Group Members: Hannah Bunyan

Photo of Hannah Bunyan


Bain Group History

MSc Student, 2008-present

Project Title

Deformation of Emulsion Droplets Using Optical Tweezers

Research Outline

My project aims to manipulate and deform microemulsion droplets using optical trapping, made possible by the ultralow interfacial tensions found in microemulsion systems.

Research A Closer Look

OPTICAL TRAPPING was first described by Ashkin in 1970.[1] A continuous wave laser beam of a few hundred milliwatts of power exerts radiation pressure on a particle, causing the particle to be accelerated along the direction of the laser beam, and also to be drawn into the centre of the beam. OPTICAL TWEEZERS consist of a laser beam directed through a microscope objective, bringing the beam to a very tight focus. Particles are drawn into the focus and held in the stable trap.

Previous work has shown that emulsion droplets of sufficiently low interfacial tension can be deformed using multiple optical tweezers.[2] Droplets adopt a spherical shape because surface free energy is minimised with a minimum interfacial area between the droplets and the dispersed phase. It is not possible to deform normal emulsion droplets using optical traps, because the force exerted by the trap (10 μN/m) is much smaller than the force exerted on the surface of the droplet (interfacial tension) causing it to retain its spherical shape.

Microemulsions have the unusual property of ultralow surface tension. At a particular temperature, known as the phase inversion temperature (PIT), a microemulsion will change from consisting of droplets of oil in water, to droplets of water in oil. In between these two stages exists a middle phase, where the oil-water interfacial tension is close to zero. The same effect can also be observed with a certain concentration of electrolyte in the system. A system close to the phase inversion boundary will exhibit interfacial tension low enough such that the emulsion droplets can be deformed by optical trapping.

The figure below (taken from [2]) shows examples of the shapes achieved in previous work by Ward et al. on the deformation of emulsion droplets using optical tweezers.

Figure showing spherical (undeformed), 'oval', 'triangular' and 'square' emulsion droplets

The project aims to use a spatial light modulator to arrange several optical traps around an emulsion droplet, and deform it into three-dimensional shapes by moving the traps apart.

A further aim of the project is to perform polymerisation reactions in the deformed emulsion droplets, in the hope that the resulting polymer beads will take on the shape of the emulsion droplet. In order for this to be successful, the emulsion system used needs to be temperature-insensitive so that it is not disrupted by the exothermic polymerisation reaction. Finding a system in which ultralow interfacial tension can be achieved within a wide temperature range is one of the challenges of the project. Potential systems employ a co-surfactant such as hexanol, mixtures of ionic and non-ionic surfactants such as sodium dodecyl sulphate (SDS) and triethylene glycol monododecyl ether (C12E3), and Gemini surfactants containing dual polar head groups and hydrocarbon chains.


1. A. Ashkin, "Acceleration and Trapping of Particles by Radiation Pressure" Phys. Rev. Lett. 1970, 156-159 (DOI).

2. A. D. Ward, M. G. Berry, C. D. Mellor, C. D. Bain "Optical sculpture: controlled deformation of emulsion droplets with ultralow interfacial tensions using optical tweezers" Chemical Communications 2006, 4515-4517 (DOI).


My interests lie mainly outdoors, including caving and potholing with the university club and various clubs in the Yorkshire Dales, hill walking, camping, scrambling and a bit of indoor climbing, and rowing with Trevelyan College Boat Club. I love films of all kinds, especially westerns and mountaineering adventures, and I enjoy playing the violin and guitar.