Optical trapping techniques for the study of complex aerosol particles in an x-ray beamline

The elucidation of particle structure and the nature of particle dynamics is best achieved in the natural environment in which that particle exists. For aerosol particles this environment is air and this proves to be a limiting factor in the analysis of such particles when it comes to looking at analytical techniques such as x-ray diffraction. Carrying out such experiments in air (or vacuum) also improves the signal to noise ratio that can be achieved, as the x-ray attenuation is reduced. The key problem is the localisation of such particles and the ability to hold them still during an experiment. In this proposal we aim to develop optical trapping techniques that can trap a variety of complex aerosol types in air/vacuum within one of the x-ray beamlines at the Diamond facility. Our aim in doing so is to characterise particles such as volcanic ash, soot and starch grains along with the possibility of looking at protein crystals. We also aim to look at dynamic processes such as contact nucleation of supercooled water through the use of advanced beam shaping technologies using computer controlled holograms. The basic technology behind the proposal is the optical tweezers, a method by which a focussed beam of light can be used to trap micro- and nano-scopic sized particles. The difficultly that we face is the aerosols in question are often aspheric and partly or wholly absorbing, and so are difficult to trap using conventional techniques. We propose to develop a set of techniques based on feedback of position detection, beam shaping and new forms of trapping using photophoresis, as well as making use of dual beam traps to overcome these problems and deliver a tool that can robustly trap a wide variety of aerosol types, both differing in size and shape. Our ultimate goal is to develop an x-ray analysis tool that can be used by a wide range of users well beyond the duration of the project, and to open up the Diamond facility to new types of experiment. The project will form a collaboration between the optics group based in Dundee and colleagues based at STFC and Diamond and will bring together a suite of complementary skills based around expertise in trapping particles in air and the need for the x-ray analysis of such particles. The project will fund a student who will be based in Dundee but spend significant amounts of time at Diamond working to integrate trapping and x-ray techniques. As such this project is design to open up whole new avenues of analytical science.