Transport of driven colloids in optical landscapes

Roel Dullens
University of Oxford, England

Abstract:
In this talk I will discuss our experimental work on driving colloidal particles over a periodic potential energy landscape generated by multiple optical traps. The majority of the talk will be dedicated to the microscopic dynamics underlying a synchronisation phenomenon termed dynamic mode locking, which occurs when naturally oscillating processes are driven by an externally imposed modulation. In our experiments, we elucidate the nature of the dynamic mode locking steps by analysing particle trajectories in real-space and through phase portraits. Furthermore, we extend our experiments to chains of magnetically coupled particles driven over optical landscapes. We show that the synchronisation of density waves within the chain leads to the enhancement of mode locking of the chain as a whole, which underlines the close link between microscopic dynamics and synchronisation in driven systems. In the final part of the talk, I will show some preliminary results on the depinning of chains of interacting magnetic colloidal particles of various lengths in a static sinusoidal optical landscape. The mismatch between the wavelength of the landscape and the particle spacing in the chain gives rise to an Aubry-type transition. In this regime a chain exhibits zero static friction despite being subject to a potential energy landscape. This ‘zero friction’ is observed for a finite chain at regular and well-defined chain lengths depending on the commensurability between the wavelength and the particle spacing.