Chang-Hwan Choi

Current Research

Nanofabrication

While several nano-scale patterning techniques are available, it should be noted that serial lithography methods such as e-beam lithography and scanning probe lithography do not cover a large area needed for non-electronic applications. Other non-lithographic methods, for example, the use of nano-templates such as block copolymers, nano-spheres, and a porous anodic alumina membrane, or the direct growth of nano-scale structures such as carbon nano-tubes, do not provide good regularity over a large area. The primary goal of my research has been to investigate a simple but efficient fabrication method of nanostructures with superior control of pattern regularity, size, and shape over a large area.

SEM images of silicon nanostructures of ~200 nm pitch patterned by interference lithography and DRIE. Tips are sharpened by thermal oxidation and oxide etching.

Drag reduction by nano-engineered super-hydrophobic surfaces

The nano-engineered surfaces with the good regularity over a large area are well adapted to many applications. The first application of the nanostructures is to reduce drag in liquid flow. After a hydrophobic surface treatment, the sharp-tip nanostructures make the surface super-hydrophobic and can levitate liquid over the air pockets trapped between the non-wetting structures. Then the liquid is in minimal contact with the solid surface, and expected to flow with significantly reduced skin friction. It is critical that the surface structures are of nanometer scale, because otherwise the liquid would lose levitation as the applied pressure to liquid increases.

Nano-engineered super-hydrophobic surface and its application for drag reduction in liquid flow. Liquid sits on tall and sharp tips of hydrophobic nanostructures by surface tension. Majority of liquid boundary is with air where friction is negligible.

Cell adhesions on Nano-structured surfaces

Cell-matrix adhesion in vivo is a three-dimensional (3D) nano-environmental phenomenon. It differs from focal and fibrillar adhesions characterized on two-dimensional substrates in vitro. The full complement of matrix topography, molecular composition, and mechanical properties (e.g., pliability) are important for adhesions to form properly and for activation of desired intracellular pathways. However, the specific contribution of matrix topography to the formation of cell adhesions is still poorly understood. Although several cell behaviors over various surface topographies have been studied with micro- and nano-structured surfaces, there is relatively little information on the reactions to the nano-structured surfaces. The ability to control the surface topography, especially in the nano-scale, was very limited such that it was difficult to isolate the effect of three-dimensionality of nano-scale surface features on cell adhesions. Another application of the nanostructures is to characterize several cell behaviors including the composition and function of adhesions. The size and tip shape of the nanostructures can be systematically controlled to represent various three-dimensionalities of a surface and to differentiate the 3D nano-topographical effect on cell behaviors.

SEM pictures of lamellipodia and filopodia of fibroblasts cultured on the sharp-tip nano-post structures.

Eduaction

2002 - 2006 Ph. D. Mechanical Engineering, University of California, Los Angeles, CA, USA

2000 - 2002 M.S. Mechanical Engineering, Brown University, Providence, RI, USA

1995 - 1997 M.S. Aerospace Engineering, Seoul National University, Seoul, Korea

1991 - 1995 B.S. Aerospace Engineering, Seoul National University, Seoul, Korea

Work Experience

1999 - 2000 Researcher, Korea Aerospace Research Institute, Deajeon, Korea

1997 - 1999 Lecturer, Rajabhat Institute Chandrakasem, Bangkok, Thailand

1996 - 1997 Assistant Researcher, Korea Aerospace Research Institute, Daejoen, Korea

Honors/Awards

2002 California NanoSystems Institute Fellowship

2000 Brown University Engineering Research Fellowship

2000 Korean Government National Fellowship

1995 Graduated with 'magna cum laude' Honors

1994 Seoul National University Honors Fellowship

1993 Seoul National University Honors Fellowship

Publications

Thesis:

1. C.-H. Choi, "Flow Rates and Slip Velocities of Liquids in Hydrophilic and Hydrophobic Microchannels", Master Thesis, Division of Engineering, Brown University, May 2003.

2. C.-H. Choi, "Thermal Analysis and Design of 3-Axis-Stabilized Satellite in Sun-synchronous Orbit", Master Thesis, Aerospace Engineering, Seoul National University, February 1997.

Conference Papers:

1. C.-H. Choi, S.H. Hagvall, J. Dunn, B.M. Wu, C.-J. Kim, "Cell Adhesions on NanoTurf Surfaces", Proceedings of MEMS 2006: The 19th International Conference on Micro Electro Mechanical Systems, January 22-26, 2006, Istanbul, Turkey.

2. C.-H. Choi, C.-J. Kim, "Measurement of Liquid Slip on NanoTurf Surfaces", Proceedings of NANO2005 Integrated Nanosystems: Design, Synthesis and Applications, September 14-16, 2005, University of California, Berkeley, California, USA.

3. C.-H. Choi, C.-J. Kim, "Control of Sidewall Profile of Silicon Nanostructures in Bosch Process and Its Use for Sharp Tip Fabrication", Proceedings of UKC2005: The 2005 US-Korea Conference on Science, Tehcnology, & Enterpreneurship, August 11-13, 2005, Irvine, California, USA.

4. C.-H. Choi, C.-J. Kim, "Fabrication of Silicon Nanostructures with Various Side Wall Profiles and Sharp Tips", Proceedings of Transducers'05: The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, June 5-9, 2005, Seoul, Korea.

5. C.-J. Kim, C.-H. Choi, "Nano-Engineered Low Friction Surface for Liquid Flow", Proceedings of the 6th KSME-JSME Thermal and Fluids Engineering Conference, March 20-23, 2005, Jeju, Korea.

6. C.-H. Choi, J. Kim, C.-J. Kim, "Nanoturf Surfaces for Reduction of Liquid Flow Drag in Microchannels", Proceedings of NANO2004 Integrated Nanosystems: Design, Synthesis and Applications, September 22-24, 2004, Pasadena, CA, USA.

7. C.-H. Choi, K.J.A. Westin, K.S. Breuer, "To Slip or Not to Slip: Water Flows in Hydrophilic and Hydrophobic Microchannels", Proceedings of ASME IMECE, November 2002, New Orleans, LA, USA.

8. K.J.A. Westin, C.-H. Choi, P. Huang, Z. Cao, K.S. Breuer, B. Caswell, P. Richardson, M. Sibulkin, "Liquid Transport Properties in Submicron Channel Flows", Proceedings of ASME IMECE, November 2001, New York, NY, USA.

9. K.J.A. Westin, C.-H. Choi, K.S. Breuer, "Rheological Shear Experiments in Micron Scale Geometries", Proceedings of MicroTAS. October 2001, Monterey, CA, USA.

10. J.-M. Choi, S.-W. Choi, C.-H. Choi, K.-J. Lee, "Satellite Anomaly and Solar Array Temperature Variation", Proceedings of KSAS (the Korean Society for Aeronautical & Space Sciences) Spring Annual Meeting, April 2000, Korea.

11. C.-H. Choi, J.-M. Choi, S.-W. Choi, "Thermal Design & Analysis of Satellite Battery Module", Proceedings of KSAS Fall Annual Meeting, November 1999, Korea.

12. C.-H. Choi, O.-H. Rho, S.-W. Choi, "TRASYS Modeling & Thermal Analysis of KOMPSAT", Proceedings of KSAS Spring Annual Meeting, April 1996, Korea.

Journal Papers:

1. C.-H. Choi, J. Kim, U. Ulmanella, C.-M. Ho, C.-J. Kim, "Slip of Water Flow over Nano-grating Structures in Microchannels", Physical Review Letters (in review).

2. C.-H. Choi, C.-J. Kim, "Large Slip of Aqueous Liquid Flow over a Nano-engineered Super-hydrophobic Surface", Physical Review Letters (in review).

3. C.-H. Choi, K.J.A. Westin, K.S. Breuer, "Apparent Slip Flows in Hydrophilic and Hydrophobic Microchannels", Physics of Fluids 15(10), pp. 2897-2902, 2003.

4. K.J.A. Westin, C.-H. Choi, K.S. Breuer, "A Novel System for Measuring Liquid Flow Rates with Nanoliter per Minute Resolution", Experiments in Fluids 34, pp. 635-642, 2003.

Books:

Patents:

Contact Info

48-121, Engineering 4 420 Westwood Plaza Los Angeles, CA 90095-1597, USA

Phone: 310-825-3977

E-Mail: chchoi@ucla.edu