Centre for Nano Science and Engineering (CeNSE)

Indian Institute of Science

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Faculty - Rudra Pratap Faculty List  

Rudra Pratap
Chairperson

Room No. - SF 12

Ph: +91 80 2293 3250
Fax: +91 80 2360 8659
E-mail: pratap@cense.iisc.ernet.in

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Group webpage:
http://www.mecheng.iisc.ernet.in/~pratap/

Associated Departments:
Department of Mechanical Engineering (Professor)

Research Areas:

  • MEMS and NEMS Sensors
  • Transduction Targeted Material Development
  • Energy Dissipation in Oscillations of Micro and Nano Scale Structures
  • Vibratory Mechanobiology

 
Education:
  • PhD, Cornell University, Ithaca, NY, USA, 1993
  • MS, University of Arizona, Tucson, USA, 1987
  • BTech (Hons), I.I.T. Kharagpur, India, 1985
Experience:
  • Professor, IISc., Bangalore 2008 - Present
  • Associate Professor, IISc, Bangalore 2002 - 2008
  • Assistant Professor, IISc, Bangalore 1996 - 2002
  • Lecturer, Cornell University, USA, 1994 - 1996
 

Research Areas:

  • MEMS and NEMS Sensors

We design and develop dynamics MEMS and NEMS devices with applications in inertial sensing (MEMS gyros, MEMS resonators), acoustics, (MEMS microphones) and ultrasonics (capacitive and piezo electric ultrasound transducers, also known as CMUTs and PMUTs). Our focus is on exploiting appropriate transduction mechanisms that are favourable at these small scales and designing low power and high sensitivity sensors that can impart intelligence to manmade objects. All the sensors we design and develop use extremely small structures that oscillate in response to some mechanical stimulus. Such structures necessarily require elastic and inertial elements that are suspended over a substrate with appropriate clearance to enable oscillations. The dimensions involved and the compatibility of the fabrication process with CMOS make it ideal to combine these structures with electronics devices such as FETs. We are working on suspended Gate FETs where the transistor gate is a micromechanical structure capable of responding to a mechanical stimulus and causing substantial change in the drain current of the transistor. This work is in collaboration with Prof. Navakanta Bhat and the goal is to develop a platform technology for extremely sensitive dynamic displacement and pressure sensing.

 
 

Fig. 1 Dynamic characterization of MEMS gyroscope for Out of plane vibration mode and In-plane motion in frames

  Fig. 2 A circular array of PMUT devices wire bonded on patterned PCB for integration
 
  • Transduction Targeted Material Development

    Piezoelectric and piezoresistive materials are likely to play a major role in the development of micro and nano scale sensors and actuators. The transduction mechanism they provide for coupling mechanical and electrical energy domains is ideal for small scale transducers. A piezoresistor with low impedence, high gauge factor, and CMOS process compatability can lead to the development of a host of self-sensing structures. Similarly, CMOS process compatible piezoelectric materials with high coupling coefficients can help tremendously in the development of energy harvesters that can, in turn, make MEMS and NEMS sensor systems autonomous. It is this theme that guides our work on the development of such materials. We are pursuing controlled electromigration induced nanoscale perturbations in structural topology of metallic films in order to enhance their piezoresistive sensitivity. This work has opened new frontiers in the dynamics of phase transformation and material transport at very small scales. Prelimiary studies indicate exciting possibilities in electric field mediated nanoscale material transport. We are also developing ZnO nanostructures with very high piezoelectric coupling coefficients for energy harvesting as the intended application.

 
 
Fig. 3 ZnO Mirobowl is synthesized by microwave assisted growth  

Fig. 4 Graphene on electroplated copper

 
  • Energy Dissipation in Oscillations of Micro and Nano Scale Structures

Most of the devices and structures we design and develop exploit the dynamic response of some structural element for their essential operation. In studying these devices, one particular aspect of their response that has fascinated us the evaluation of energy dissipation in their micro and nano scale motions. Any dissipation of energy from the oscillating structure directly impacts its quality factor, Q. With so much interest in developing high-Q MEMS and NEMS devices, it becomes imperative to have a good understanding of energy transport from such oscillating structures. It turns out that in most cases, the fluid- structure interaction with the surrounding fluid (mostly air) is responsible for maximum energy dissipation. In particular, we have been investigating squeeze film damping and acoustic radiation losses, and their effect on the Q of MEMS devices. Squeeze film damping refers 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 and a fixed substrate. We have been interested in understanding squeeze film flow and associated energy dissipation in all kinds of flow conditions - from continuum to molecular flow. We are also working on understanding acoustic radiation from these tiny structures and the associated energy losses particularly relevant to 2-D structures.

 
   

Fig. 5 FEM generated pressure distribution at a) low squeeze number (sigma = 1) and b) high squeeze number (sigma = 100) for a square plate with OCOC configuration.

  Fig. 6 Computation of acoustic losses and corresponding Q in various modes of a micro drum resonator with 3 different levels of residual stress. (Ref [1])   Fig. 7 Voltage output from PVDF bimorph for 1 MO load resistance and 0.5 g input acceleration swept from 20 Hz to 100 Hz.
 
  • Vibratory Mechanobiology

Our studies of MEMS and NEMS devices have led our attention to some of the small scale sensors and transducers that nature exploits. Nature's design is particularly very elegant and most of the times awe inspiring. We, in collaboration with colleagues from biology, are studying the mechanics of insect hateres as gyroscopic sensors, cricket harp as an efficient sound radiator and cell oscillations for signals of cell pathology. These are all incredibly complex and fascinating "devices" whose understanding has a huge bearing on our design of micro and nano scale systems.

 

Fig. 8 Finite element modelling and simulation of the cricket harp- the primary sound radiating structure

 

Fig. 9 Haltere of a Soldier fly

 

Selected Publications:

  1. S. D. Vishwakarma, A. K. Pandey, J. M. Parpia, D. R. Southworth, H. G. Craighead, and Rudra Pratap, “Evaluation of Mode Dependent Fluid Damping in a High Frequency Drumhead Microresonator”, (to appear),Journal of Microelectromechanical Systems, 2013.
  2. S. Talukdar, P. Kumar, and Rudra Pratap, “Electric Current Induced Mass Flow in Very Thin Infinite Metallic Films”, IEEE Transactions on Electron Devices, Vol 60, No. 9, pp 2877-2883, September 2013.
  3. Thejas, Navakanta Bhat,Rudra Pratap and K. N. Bhat,“Fringe Field Junctionless FET as a Sensitive Displacment Sensor”, Journal of ISSS, Vol. 2, No. 1, April 2013.
  4. F. Jiang, A. Keating, M. Martyniuk, Rudra Pratap, L. Faraone and J. M. Dell, “Process control of cantilever deflection for sensor application based on optical waveguides”, Journal of Microelectromechanical Systems, Vol. 22, No. 3, pp 569-579, 2013.
  5. S. M. Mohanasundaram, R. Pratap and Arindam Ghosh, “A cantilever resonator with integrated actuation and sensing fabricated using a single step lithography”, IEEE Sensors Journal, Vol. 13, No. 2, February 2013.
  6. M. Mohanasundaram, Rudra Pratap and Arindam Ghosh, “Two orders of magnitude increase in metal piezoresistor sensitivity through nanoscale inhomogenization”, Journal of Applied Physics, Vol. 112, Issue 8, pp 084332-9, 2012.
 
 
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