Centre for Microfluidics and Microsystems Modelling
Introduction to micro-devices
Rapid progress in micro-fabrication and assembly techniques has
led to the development of extremely small-scale devices commonly referred to as
MEMS (Micro-Electro-Mechanical Systems) or mTAS (Micro-Total Analysis Systems).
These devices can combine electrical and mechanical components down to a characteristic length
scale of 1 micron. Most of these devices are currently manufactured in silicon
but recent developments have demonstrated that materials such as glass, quartz,
ceramics and polymers can also been used for MEMS. These advances have enabled
mechanical devices to be constructed that are at least three orders
of magnitude smaller than conventionally machined
components.
The figure illustrates length scales typically encountered in micro-systems
Microfluidics
An increasing number of micro-devices involve the manipulation
of fluids and has led to an exciting new field of research called
microfluidics. Examples include miniaturised heat-exchanges to
cool integrated circuits, micro-reactors to generate small quantities of
dangerous or expensive chemicals, ‘lab-on-a-chip’ bio-chemical sensors which
perform complex biological assays on sub-nanolitre samples and hand-held gas
chromatography systems for the detection of trace concentrations of air-borne
pollutants. In addition, it is envisaged that arrays of micro-sensors and
micro-actuators could eventually be placed on aircraft wings to sense and
control the small-scale vortex structures within the turbulent boundary-layer
thereby increasing aerodynamic efficiency and manoeuvrability. A common link
between these examples is the requirement to transport fluid through or over the
device in a controlled manner. However, one of the emerging issues is the
realisation that the fluid mechanics at such small scales is not the same as
that experienced in the macroscopic world.
Successful fluid manipulation is the most critical factor in
the design of any microfluidic device. The methods used for fluid transport are
diverse and include pumps and valves (pressure-based systems), electrokinetics
(electrophoresis or electro-osmosis) and magneto-hydrodynamics. Other related
approaches can be used to manipulate the fluid: for example, centrifugal forces
on CD diagnostic systems. The fluid may have to traverse a complex set of
micron-sized channels and undergo processes that may involve reaction zones,
separation and mixing. This presents a significant modelling challenge because
the fluid or the sample may be interacting with electric and/or magnetic fields.
Moreover, the simulation of micro-flows is by no means trivial because the
continuum hypothesis of the Navier-Stokes equations begins to break down when the
characteristic dimensions of the flow geometry are comparable to the mean free
path of the molecules. As a consequence, microfluidic systems which are simply
scaled down versions of macro-scale devices may not always function as
intended.
Current Research Activities
The Centre for
Microfluidics was established at CCLRC Daresbury Laboratory
in 1999 with the aim of providing a microfluidic design and simulation service
for the academic and industrial community. Initial work has concentrated on
understanding the fundamental issues to be addressed when analysing fluid flows
in micro-devices. To reflect our increasingly broader role in the
simulation of microdevices the Centre is now known as the
Centre for Microfluidics and Microsystems Modelling
.
The Centre has experience in modelling a variety of pressure
driven and electrokinetic flow regimes and is in close collaboration with
leading UK universities on developing novel high-throughput bio-chemical
screening devices. The design work employs a variety of flow simulation tools
including analytical and numerical models and makes use of commercial and
in-house Computational Fluid Dynamics (CFD) codes. Numerical modelling provides
an invaluable role in optimising the geometry of proposed microfluidic devices
prior to fabrication, considerably reducing the overall development costs. In
addition, numerical modelling can play a significant role in generating novel
ideas and developing the resulting concepts into commercially viable design
solutions.
The Computational
Engineering Group has developed two body-fitted multi-block
Navier-Stokes solvers: THOR-2D and THOR-3D which have been specially adapted for
microfluidic simulations. In addition, the Centre makes use of the commercial
Computational Fluid Dynamics code CFD-ACE+ which can handle a variety of complex
flow problems including non-Newtonian rheology, volume of fluid (VOF)
simulations, air bubbles and moving boundary problems.
Technical Advantages of Microfluidics:
- Minimal device size for hand-held instrumentation and point-of-care
testing.
- Efficient use of expensive chemical reagents.
- Low production costs per device allowing disposable microfluidic systems.
- Precise volumetric control of samples and reagents leading to higher
sensitivities in analytical applications.
- High-throughput biological screening made possible by faster sampling
times through parallel processing of samples.
- Precise thermal control of reagents due to high surface to volume ratios.
- Rapid heat extraction enabling hazardous exothermic reactions to be
conducted in safety.
- Low power consumption for satellite and remote sensing applications.
- In-situ production of unstable compounds for biological assays.
Current research work can be divided into a number of areas:
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Image shows splitting of droplet using EWOD
Contact Details
For further information on this work please contact:
Professor David Emerson
Science and Technology Facilities Council
Daresbury Laboratory
Daresbury Science and Innovation Campus
Warrington WA4 4AD
Cheshire
United Kingdom
Tel. +44 (0)1925 603221
Fax. +44 (0)1925 603634
Email: david.emerson@stfc.ac.uk
Web Page: Computational Engineering Group's Home Page
