Multiscale Interfacial Transport (MIT) Laboratory
in Space-Energy-Environmental-Materials Systems
Micro- and nano-scale transports have gained much more attention recently due to their increasing applications in clean energy, environment, aerospace, nanomaterial, and biomedical engineering. The minuscule characteristic length yet large surface-to-volume ratio of the micro/nano systems often change the fluid flow and energy transport behavior fundamentally from those systems at macro-scales. Analyzing such transports is both challenging and significant for science and for practical applications. My research interests lie in building up two bridges: one is a “scale-bridge” between the concerned microscale transports and the well-known macroscale transports; the other is a “discipline-bridge” between the fundamental theories of physics and various engineering practices.
In the nearly ten years, my researches have been focusing on microscale fluid mechanics and energy transport, including developing theoretical models and mathematical algorithms, and designing experiments for primary validations. My objectives are to improve the understanding of new inherent mechanisms of fluid dynamics and thermal science at micro/nanoscales, and to use these insights to provide novel and optimal design schemes for practical engineering. My researches can be roughly summarized into such three aspects as: multiphysical transports in complex multiphase media, micro- and nanoscale electro-kinetic flow, and microscale aerodynamics and thermophysics.
Multiphysical transports in complex multiphase media
Although transports in porous media have been studied for over 100 years, the inherent mechanisms have far from been completely discovered due to the complexities of multiscale structures and multiphysical effects. Especially in micro porous media, the size effects and stochastic factors break down most of the existing theoretical and empirical models. New methods for reliable modeling and prediction of such phenomena are exigent for new material designs, energy developments, environmental and even biological applications.
My past works include:
(1) developing a random generation-growth methodology for reproducing microstructures of natural porous media with different morphologies (granular, fibrous and network, see Figure 1-b) [Phys. Rev. E, 2007; Int. J. Thermal Sci., 2007b; Int. J. Heat Mass Transfer, 2008];
(3) investigating the effects of structure morphology and phase interaction on the effective thermal, electrical and mechanical properties of composite materials and porous media [J. Colloids Interface Sci, 2007 a ; J. Appl Phys, 2007a &b; J. Comput. Phys., 2007b], with Figure 1-a demonstrating the effective thermal conductivity of two-phase composite materials for different microstructure morphologies;
Future plans are (1) to develop the random generation method for complex multiscale microstructures of multiphase porous media; (2) to investigate the water, ions, concentration and thermal transports in micro fuel cell and biofuel; (3) to model the behavior of electroseismic responses in porous stratums for petroleum prospecting; (4) to study the phase distribution and interaction effects on the effective properties of nanomaterials and biomaterials for design and optimization of novel materials.
Figure 1: (a) The effective thermal conductivity of two-phase materials for different microstructure morphologies. The phase properties used in the two-dimensional modeling are =100 W/m K and =1 W/m K. (b) six compared microstructures. The upper three are granular, fibrous and network microstructures respectively, generated using my random generation-growth methods; the lower three are regular structures: spheres array, parallel mode and series mode respectively.
Micro- and nanoscale electro-kinetic transport
Electro-kinetic flows, including electroosmotic and electrophoretic flows, have very important application significance in bio-chemical-medical engineering since almost all cells are living in electrolyte solution surroundings. Also the electroosmotic flow is more and more used as a key tool in actuators for micro/nano systems, such as micro-pumps and micro-mixers. Since the electrokinetic flow highly depends on the electrical double layer adjacent to the charged surface, there rise two challenges in theory: (1) whether the continuum theories are still valid for micro- and nanoscale electro-kinetic flows; (2) how the nano-structured surface affects the microscale flow behavior. At an engineering level, as the electric force is a long-distance force, thus leading to very strongly non-linear governing equations of electric potential. Such coupled governing equations are very difficulties to solve, flows especially for those involving complex geometries.
My past contributions dealing with these problems include:
atomistic methods (NEMD, P
(2) developing a high-efficiency lattice Poisson-Boltzmann method for electroosmotic flows in microchannels [J. Colloids Interface Sci, 2006 a ] and investigating the fluid mechanics in charged rough channels [J. Comput. Phys., 2007] and the performances of electroosmotic micropump and mixers [J. Colloids Interface Sci, 2006a &b].
Future plans are (1) to develop a novel multiscale/hybrid model for the electro-kinetic flow to integrate the models for different scales to capture the nano-structured surface effects without resolving in the entire computational domain (2) to investigate the electro-kinetic flow behavior inside bio-cells (such as ion channels), drug delivery process and M/NEMS devices; (3) to extend the methodology to electro-kinetic flow of complex fluids, such as suspensions and gels, or to complicated flow behavior, such as electrical nanowetting.
Microscale aerodynamics and thermophysics
Due to the very small characteristic length micro- and nanoscale gas flows and thermal transports show drastically different behaviors at the fluid-solid interfaces and are beyond the scope of continuum theories. Therefore new theories and computational methods are necessary for accurate predictions of such behavior since they have very important applications in aerospace and biomedical engineering. For instance, the performance analysis of micro nozzles is the base of design for a propulsion system of microsatellites for position orientation or attitude adjustment. The micro aerodynamics in human trachea is responsible for the origin and spreading mechanism of some pulmonary disease, drug delivery and the design of artificial lung. Recently, as the nanotechnology has been greatly developed, the “nano-pollution” problem is rising as a public health concern. The research on air flow interaction with artificial nanoparticles, including nanotubes and nanofibers, will be very helpful for predicting, controlling and preventing the nano pollutions.
My past contributions in this area include:
(1) theoretically analyzing the similarity between the micro gas flow and the rarefied gas flow, and numerically validating the critical conditions of the similarity for various usual gases [Sci. China, 2003; Int. J. Heat Mass Transfer, 2007];
(2) developing high-efficiency boundary treatments for direct simulation using Monte Carlo (DSMC) method and investigation of high-Kn gas flow in complex microgeometries [Int. J. Heat Fluid Flow, 2004];
Future plans: (1) the short-term plan is to develop a full-scale hybrid numerical model for the high-Kn, high-speed and high-pressure-gradient gas flow in micro nozzle array and to analyze the propulsion performance; (2) the long-term (2~4 years) plans in this area are to simulate the gas flow and mass exchange in micro bronchia in lung, and to predict the movements and interactions of nanoparticles in air flow and then to analyze the fluid dynamics of nanoparticle aerosols.
(1) Phase-change micropump
Experimental setup schematic Simplified physical model
The flow rate changing with the heating current The flow rate changing with the switch time
See Ref [Chin. Sci. Bull., 2002]
See Ref. [Tsinghua Sci. Tech., 2004]
(2) Air bearing slider
Hard disk driver Simplified model
DSMC modeling results compared with other theoretical models
See Ref [Tribology, 2005]