Microfluidic fluid mechanics

The small characteristic length scales found in microreactors significantly affect flow behavior in these systems and, consequently, their design and performance. In the section that follows, we will give an overview of some of the important characteristics of fluid mechanics at the microscale. The reader is referred to one of the many texts or review articles on microfluidics for a more thorough discussion of the topic. " " ... 

H.A. Stone, A.D. Stroock, and A. Ajdari, Engineering flows in small devices Microfluidics toward a lab-on-a-chip. Annual Review of Fluid Mechanics, 36, 381-411,(2004). 

M. De Menech, P. Garstecki, F. Jousse, and H.A. Stone, Transition from squeezing to dripping in a microfluidic T-shaped junction, Journal of Fluid Mechanics, 595, 141-161, (2008). 

IJ.-TAS) [4, 9,10], which strive to develop true lab on a chip , or fully integrated analytical systems involving sample introduction, preparation, analysis (may or may not include a separation) and result reporting on a single substrate. This field is still in its infancy, but has attracted many hundreds of researchers and now is the main topic of numerous international conferences and journals. Taken as a whole, development of chip-based separation capabilities helped establish the field of microfluidics, a branch of fluid mechanics devoted to fluid behavior in sub-millimeter diameter channels interconnected in simple and complex ways. Channels in this size range are commonly referred to as microchannels, whose cross sections are continually shrinking. A current hot research area deals with fluid behavior in nanochannels. 

GERF (discovered by Wen et.al.) has a yield stress up to 300 kPa in response to an electric field, which provides an alternative choice of digitally controllable microvalve that can respond within 10 pm [13, 43]. Its solid-like behavior sustains shear in the direction perpendicular to the applied electric field, the shear stress can be enhanced when the applied electric field increases, and its rheological variation is reversible upon removal of the electric field (Fig. 4). These marvelous features qualify GERF as an electric-fluid-mechanical interface for digital fluid control in microfluidics [55, 56]. 

Cocalia, V. A., Jensen, M. P., Holbrey, J. D., Spear, S. K., Stepinski, D. C. Rogers, R. D. (2005). Identical extraction behavior and coordination of trivalent or hexavalent f-element cations using ionic liquid and molecular solvents. Dalton Transactions, 2005, 1966-1971. de Menech, M., Garstecki, P., Jousse, F., Stone, H. (2008). Transition from squeezing to dripping in a microfluidic T-shaped junction. Journal of Fluid Mechanics, 595, 141-161. Dehkordi, A. M. (2001). Novel type of impinging streams contactor for liquid-liquid extraction. 

Kirby BJ (2010) Chapter 6 Electroosmosis. In Micro- and nanoscale fluid mechanics transport in microfluidic devices. Cambridge University Press, New York... 

In micro- and nanoscale fluid mechanics, measurements of mass transport and fluid velocity are used to probe fundamental physical phenomena and evaluate the performance of microfluidic devices. Evanescent wave illumination has been combined with several other diagnostic techniques to make such measurements within a few hundred nanometers of fluid—solid interfaces with a resolution as small as several nanometers. Laser Doppler velocimetry has been applied to measure single-point tracer particle velocities in the boundary layer of a fluid within 1 pm of a wall. By seeding fluid with fluorescent dye, total internal reflection fluorescence recovery after photobleaching (FRAP) has been used to measure near-wall diffusion coefficients and velocity (for a summary of early applications, see Zettner and Yoda [2]). 

Hot-wire anemometers ( Micro/Nano-Anemometers ) have been developed for a wide spectrum of applications from experimental fluid mechanics to aerospace engineering to measure physical parameters such as temperature, flow rate, and shear stress. The advent of microelec-tromechanical systems (MEMS) and nanoscale thermal sensors has provided an entry point to microfluidics, biomedical sciences, and microcirculation in cardiovascular medicine. These MEMS and nanoscale devices are fabricated... 

Flow visualization is a branch of fluid mechanics that provides visual perception of the dynamic behavior of fluids flows. The fundamental principle of any flow visualization technique lies in the detection of fluid transport by altering the fluid properties while leaving the fluid motion unaltered. Microscale flow visualization focuses on imaging microfluidic flows, with the most common techniques broadly classified into particle-based and scalar-based methods. 

Microfluidic and nanofluidic phenomena arising from Lab-on-a-Chip microdevices are characterized by hierarchal multiscale nature with respect not only to space but also to time. In the area of microfluidics and nanofluidics, multiscale modeling aims to develop a viable multiscale computational methodology to study fluid mechanics in spatial and temporal domains ranging from the molecular level to the continuum level. 

Fluid flow in small devices acts differently from those in macroscopic scale. The Reynolds number (Re) is the most often mentioned dimensionless number in fluid mechanics. The Re number, defined by pf/L/p, represents the ratio of inertial forces to viscous ones. In most circumstances involved in micro- and nanofluidics, the Re number is at least one order of magnitude smaller than unity, ruling out any turbulence flows in micro-/nanochannels. Inertial force plays an insignificant role in microfluidics, and as systems continue to scale down, it will become even less important. For such small Re number flows, the convective term (pu Vu) of Navier-Stokes equations can be dropped. Without this nonlinear convection, simple micro-/ nanofluidic systems have laminar, deterministic flow patterns. They have parabolic velocity... 

In the case of flow in microscale pathways, some important parameters in macroscale fluid mechanics remain important, but the features of the flow are very different. The influences of viscosity and surface effects are much more significant compared with fluid flow in traditional pipelines or channels. That is, the ratio of the inertial force to the viscous force (Reynolds number) is in general smaller than unity, and the ratio of the inertial force to surface tension (Weber number) is also small, so that accurate microfluidic metering is somewhat more difficult than metering for macroscopic flows. For gas flows, the effect of the Knudsen number, defined as the ratio of the molecular mean free path to the channel size, is high and cannot be ignored for microscale channels, and hence the fluid metering discussed here will be for liquids only. The Reynolds number Re, the Weber number We, and the Knudsen number Kn are expressed as follows ... 

WILKES Solutions Manual for Fluid Mechanics for Chemical Engineering, 2nd edition, with Microfluidics and CFD... 

Huh D, Fujioka H, Tung Y, Futai N, Paine III R, Grotberg JB, Takayama S. 2007. Acoustically detectable cellular-level lung injury induced by fluid mechanical stresses in microfluidic airway systems. PNAS 104 18886. 

In chemical micro process technology there is a clear dominance of pressure-driven flows over alternative mechanisms for fluid transport However, any kind of supplementary mechanism allowing promotion of mixing is a useful addition to the toolbox of chemical engineering. Also in conventional process technology, actuation of the fluids by external sources has proven successful for process intensification. An example is mass transfer enhancement by ultrasonic fields which is utilized in sonochemical reactors [143], There exist a number of microfluidic principles to promote mixing which rely on input of various forms of energy into the fluid. 

---