Microfluidic channels with a spiral geometry are extensively researched for their use in particle focusing, separation and identification. Instead of using electrophoresis, magnetophoresis, etc., spiral channel takes advantage of the Inertial Lift Force along with the Viscous Drag to achieve size based separation of particles. Inertial microfluidic channel can have high throughput and are much safer to use for live cell separation and other physiological fluids processing. A particle flowing freely in a spiral microchannel at low Reynolds number inertial flow, attains lateral equilibrium due to balance of Inertial Lift force and the viscous Dean Drag. The inertial lift forces are primarily due to the wall effect and the shear gradient of the fluid flow profile. Much theoretical research has been done in this field to explain the lateral migration of a particle in an inertial fluid flow. Notable contributions were made by Saffman (1965), Ho and Leal (1974) and later Vasseur and Cox (1976) in explaining the lift force on a particle theoretically. All these and many other theoretical models developed in the last few decades discuss Lift force being dependent on the particle slip velocity. Additionally many models including the one developed by Saffman predicts a linear dependence of Lift force on the slip velocity of particle. But it seems that the microfluidic community has ignored this dependence with the result that several hypotheses and models exist in which the slip velocity is nonexistent. The measurement of slip velocities for particles has never been done in the field of microfluidics. The current study aims to do so and bridge the gap in understanding the Lift force responsible for the lateral migration of particles. The focused particle’s velocity when it passes through the outer arm of the spiral microfluidic device is measured experimentally followed by a computational study (using COMSOL Multiphysics) to obtain the undisturbed fluid flow velocity through the spiral arm. To calculate the slip velocity, identification of focusing positions in the horizontal and vertical plane of the channel is necessary. Identification in horizontal plane is easy by simply observing the channel under microscope. To identify the vertical focusing positions, a high speed camera (Photron SA-4) coupled with a Nikon microscope and a 50x objective lens (depth of focus = 0.9 um) is used. The narrow depth of focus of objective lens coupled with the precise movement of microfluidic device in the vertical plane is used to identify the height of focused particles from the channel bottom. A focus-measure of all the acquired images is calculated (using a Matlab script which calculates the global variance of an image as a focus-measure) followed by its statistical distribution to obtain the particle’s vertical location within an error of ±5 um. Velocity of the particles for all the focused positions is now calculated using a Matlab script which detects the particles from the acquired images and traces it across successive frames. At the focused position, particle is in equilibrium due to a balance of Dean Drag and the Inertial Lift force. Velocity components of Dean Flow are obtained from the computational study, followed by calculation of Dean Drag acting on the focused particles. The Lift force acting on the particle is now known and equating it with the slip velocity of particles, numerical values of Lift coefficient are obtained for the first time. These Lift coefficients are obtained for various focusing positions in the vertical plane of channel for two sets of Reynolds number
