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AISRF Research overview
The relentless trend towards miniaturisation in modern technologies
extends to devices in which solid parts and/or fluids are required to
move. Examples include hard disk drives, microelectromechanical systems
(MEMS), and microfluidic devices. These devices all start from normal
design considerations, but some of the concepts that are routinely
applied when engineering macroscopic devices may become questionable as
the scale of devices shrinks. Consider some examples:
- Read/write magnetic heads in hard drives now “fly” over the disk
surface at a height of a few nanometres at speeds of tens of metres per
second. For this to occur without disastrous head-disk crashes, the disk
surface must be remarkably smooth, and the lubricating film applied to
protect it can be only a monolayer thick. The film performance is
considered in terms of molecular rather than bulk fluid behaviour.
- In contrast to macroscopic machinery, the inertia of moving parts in
MEMS is small compared to forces acting on their surfaces including
hydrodynamic forces and adhesion, and in fact many MEMS fail after a
small number of cycles due to unwanted surface forces that cannot be
overcome by the driving force normally applied (e.g. electrostatic
actuation).
- Liquids in microfluidic devices are required to flow through very
narrow channels, but ultimately there must be a size limit below which
normal hydrodynamic concepts (such as viscosity) are no longer valid or
useful. What is that size limit, and how will we understand fluid flow
when the device dimensions shrink below it? What are the conditions for
slip versus non-slip hydrodynamic boundary conditions?
In fact questions of fluid flow in narrow gaps are not confined to tiny
devices. Lubricating films in machinery of any size are often submicron in
thickness, and similar questions about the flow behaviour on a microscopic
or even molecular scale arise. In this case the fluids involved are
generally complex, multiphase fluids, and little is known about the
comportment of the various phases under operating conditions of the
lubricant. Furthermore, the surfaces are generally not smooth and uniform in
composition. Examples include:
- Metal-working and cutting fluids flow in submicron confined spaces
under great pressure. These are complex 3 or 4 phase fluids having a
range of functionalities. Flow of these fluids and their interaction
with die and workpiece is little understood and is a matter of great
interest to the engineering industry.
- Many lubricating fluids are emulsions which flow in submicron
channels. Boundary layers are generated dynamically on the nanometre
scale by amphiphilic molecules attached to oil droplets. The fluid
composition and phase behaviour are affected by the flow and confinement
conditions, and conversely the phase behaviour (e.g. switching from
micellar structure to inverse micelles) has a major influence on the
lubrication properties.
- Engine oils carry dispersed nanoparticles including wear debris. How
do these behave in confined spaces when the oil is compressed in a
bearing? How do fine particles affect the distribution of adsorbate
molecules that are intended to act as boundary lubricants?
- Lubricated surfaces may be metal alloys, ceramics or composite
materials in which graininess is inherent, so the behaviour of a
lubricant will not be the same at all points on the surface but will
depend on whether it is contacting the material’s matrix, grains, or a
boundary between them. If the lubricant sticks to some regions of the
surface but slips over other regions, what is the overall behaviour?
These and the last question in particular, are the realms we will explore
in this project, through a combination of experimental research, theoretical
modelling, and focussed workshops involving specialist research staff from
the partner institutions in India and Australia. The project will make use
of outstanding experimental facilities and research expertise that are
available at both institutions. In particular, it will capitalise on
facilities for fabricating and characterising surfaces of controlled
heterogeneous structure at UniSA, with facilities for and expertise in
tribological measurements on a sub-micron scale at the Indian Institute of
Science (IISc). Part of the research will address basic questions with
simple (one-component) liquids, and part will address complex
multi-component fluids.
By the end of the project the goal will be to synthesize the knowledge
gained from these two approaches into a coherent picture that will apply
both to microdevices and to lubricating films in general machinery. Improved
understanding of fluid flow over heterogeneous surfaces will be of great
interest to numerous industries seeking to maximise energy efficiency and
maximise longevity of mechanical parts, and to optimise fabrication and flow
conditions for mixing, chemical reaction and/or analytical applications in
microfluidic devices.
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