Supervisor: Prof. dr. Alfons van Blaaderen
Funding: MCEC
Employed: 10 April 2016 – 10 April 2019

Shearing long-ranged repulsive colloidal system

Investigating colloidal fluids under steady and oscillatory shear in real space is an interesting and fascinating aspect. There has been very few experiments reported [1] hard sphere crystallization and melting upon application of steady shear and cessation of shear respectively. Recently [2] four different phases also have been identified in hard sphere system both in experiment and simulation upon application of oscillatory shear. Those fascinating phases are an oscillating twinned face-centered-cubic (FCC) phase, a sliding layer phase, a string phase and a tilted layers phase. However there is no such attempt on long ranged soft colloidal system in which body-centered-cubic (BCC) crystal can be investigated.

In this work, we investigate colloidal dynamics and phase behaviours in soft colloidal crystals (BCC crystal (Figure.1)) submitted to a oscillatory shear. We use a home-built shear cell [3] mounted on state of the art confocal microscope and particle tracking techniques to study systems on the single particle level. The novel set up provides image of 3D structure of colloidal crystals, shear induced ordering and alignment of crystalline layers in both vorticity-velocity plane and their perpendicular direction.  In addition it has the ability to provide structural information of colloids at zero velocity plane during the time of shear. Interestingly on studying above system, We observe BCC crystal formation (Figure.1) under application of oscillatory shear at a particular frequency. We obtain also the melting condition of the crystal and a phase diagram for crystallization based on shear rate and frequency.

Figure.1: a) Radial distribution function of  BCC crystal (b) Bond order parameters for observed crystal

[1]  Y. L. Wu, D. Derks, A. van Blaaderen and A. Imhof, Proc. Natl. Acad. Sci. U. A., 106, 10564 (2009).

[2] T. H. Besseling et al, Soft Matter, 8, 6931 (2012)

[3] D. Derks, Y. L. Wu, A. van Blaaderen and A. Imhof, Soft Matter, 5, 1060(2009)