Part II Undergraduate Student, University of Oxford, 2003–2004
Postgraduate Research Assistant, University of Oxford, 2004–2005
Postgraduate Research Assistant, Durham University, 2005
Ph.D. Student, Durham University, from 2006
Surfactant Adsorption and Marangoni Flow in Liquid Jets
The primary focus of my research has been the use of LASER DOPPLER VELOCIMETRY (LDV) and ELLIPSOMETRY to study how different surfactants added to the bulk phase of a solution affect the detailed hydrodynamics of an air–water interface at conditions far from equilibrium. A lot of literature exists on the properties of different systems at near equilibrium conditions, but using a millimetre scale liquid jet to create a nascent water surface with expansion rates up to 300 s–1 allows us to probe adsorption on the millisecond timescale.
Most of my work has been done using a 1.58 mm diameter liquid jet with mean velocity of the order of 1 ms-1. Using a laser Doppler velocimeter with a spatial resolution of < 10 μm allows very accurate point by point mapping of the velocity profile within the jet. Ellipsometry can also be carried out on the jet surface giving direct information about the amount of surfactant adsorbed.
When the jet exits the nozzle any surfactant added to the bulk diffuses to the nascent water surface, reducing the surface tension. The resulting surface tension gradient generates a shear stress at the surface, which must be balanced by a viscous stress resulting from a velocity gradient in the bulk fluid. What we observe is a reduction in the surface velocity of the jet; an example of the Marangoni effect. By combining the surface velocity data with surface tensions (derived from ellipsometry) we can correlate the reduction in surface velocity with the surface tension gradient in the jet.
Because the jet allows the steady-state study of adsorption on a millisecond timescale it is an ideal platform to help understand surfactant behaviour. For instance, it is well known that surfactants aggregate to form micelles in solutions that exceed the critical micelle concentration (CMC). Historically it has been assumed that micelles must first break down in the sub-surface before the component monomers adsorb to the air–water interface.
Non-ionic surfactants of the polyoxyethylene glycol ether series (CnEm) studied in the jet have such low CMCs that under experimental conditions they exist almost entirely in micelle form. The surface excesses observed in the jet cannot be accounted for by the standard micelle breakdown mechanism because the micellar lifetimes of these species are too long (> 1 s).
We hypothesised that direct adsorption of non-ionic micelles to the air–water interface, a previously neglected pathway, must be occurring. This was validated by doping the non-ionic micelles with small amounts of a cationic surfactant. By adding this slight positive charge to the micelle we increase the micellar diffusion coefficient but also create an adsorption barrier for positively charged micelles. If the micelles were somehow able to break down in the sub-surface then the uncharged non-ionic monomer should still adsorb, with an overall increase in surface excess as a result of the increased diffusion coefficient. If, as we hypothesised, the micelles were adsorbing directly then the surface excess should decrease as a result of the adsorption barrier. The latter case was observed.
D. M. Colegate and C. D. Bain "Adsorption Kinetics in Micellar Solutions of Nonionic Surfactants" Physical Review Letters 2005, 95, 198302/1–4 (DOI).
D. M. Colegate and C. D. Bain "Marangoni Effects in Liquid Jets of Non-Ionic Surfactants" Australian Journal of Chemistry 2005, 58, 678–682 (DOI).
I grew up playing ice hockey since I was about 12 and managed to carry on playing at university for the varsity team and had some excellent European tours and varsity matches against Cambridge. I also spent a lot of time rowing for Wadham College and managed to compete at Henley a couple of times. When I finished my degree I found it very hard to stay involved in team sports because of time commitments and commuting to non-university venues so I ended up falling for cycling in a big way. The problem is that I now enjoy all three sports very much and spend a lot of time trying to decide between them.