induced particle migration in microfluidics devices. If the channel walls are deformed by the flow, the resulting measured value of the relaxation time could be not reliable. In this work, we study the effect of channel deformation on particle migration in square-shaped microchannel. Experiments are carried out in several PolyEthylene Oxyde solutions flowing in two devices made of PDMS and Poly(methylmethacrylate) (PMMA). The relevance of wall rigidity on particle migration is investigated, and the corresponding importance of wall rigidity on the determination of the relaxation time of the suspending liquid is examined.
10(2016); http://dx.doi.org/10.1063/1.4948235View Description Hide Description
Understanding the elongational rheology of dilute polymer solutions plays an important role in many biological and industrial applications ranging from microfluidic lab-on-a-chip diagnostics to phenomena such as fuel atomization and combustion. Making quantitative measurements of the extensional viscosity for dilute viscoelastic fluids is a long-standing challenge and it motivates developments in microfluidic fabrication techniques and high speed/strobe imaging of millifluidic capillary phenomena in order to develop new classes of instruments. In this paper, we study the elongational rheology of a family of dilute polymeric solutions in two devices: first, steady pressure-driven flow through a hyperbolic microfluidic contraction/expansion and, second, the capillary driven breakup of a thin filament formed from a small diameter jet (). The small length scale of the device allows very large deformation rates to be achieved. Our results show that in certain limits of low viscosity and elasticity, jet breakup studies offer significant advantages over the hyperbolic channel measurements despite the more complex implementation. Using our results, together with scaling estimates of the competing viscous, elastic, inertial and capillary timescales that control the dynamics, we construct a dimensionless map or nomogram summarizing the operating space for each instrument.
10(2016); http://dx.doi.org/10.1063/1.4953863View Description Hide Description
Microfluidic stickers are used as a sample environment to measure the microrheology of monoclonal antibody (mAb) protein solutions. A Peltier-based microscope stage is implemented and validated, and is capable of controlling the sample temperature over the range 0.9–40 °C. The design accounts for heat transfer to and from the objective, controls the sample environment humidity to mitigate condensation, and provides adequate damping to reduce vibration from the cooling system. A concentrated sucrose solution is used as a standard sample to provide an in situ temperature measurement by the Stokes-Einstein-Sutherland relation. By combining microfluidic stickers and microrheology, 72 temperature-concentration viscosity measurements of mAb solutions can be made in 1 day, a significant increase in throughput over conventional rheometry.
10(2016); http://dx.doi.org/10.1063/1.4954194View Description Hide Description
We use a microfluidic flow-focusing device with integrated electrodes for controlling the production of water-in-oil drops. In a previous work, we reported that very long jets can be formed upon application of AC fields. We now study in detail the appearance of the long jets as a function of the electrical parameters, i.e., water conductivity, signal frequency, and voltage amplitude. For intermediate frequencies, we find a threshold voltage above which the jet length rapidly increases. Interestingly, this abrupt transition vanishes for high frequencies of the signal and the jet length grows smoothly with voltage. For frequencies below a threshold value, we previously reported a transition from a well-behaved uniform jet to highly unstable liquid structures in which axisymmetry is lost rather abruptly. These liquid filaments eventually break into droplets of different sizes. In this work, we characterize this transition with a diagram as a function of voltage and liquid conductivity. The electrical response of the long jets was studied via a distributed element circuit model. The model allows us to estimate the electric potential at the tip of the jet revealing that, for any combination of the electrical parameters, the breakup of the jet occurs at a critical value of this potential. We show that this voltage is around 550 V for our device geometry and choice of flow rates.
10(2016); http://dx.doi.org/10.1063/1.4954193View Description Hide Description
We analyze the effective rheology of a dilute suspension of self-propelled slender particles confined between two infinite parallel plates and subject to a pressure-driven flow. We use a continuum kinetic model to describe the configuration of the particles in the system, in which the disturbance flows induced by the swimmers are taken into account, and use it to calculate estimates of the suspension viscosity for a range of channel widths and flow strengths typical of microfluidic experiments. Our results are in agreement with previous bulk models, and in particular, demonstrate that the effect of activity is strongest at low flow rates, where pushers tend to decrease the suspension viscosity whereas pullers enhance it. In stronger flows, dissipative stresses overcome the effects of activity leading to increased viscosities followed by shear-thinning. The effects of confinement and number density are also analyzed, and our results confirm the apparent transition to superfluidity reported in recent experiments on pusher suspensions at intermediate densities. We also derive an approximate analytical expression for the effective viscosity in the limit of weak flows and wide channels, and demonstrate good agreement between theory and numerical calculations.
10(2016); http://dx.doi.org/10.1063/1.4954936View Description Hide Description
A geometrically simple flow cell is proposed to generate different types of stagnation flows, using a separation flow and small variations of the geometric parameters. Flows with high local deformation rates can be changed from purely rotational, over simple shear flow, to extensional flow in a region surrounding a stagnation point. Computational fluid dynamic calculations are used to analyse how variations of the geometrical parameters affect the flow field. These numerical calculations are compared to the experimentally obtained streamlines of different designs, which have been determined by high speed confocal microscopy. As the flow type is dictated predominantly by the geometrical parameters, such simple separating flow devices may alleviate the requirements for flow control, while offering good stability for a wide variety of flow types.