Research

Autonomous swimming devices

What?

Swimming devices are miniaturised machines capable of moving through fluid environments.

Why?

These devices have the potential to

• deliver drugs to targeted locations in the body

• sense and clean up environmental contamination

• rapidly collect diagnostic chemicals in medical diagnostic devices

• enhance reaction rates by mixing solutions at small scales

• assemble materials at small scales.

How?

Autonomous swimmers are powered by chemical decomposition reactions, and rely on an asymmetric distribution of catalyst at their surface:

Controlling device trajectory

To use small-scale swimming devices to perform useful tasks we need a way to exert control over their pathways. This is because Brownian phenomena cause small objects immersed in fluid to be randomly jostled and rotated:

We have used a number of methods to overcome this limitation including:

Boundary steering

We have shown that when swimming devices are near to a solid boundary, their Brownian rotations are quenched, and this causes them to show directed motion along a well defined path. This video shows a 5 micron sized catalytic swimmer following a curved edge:

Listen to Dr Ebbens speak about the implications of this work to the BBC world services Newsday radio program (MP3, 782KB)

Gravity

Due to the asymmetrical distribution of mass at the surface of swimming devices, we have shown that devices interact with the gravitational field to produce a 'gravitaxis' effect, meaning that certain sized devices autonomously swim upwards.  

This effect is also seen in simple bacteria.

Catalytic patch shape control

If the shape of the catalyst patch is varied in a way that reduces it's symmetry, rather than producing trajectories that are intrinsically linear, rapidly spinning devices are made. Spinning may be useful to mix fluids at small scales.

Glancing angle catalyst deposition can control the cap shape.

This leads to a range of increasingly rapidly spinning trajectories:

Self-assembly

By allowing individual swimming units to assemble together, we can produce trajectories ranging from spiralling to linear motion:

Size-changes

We have shown using a combination of simulations and experimental data that swimming devices that can change size in response to a signalling chemical will be able to move towards that signal autonomously. 

This will potentially allow devices to find their way towards a particular location, for example a disease site where they could deliver a drug payload.

An even distribution of swimming devices cluster towards the red region, which contains a signalling chemical which causes part of the device to expand.

Understanding the propulsion mechanism

Another key requirement for developing applications for swimming devices is to fully understand how asymmetrical catalytic reactions produce motion.

Bubble propulsive trajectories

The link between catalyst coverage and trajectory had been poorly understood for larger devices driven by bubble release from their surface. 

In a systematic investigation we demonstrated that while catalyst coverage does affect trajectory, bubble release driven devices exhibit far more chaotic motion than smaller devices which are driven by other surface phenomena.

The persistence length for bubble propulsion driven swimming devices is shown to vary with catalyst coverage.

Propulsion velocity scaling

Experimental data reporting how propulsion velocity scales with size has been used to inform the development of theories of motion.

Electrostatic effects

We have shown that the addition of low levels of salt will dramatically reduce swimming velocity, an observation that is motivating the modification of existing theories for catalytic swimming.

Autonomous micro-stirring devices

We have invented printable micro-stirring devices that can efficiently mix small volumes of fluid. Examples of possible applications include speeding up medical diagnosis assays which currently rely on diffusion of an analyte to surface bound detection chemistry.  

Self-optimising manufacturing research

Making materials that can enable new energy storage and generation systems, such as batteries and solar cells, increasingly requires the ability to produce coatings that have well controlled small scale structures.  To enable efficient manufacturing, ideally these coatings will be deposited continuously onto flexible sheets as they rapidly move between rollers, a process called Roll to Roll (R2R) manufacture.

Following on from the support of a EPSRC adventurous manufacturing grant  we are currently developing methods that can control and optimise these coating processes with assistance from Machine Learning.  For example, we are deploying optical imaging to reveal micron scale details of the coating process as it happens, so that we can "learn" the best operating conditions, and maintain particular critical structures during manufacture.  This work is being performed using a custom R2R slot die coater:

An additional feature we are exploiting is the ability for particles embedded in these coatings to self-assemble into regular structures that can have useful optical properties, and be used to make porous materials for use as components within batteries.  The below image shows the way in which regular spherical particles can organise into crystalline arrangements

Techniques and instrumentation

We employ and develop a wide range of colloidal and surface analytical instrumentation to support our work.

Colloidal synthesis

We have a well equipped synthetic laboratory to allow the manufacture and surface modification of colloids.

Metal evaporation

Our lab is equipped with a Moorfield metal evaporator, able to deposit a wide range of metals onto colloids with a high degree of thickness control either by e-beam evaporation or sputter coating.

Surface analysis

We routinely employ scanning electron microscopy and contact angle measurements to determine the properties of the colloids we make. 

We also enjoy access to state of the art X-ray photoelectron spectroscopy instrumentation and other analytical capability across the University.

Microscopy

Our key method for determining the motion of our miniaturised swimming devices, and characterising coating processes is optical microscopy. 

We have three fixed microscope platforms currently in the group, equipped with high spatial resolution automated stages, a range of fluorescence filters, and combined with high performance camera equipment capable of high quantum efficiency and high frame rate data acquisition.  .

In addition, we enjoy access to Confocal Laser Scanning Microscopy via the Kroto centre.

Image analysis

We develop image analysis algorithms, commonly using the Labview Vision software platform.

Particle Sizing and Zeta Potential Measurements

To extend our colloidal sizing beyond the optical limit we also have a Nanosight particle tracking system which uses a laser to allow the trajectories of nanoscale particles to be determined.  Additionally we make use of a NanoFlex platform that allows both DLS particle sizing and Zeta Potential measurements to be performed.