It’s known that there is not a scientific way of describing what is microfluidics because it is not at all a scientific phenomenon. So, microfluidics does not include anything from pure scientific terminology and its considerations. Basically, microfluidics is the use of devices that apply fluid flow to channels smaller than 1 millimeter in at least one size. The term deals with the behavior or controlling of the fluids (generally in sub-millimeter scale) at which surface physical characteristics dominate volumetric characteristics.

Terminology of Microfluidics

For the better understanding microfluidics, terminology of “fluid” must be known very well. The term of ‘fluid’ is defined as a substance that shows continuous shear deformation in response to an applied shear force. In physics, the behaviour of the fluid is mathematically expressed in Navier Stoke’s Equation which is generated from applying Newton’s second law to fluid motion.

Microfluidics 1

where ρ = density, μ = viscosity, ν = velocity (vector), P=pressure, g = gravity.

In that equation, it’s shown that fluid is affected by three main forces which are pressure gradient, viscous forces, and gravity.

Fluid mechanics are different from solid mechanics. There is a specific basic physical difference between them: While the solids become deformed under a force which is apllied on, fluid is just flow. In other words, fluids cannot to resist deformation that is caused by applied force and continue to flow as long as the force is applied.

Solid state A Fluid state B shear strain under shear stress

The angle of deformation α (refers the shear strain or angular displacement) increases in directly proportional to the applied force F (Figure 1. A). Using the assumption that there is no slip between the plate and the rubber, while the bottom surface is in a fixed position, the upper surface is displaced in direct relation to the displacement of the plate. In figure 1. B, the velocity at the bottom side (boundary plate) is “0” and the velocity profile is getting large as the flow goes away from the boundary.

In solid-state shear stress applied is proportional to shear strain. The proportional factor is called the shear modulus. When equilibrium is reached solid material ceases to deform. But in the liquid phase shear stress applied is proportional to the time rate of strain. Proportional factor is called dynamic (absolute) viscosity. Equilibrium is out of the question because liquid continues to deform as long as the stress is applied.

If the fluid shows a proportional shear rate under shear stress and if it’s in a cylindrical channel, the velocity profile is observed parabolic inside the channel. Fluid next to the wall have zero velocity by sticking to the wall at that point. Moving away from the wall the velocity increases to a maximum. The flow characteristic may liken the flow in a pipe.

A flow gradient is observed from the surfaces of the fluids towards the centre. Under normal conditions, any part of the fluid has a different velocity than another part next to it. Particles exert forces on each other (due to intermolecular action) as a result of having different velocities between neighbour particles. If there is not a wall, the velocity gradient would not be made because of the absence of shear forces.

Velocity profile of fluid in a pipe

As a definiton of viscosity, it is a measure of a fluid’s ability to resist gradual deformation by shear or tensile stress.

Microfluidics 2

where ν= fluid velocity, y= distance from solid surface, dν /dy= rate of strain, µ=dynamic viscosity, and τ= shear stress.

When the viscosity is considered as resistance to flow, velocity distribution next a boundary graph is shown below:

Velocity distribution next a boundary graph Left Viscosity graph of fluids Right

Viscosity, is the ratio between shear stress applied and the rate of deformation. For example, walking through a 1-m pool of water is easier than oil. Because water has less friction. Water moves out of your way at a quick rate when you applying shear stress by mean of walking through it. But oil moves out of your way more slowly when you apply the same stress.

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