Small scale devices are predominant in many contemporary technological applications. They are widespread across many streams of science and engineering. The study of convective flow field around micro-organisms has critical importance for environmental and biological studies. The characterization of flow inside micro channel is essential for proper design of micro heat exchanger used in electronics cooling applications. The detailed study of flow inside micro nozzle is important for improvement in quality of ink jet printing. The detailed analysis of flow inside complex pores is essential for modeling and prediction of enhanced oil recovery mechanism in petroleum engineering. The study of flow inside electro-osmosis based micro pump is required for design of micro fluidic dosing applications. The proper design of artificial heart valve needs flow study in models of human heart. The design of chemotherapeutic drug for treatment of brain tumor requires proper characterization of flow inside human ventricular system. According to a recent survey by systems planning corporation, the fluid flow plays a critical role on the performance of about 30% of MEMS devices. The study of fluid flow phenomena in these small scale devices is quite challenging from both modeling and measurement point of view. The traditional model based on continuum approach and Fourier laws of conduction etc. used for engineering/scientific analysis breaks down for small scale devices. Similarly, the intrusive techniques for measurements increase the system complexity and inaccuracy of measurements in small scale devices. The optical techniques are ideal for study of these devices due to their non-intrusive nature, high spatial and temporal resolution and field measurement capability.
Dr. P. K. Panigrahi from Mechanical Engineering Department of IIT Kanpur has been involved in development and implementation of various emerging optical techniques i.e. liquid crystal thermography, particle image velocimetry, laser schlieren and colour schlieren. These techniques have proved to be highly useful for various technological development projects related to aeronautics, naval, laser and nuclear engineering applications. His current research interest is the extension of above optical techniques for measurements in small scale devices and study of small scale phenomena. The Fluid Mechanics Laboratory of Mechanical Engineering Department is already equipped with a stereo PIV system, which is capable of 3-component velocity field measurement. The resolution of the conventional PIV system is of the order of 1-mm. The study of small scale devices requires the measurement resolution in the order of 1-μm and less. To overcome this shortcoming, a micro-particle-image-velocimetry ( -PIV) setup is being implemented in the laboratory.
The -PIV can be capable of flow field characterization with micron resolution. The small scale nature of the flow field brings additional difficulties for -PIV technique. Based on the Einstein's formula, the diffusion coefficient is inversely proportional to the particle diameter. Therefore, the mean square distance traveled by the particle due to diffusion is higher for -PIV. Hence, there is probability of greater loss in particle pairs from the interrogation zone. Saffman has observed the migration velocity away from the wall to be proportional to the velocity gradient. The blood cells flowing through capillaries stay away from the walls. This has been attributed to the shear induced migration of the particles away from the wall. The use of high magnification microscopic objective leads to low visibility of the particle and inaccuracy in the particle location due to diffraction limited imaging. In conventional PIV, the out of plane velocity is defined by the thickness of the light sheet. In contrast, for micro PIV the volume illumination is used and the depth of field of the lens defines the out-of-plane thickness of the measurement plane. This leads to higher noise by the out of focus particles and hence drop in correlation peak. The typical diameter of the particle to be used for micro PIV is smaller leading to the scattering in the Rayleigh regime. Therefore, there is a significant drop in the scattered light intensity for the -PIV compared to the traditional PIV leading to lower signal to noise ratio.
One drawback of existing PIV techniques is the inability for measurements of all components of velocity stress tensor and vorticity, which is required for complete characterization of the flow field. The three-dimensionality is stronger in small scale devices due to greater wall effect. Therefore, the concept of digital holography will be introduced with the -PIV technique for three dimensional flow field characterizations. This technique can also be extended to many other exciting applications i.e. nano-scope capable of measuring the internal shape of small channels and temperature measurement of flow inside small devices.