Separation Techniques Using Microfluidics Mohanad Radhi

Microfluidics Definition: is the use of procedures for controlling and manipulating fluid flows in channels with dimensions of order of 1 millimeter or less. Possibility of manufacturing devices with accuracy up to many micrometers The exponential developments in biology and biotechnology where samples sizes are very small. The increasing demand for mobile devices able to achieve inexpensive analysis Powerful use of microscale systems to do principal studies in major fields.

Problems On the microscale, membrane filtration techniques suffer from various disadvantages including fabrication of complex 3-D structures to define the pore sizes and issues arising from membrane clogging. These factors have limited the popularity of this technique at the microscale leading to the development of numerous membraneless separation techniques. The use of chemical substances is harmful for biomedical applications within this scale in addition to their catastrophic effect on the enviroment; Therefore, passive separation devices are needed to avoid or at least to minimize the adversary effects.

1-Suspensions in Liquids Hydrodynamic Passive Separation: A sample inlet was placed in locations relative to the inlet reservoir, Fig. 1. Particles of different sizes were able to migrate along different trajectories out of the reservoir based on their preferred streamline due to viscous drag force as result of Stokes Flow.

1-Suspensions in Liquids-Continued B. Separation Using Dielectophoresis(DEP):To generate nonuniform electric fields, most DEP-based devices integrate various types of electrodes by different techniques into their glass/Si substrates. DEP force is dependent on the size and shape of particles, the electrical properties of liquid and particles, and the magnitude and frequency of applied nonuniform electric field.

1-Suspensions in Liquids-Continued C. Separation Using Acoustophoresis: an acoustic radiation force acts on the cells or particles due to the differences in density and compressibility of the cells and particles with respect to the surrounding fluid medium. suspended particles with positive contrast factors will move towards the pressure node; and particles with negative contrast factors, towards the pressure anti-node.

1-Suspensions in Liquids-Continued D. Inertial Microfluidics with Spiral Microchannels:Particles flowing in a spiral microchannel with rectangular cross-section experience a combination of inertial lift and Dean drag forces. Magnitude and direction forces depend on the particle size and position across the microchannel. Position at which particles of different sizes equilibrate is dependent on the ratio of lift and Dean drag forces.

2-Separation of Two Immiscible Fluids One liquid is typically used as the solvent for the chemical species of interest, while the other acts as an inert carrier that maintains the discrete nature of the solvent flow. The carrier fluid is chosen to have a greater affinity for the porous tubing than the solvent, causing it to wet and subsequently permeate the porous wall. To achieve reliable phase separation an appropriate back pressure must be established within the separator.

3-Separation of Gas from Liquid by Microdistillation Using Carrier Gas: performed using a dispersed phase driven by carrier gas like helium or nitrogen employed specifically to derive the vapor gas flow through the distillation chip Limitation:no reflux can be circulated back to the microchannel because of the uni-directional flow of the carrier gas. additional gas–liquid separator is required after the distillation process to remove the carrier gas.

3-Separation of Gas from Liquid by Microdistillation-Continued B. Using Vacuum: vacuum can be applied to drive the vapour flow through the microchannel. Incorporating a microporous membrane in a multilayer configuration. membrane is used to maintain and stabilize the vapor–liquid interface. a higher temperature gradient is needed when a membrane with smaller pore diameter is used

3-Separation of Gas from Liquid by Microdistillation-Continued C. Using Capillary Force: vapor flow can be established as a result of a pressure difference created by the continuous vaporization and condensation in the heating and cooling regions of the device, respectively. Mass transfer takes place at the liquid–vapor interface, leading to a separation based on vapor–liquid equilibrium

Conclusions: It is possible to achieve 100% separation efficiency by controlling the main parameters like pressure, flow rate, current intensity..etc Clean separation processes can be performed depending on the physical manipulations rather than chemical reactions. Complex mixtures can be separated by integrating two or more separation techniques leading to a multistage separator. Passive separation using microchannels is a promising area that needs to be explored deeply. Massive separation may be obtained by connecting large number of microchannels in parallel.

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