Friday, 23 July 2010

Nuffield Bursary Update (2)

Throughout the course of her Nuffield Foundation project this summer Radhika Patel will be making updates of her progress on our blog.  Her second instalment is below.

Radhika writes: Since working on the optical tweezers setup with Susan Skelton, I have helped install and refine a “particle tracking system” which tracks the particle’s Brownian motion in the trap using the backscattered light from a 'probe' laser beam. We can use this method to measure the spring constant of the optical trap, and calibrate the detector sensitivity so that we can make measurements of the particle’s displacement and the absolute force applied.

We have been visited by Maria Grazia Donato from IPCF-CNR (Messina) and Gianluca Memoli from NPL this week also. Having worked with optics for many years, they have helped improve the experiment as well as my understanding of ray optics and optical tweezers.

Monday, 19 July 2010

Richard Berry Seminar

Richard Berry (Oxford) will be giving the CoMPLEX seminar on Tuesday 27 July on Single-molecule observations of turnover, co-operativity and mechanochemistry in a macromolecular complex.

Abstract: The bacterial flagellar motor has long been a canonical macromolecular complex because of the relative ease with which its output, rotation of the extracellular flagellar filament, can be observed.  In vivo imaging of GFP-labelled components of the motor has revealed that the motor is not a static structure, but that individual proteins are constantly replaced at rates on the order of 1/minute.  Localization at nanometre precision of labels attached to the motor, at frame rates of several to many kilohertz, has also allowed observations of the mechanism of co-operative directional switching in the motor and of its mechanochemical cycle.

Monday, 12 July 2010

Visit by Maria Grazia Donato

Maria Grazia Donato of the Istituto Processi Chimico-Fisici (IPCF-CNR) in Messina is be visiting the group this week.  Maria studied at the University of Messina, and recently joined the CNR where she works on Raman and photoluminescence spectroscopy of both amorphous and crystalline material, and steady-state and time-resolved fluorescence anisotropy of molecular aggregates.  She will be contributing to the work of our Royal Society International Joint Project with Dr Onofrio Marago.

Friday, 9 July 2010

Nuffield Bursary Update

Throughout the course of her Nuffield Foundation project this summer Radhika Patel will be making updates of her progress on our blog.  The first instalment is below.

Radhika writes:
  Having joined the UCL Optical Tweezers Group at the end of June, I have been trapping micron sized particles using a diode laser and measuring the spring constant of the optical trap.  The objective was to familiarise myself with the apparatus and theory of optical trapping, and learn about different methods for calibration. Alongside this I have been learning about the experiment that Susan Skelton (PhD student) is working on which is an inverted optical tweezers.  We have been calibrating the optical tweezers for use in Photonic Force Microscopy (PFM) which is the project put forward to the Nuffield Foundation for the Undergraduate Research Bursary I was granted this summer.

Monday, 5 July 2010

Optical Binding

In a new experiment at UCL we have realised evanescent wave optical binding of microscopic spheres.  A drop of liquid containing a suspension of microspheres is placed on top of a prism and a laser beam focused onto the prism surface from below.  When the beam is incident on the glass-water interface at just greater than the critical angle for total internal reflection an evanescent field penetrates a short distance into the water.  Under these conditions we see the microparticles drawn together into short optically bound chains.

Part (a) of the diagram represents out set-up.  The beam is retro-reflected and re-focused by the concave mirror in order to balance the radiation pressure in each direction.  Part (b) shows a photograph of optically bound chains of 2 micron diameter microspheres.