**Multiscale
Interfacial Transport (MIT) Laboratory**

in** S**pace-

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];

(2) developing
high-efficiency multiple lattice Boltzmann models for multiple physical
transports through porous structures [*Modern Phys. Lett. B, 2005*;
*J. Comput. Phys*. 2007

; *Int. J. Thermal Sci.*,
2007a ];

(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;

(4) modeling
the electroosmosis in granular porous media for the first time [*J. Colloids Interface Sci*,
2007b].

* 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.

(a) (b)

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:

(1) developing
atomistic methods (NEMD, P^{3}M*Mol.
Simulation*,
2007 a ] and discussing the applicability of Poisson-Boltzmann theory for
nanoflows [*Mol. Simulation*,
2007b];

(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];

(3) developing
algorithms and studying high-Kn and high-density gas flow, for the first time [*Phys. Rev. E, *2003;*
J. Micromech. Microengin.,*
2004;* Sci.
China , *2005;*
Computers & Fluids*,
2007];

(4) simulating
high-Kn high-speed gas flows in a single micro *Microfluidics**
and Nanofluidics*, 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.

** **

**Other
topics **

(1) Phase-change micropump

a) Experiments

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*]

b) Theory

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*]