KHAN, Saif A.

Assistant Professor

PhD (Chem. Engg): MIT 2006
B.Chem.Engg UICT (Mumbai) 2001

Contact information
Blk E5, 4 Engineering Drive 4, #02-28, Singapore 117576
Tel: (65) 6516 5133    Fax: (65) 6779 1936
Email: chesakk@nus.edu.sg

RESEARCH

Our research primarily focuses on the application of microfluidic technology to solve problems of fundamental scientific interest as well as industrial relevance. Application areas include chemical, biochemical and nanomaterials synthesis, multiphase fluid physics of droplets and bubbles, analysis and discovery tools for cell biology, and low-cost health monitoring systems for developing economies.

Microreactors for chemical, biochemical and nanomaterials synthesis

Scientific and commercial interest in microfluidic systems for analysis, synthesis, catalyst testing, kinetic studies, rapid reaction optimization, process intensification and production has increased tremendously since the early 1990’s. Microreactor technology utilizes chemical reactors having sub-millimetre characteristic dimensions, fabricated using well-established fabrication techniques from the silicon microelectronics industry. Photolithography-based microfabrication enables the application of classical techniques of chemical reaction engineering to design continuous chemical reactors that cannot be realized easily at the macroscale, and that closely approach theoretical ‘idealized’ reactor configurations. Reactor miniaturization confers several advantages over conventional macroscopic systems. Heat transfer is vastly improved and mixing times are reduced by orders of magnitude as a result of the decrease in linear dimensions. In addition, multiphase reactions are benefited by high surface-to-volume ratios and intimate inter-phase mass transfer. Thus reactions can be performed under more aggressive conditions with higher yields than can be achieved with conventional reactors. Scaling-up to larger amounts of production is achieved by using multiple reactor copies operating in parallel (also known as ‘numbering-up’ or ‘scale-out’) to create a high throughput system while maintaining the control and selectivity of the individual reactors. The scope of our research encompasses both single phase and multiphase chemical reactions yielding products ranging in size from small molecules to large colloidal particles. We design, fabricate and operate microreactors that not only enhance fundamental understanding of the effect of synthesis conditions on product distribution, yield, and selectivity, but also facilitate larger amounts of production.

Micro-scale multiphase fluid dynamics

Multiphase flows in microfluidic channels have received increasing interest in recent years for a variety of applications such as chemical/biological synthesis and analysis, kinetic studies, and small heat-exchangers for cooling microprocessor chips. ‘Segmented’ gas-liquid flows involve alternating flow of gas bubbles and liquid segments, and overcome the large axial dispersion effects associated with single-phase laminar flows. The important parameters involved in generating stable flows are fluid properties (viscosity, density), interfacial tension and contact angles, gas and liquid flow velocities, device dimensions and inlet geometries. Such flows are particularly interesting and exhibit rich non-linear behaviour when even small changes are made in the above parameters. We focus on the design, fabrication and characterization of microfluidic devices operating in segmented gas-liquid flow with particular emphasis on flow stability, sensitivity to device design parameters, non-linear pressure-drop versus flow behaviour, and effects of microchannel surface properties on the dynamics. We aim to use this understanding for rational design of multiphase microreactors for heterogeneous catalysis-based chemical synthesis.

SELECTED PUBLICATIONS

Khan, S. A. and Jensen, K. F.,"Microfluidic synthesis of titania shells on colloidal silica", Advanced Materials, 19, pp.2556-2560 (2007).

Khan, S. A., Günther, A., Schmidt, M. A. & Jensen, K. F. "Microfluidic Synthesis of Colloidal Silica", Langmuir 20, 8604-8611 (2004).

Günther, A., Khan S.A., Trachsel F., Thalmann M., and Jensen K.F. "Transport and reaction in segmented gas-liquid flow", Lab on a chip 4, 278-246 (2004).