Microfluidic devices shearing force

Liquid viscosities have been observed to increase, decrease, and remain constant in microfluidic devices as compared to viscosities in larger systems. ° Deviations from the no-slip boundary condition have been observed to occur at high shear rates. One important deviation from no-slip conditions occurs at moving contact lines, such as when capillary forces pull a liquid into a hydrophilic channel. The point at which the gas, liquid, and solid phases move along the channel wall is in violation of the no-slip boundary condition. Ho and Tai review discrepancies between classical Stokes flow theory and observations of flow in microchannels. No adequate theory is yet available to explain these deviations from classical behavior. ... 

Despite the many advantages of microfluidic devices as appHed in che-motaxis assays, some problems have appeared. First, a constant flow rate is required to maintain a stable chemokine gradient, and the shear force... 

Xu, J., Luo, G., Li, S., Chen, G. (2006). Shear force induced monodisperse droplet formation in a microfluidic device by controlling wetting properties. Lab on a Chip, 6, 131-136. 

For mechanical lysis, nanostructured filter-Uke contractions are employed in microfluidic channels with pressure-driven cell flow. Prinz et al. utilized rapid diffusive mixing to lyse Escherichia coli cells and trap the released chromosome via dielectrophoresis (DEP). Kim et al. developed a microfluidic compact disk platform for mechanical lysis of cells using spherical particles with an efficiency of approximately 65 % however, this method is difficult to be apphed for single-cell analysis. Lee et al. fabricated nanoscale barbs in a microfluidic chip for mechanical cell lysis by shear and frictional forces. Munce et al. reported a device to lyse individual cells by electromechanical shear force at the entrance of 10 mm separation channels. The contents of individual cells were simultaneously injected into parallel channels for electrophoretic separation, which can be recorded by laser-induced fluorescence OLIF) of the labeled cellular contents. The use of individual separation channels for each cell separation eliminated possible cross-contamination from multiple cell separations in a single channel. 

One of the main applications of microfluidic electroporation devices is cell lysis. In these devices, the mechanical (shear force) or electrical forces are applied to rapture the cell membrane and release its intercellular contents. 

The generation of a stable and controllable fluid flow in microfluidic devices is a major issue, and a lot of research work has been put into optimizing the flow driving methods. Not only conventional methods (derived from macroscopic applications) like pressure-driven and electroosmotic flows have been scaled down, but also novel methods like shear-driven flows (SDF) have been introduced. There are several problems associated with the conventional flow driving methods pressure-driven flows suffer from pressure drop limitations, while electroosmotic flows suffer from Joule heating, fluctuations of flow velocity, and double-layer overlap [1]. Therefore, other approaches to evade these problems and limitations have been proposed (centrifugal forces, magnetohydrodynamic forces, etc.). 

For cells adhered within microfluidic devices, the easiest mode of detachment is by means of fluid flow. As demonstrated by Murthy et al. [5] (Fig. 3), the cell adhesion within microfluidic devices is dependent on the magnitude of flow-induced fluid shear forces. Under most conditions, increasing the magnitude of fluid shear forces (by, for example, increasing the flow rate) will result in lower cell adhesion. Therefore it is possible to design a system wherein cells are captured selectively while flowing through the device at a slow rate, and these captured cells... 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

Microfluidics handles and analyzes fluids in structures of micrometer scale. At the microscale, different forces become dominant over those experienced in everyday life [161], Inertia means nothing on these small sizes the viscosity rears its head and becomes a very important player. The random and chaotic behavior of flows is reduced to much more smooth (laminar) flow in the smaller device. Typically, a fluid can be defined as a material that deforms continuously under shear stress. In other words, a fluid flows without three-dimensional structure. Three important parameters characterizing a fluid are its density, p, the pressure, P, and its viscosity, r. Since the pressure in a fluid is dependent only on the depth, pressure difference of a few pm to a few hundred pm in a microsystem can be neglected. However, any pressure difference induced externally at the openings of a microsystem is transmitted to every point in the fluid. Generally, the effects that become dominant in microfluidics include laminar flow, diffusion, fluidic resistance, surface area to volume ratio, and surface tension [162]. 

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 pUL/jj, 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 Vm) of Navier-Stokes equations can be dropped. Without this nonlinear convection, simple micro/nanofluidic systems have laminar, deterministic flow patterns. They have parabolic velocity profile in pressure-driven flows, plug-like velocity profile in elec-froosmotic flows, or a superposition of both. One of the benefits from the low Re number flow is that genomic material can be transported easily without shearing in Lab-... 

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