Education:
- PhD, Cornell University, Ithaca, NY, USA, 1993
- MS, University of Arizona, Tucson, USA, 1987
- BTech (Hons), I.I.T. Kharagpur, India, 1985
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Experience:
- Professor, IISc., Bangalore 2008 - Present
- Associate Professor, IISc, Bangalore 2002 - 2008
- Assistant Professor, IISc, Bangalore 1996 - 2002
- Lecturer, Cornell University, USA, 1994 - 1996
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Research Areas:
- Micro and Nano-Electro-Mechanical Systems (MEMS / NEMS) - Modeling, design, fabrication, and characterization of MEMS and NEMS devices
MEMS technology has given an unprecedented push to the development of sensors and actuators of extremely small size. The adoption of electronic manufacturing techniques has led to not only tighter integration with front end electronics but also higher reliability and the obvious cost advantage of batch fabrication. These advantages are likely to result into development of a large class of MEMS & NEMS sensors that could become ubiqutous in applications all around us, imparting intelligence to all man made devices, big and small, from aeroplanes to medical implants.
We design and develop dynamic MEMS and NEMS devices with applications in inertial sensing (MEMS gyros, mass detectors), acoustics (MEMS and NEMS microphones), and ultrasonics (capacitive and piezo micromachined ultrasound transducers, also known as CMUTs and PMUTs). In studying these devices, one particular aspect of their response that has fascinated us the most is the energy dissipation mechanism at the micro and nano scales. In particuar, we have been investigatiing squeeze film damping and its effect on the dynamic response of micro and nanomechanical structures. Squeeze film damping referes to energy dissipation in fluid flow caused by “squeezing” of a very thin film of air or some other gas trapped between an oscillating mechanical structure (e.g., a vibrating membrane) and a fixed substrate. We have been interested in understanding squeeze film flow and the associated energy dissipation in kinds of flow conditions—from continuum to molecular flow. We have carried out careful analytical, computational, and experimental studies on squeeze film damping and continue to pursue our invetigation to get a better understanding of this phenomenon. These studies have a direct bearing on better design of MEMS and NEMS resonators and several other dynamic devices that must use oscillating structures in confined spaces for their basic operation.
In MEMS and NEMS research, we are currently pursuing the following projects:
- Design and development of a suspended gate field effect transistor (SGFET) coupled MEMS microphone and a MEMS gyroscope.
- Design and development of CMUTs for subsurface crack detection, and development of PMUTs for level sensing applications.
- Study of the effect of fluid flow boundary conditions on squeeze film dynamics and evaluation of its effect on the response of MEMS structures.
- Study of the effect of nanoscale nonhomogenization on the piezoelectric response of thin metalic films and development of nanoscale metalic strain gauges.
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Fig. 1 A 50µm long slotted cantilever beam
made of SiO2 with 70 nm thick Au film on top. |
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Fig. 2 A hexagonal CMUT cell with 55 µm side length, made up of 1µm thick polysilicon membrane |
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Fig. 3 A dual axis MEMS gyroscope |
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- Dynamics of Micro-Mechanical Structures and Vibroacoustics - Experimental and theoretical studies of vibration of very small mechanical and biological structures
There is something special about vibration of very small scale structures. There are, at times. several competing forces that we do not care about at macroscales but they play significant and confusing roles at micro and nano scales, and there are some familiar forces that baffle us because of their conspicuous absence. A net result is that we observe motions at these scales that are normally out of reach at macroscales. We carry out experimental work using laser Doppler vibrometry that is capable of detecting picometer amplitude oscillations. We use Microsystem Analysers, MSA 400 and MSA 500 from Polytec, for these experimental studies. We have been able to capture upto 28 modes of vibration of a microheater structure—the highest number of modes ever recorded from a single measurement—and verified them independently with FEM simulations. These kinds of studies have encouraged us to investigate the dynamics of biological structures.
In particular, we are currently involved in the following studies:
- Dynamics of cells under healthy and unhealthy states, signatures of pathology in recorded spectra, and possibility of developing mechanical diognostic tools.
- Study of the dynamics of cranefly halteres with a special emphasis on understanding their role in angular motion sensing.
- Study of the mechanism of sound production in crickets and invetigation of the frequency and pitch variation with structural and nonstructural controls.
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Fig. 4 The first mode of
vibration of a CMUT Cell
constructed from experimental data obtained using laser vibrometry. |
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Fig. 5(a) A MEMS thermal actuator. |
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Fig. 5(b) Dynamic response of the actuator (shown on the right) obtained from MSA 400 capturing 28 modes of the structure.A dual axis MEMS gyroscope |
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Fig. 6(a) A torsional mirror |
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Fig. 6(b) Experimental investigation of the effect of air pressure on the quality factor of the mirror over the entire range of squeeze film flow from continuum to molecular flow. |
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- Modeling and Computational Mechanics - Modeling of dynamic processes at micro and nano scales
Modeling is an indispensable tool in understanding the physical world at any scale. Mirco and nanoscale systems provide an exciting opportunity for modeling since we know so little about things at this scale and our ability to observe things at this scale is severely limited. Modeling at ths scale is particularly challenging because we have so little intuition at this scale. We keep pushing the limits of physical laws we know and use in our modeling with a hope that they do not betray us. But, it is excitement unlimited when they do. Modeling of these systems often involve multiple energy domains requiring multi-physics modeling and often lead to multiscale modeling. In our work, we often use commercially available tools, such as COMSOL, ANSYS Multiphysics, and CoventorWare. However, the most basic tool we use in our lab all the time is MATLAB. We create models, carry out extensive simulations, try to validate at least some of them experimentally, and build predictive tools for design. We have been involved in modeling mostly dynamics of small scale structures and phenomena such as squeeze film damping associated with such dynamics.
Currently, we are carrying out following studies in this area:
- Creation of a comprehensive FEM based comupational package for squeeze film analysis in MEMS and NEMS devices
- Modeling of dissipation in moleculra flow regime using DSMC (Direct Simulation Monte Carlo) in order to interpret what a coninuum concept like viscosity means at that scale.
- Modeling of the dynamics of ATP synthase motor in order to understand the forces and torques generated from multiple domains for driving this motor.
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Fig. 7 The first three modal frequencies and the corresponding mode shapes of a simple MEMS cantilever captured experimentally by laser Doppler vibrometry. |
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Fig. 8 The simulated velocity distribution of air flow around a square cavity open on all four sides due to transverse oscillations of an elastic plate clamped on all sides. |
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Fig. 9 The simulated fluid pressure distribution under an elastic micro mechanical cantilever beam oscillating in the first three bending modes. A dual axis MEMS gyroscope |
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Selected Publications:
- Venkatesh K.P. and Rudra Pratap, “Capturing Higher Modes of Vibration of Micromachined Resonators”, Journal of Physics: Conference Series Vol. 181, 2009
- Balaji Jayaramana, Navakanta Bhat and Rudra Pratap, “Thermal Characterization of Microheaters from the Dynamic Response”, Journal of Micromechanics and Microengineerng,, Vol. 19, pp 11085006, 2009
- Suhas S. Mohite, Venkata R. Sonti, and Rudra Pratap, “A Compact Squeeze-film Model including Inertia, Compressibility and Rarefaction Effects for Perforated 3D MEMS Structures”, Journal of Microelectromechanical Systems, Vol. 17, No. 3, pp. 709-723, June 2008
- A. K. Pandey, Rudra Pratap and Fook Siong Chau, “Effect of Pressure on Fluid Damping In MEMS Torsional Resonators with Flow Ranging from Continuum to Molecular Regime”, Experimental Mechanics , Vol. 48, No. 1, 2008
- A. K. Pandey and R. Pratap, “A Comparative Study of Analytical Squeeze-Film Damping Models in Rigid Rectangular Perforated MEMS Structures with Experimental Results”, Microfluidics and Nanofluidics, Vol. 4, No. 3, 2008
- Ashok Kumar Pandey and Rudra Pratap, “Effect of Flexural Modes on Squeeze Film Damping In MEMS Cantilever Resonators”, Journal of Micromechanics and Microengineering, Vol. 17(12): pp 2475-2484, 2008
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