Sommaire:

On a coarse-grained level, colloids often interact via so-called ultrasoft potentials, which assume at short interparticle distances values in the order of a few k$_{\rm B}T$; thus, these particles are able to overlap at the cost of a relatively small energy penalty. Under equilibrium conditions such ultrasoft particles are able to form aggregates (clusters) of overlapping particles which can then either form a disordered or an ordered cluster phase. In the latter case, these aggregates populate the positions of a regular fcc or bcc lattice. Cluster crystals display rather unconventional properties, such as a density-independent lattice constant [1]. Also under out-of-equilibrium conditions, ultrasoft systems show unexpected features. Exposing a cluster crystal to shear leads – with increasing shear rate $\dot \gamma$ – to the following novel response-scenario: for small $\dot \gamma$-values the crystal melts; then gradually strings parallel to the flow direction form which are arranged in a hexagonal grid in the gradientvorticity plane. Upon further increasing $\dot \gamma$ this lattice eventually melts [2]. Exposing a cluster crystal to Poiseuille flow the emergence of a quantized flow pattern is observed where the height and the width of the fluid stream display well-defined plateaus, indicating a successive fluidization of crystal layers adjacent to the channel walls [3].
[1] B.M. Mladek et al., Phys. Rev. Lett. 96, 045701 (2006).
[2] A. Nikoubashman et al., Phys. Rev. Lett. 107, 068302 (2011).
[3] A. Nikoubashman et al., Soft Matter 8, 4121 (2012).

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