Short and soft: multi-domain organization, tunable dynamics and jamming in suspensions of grafted colloidal cylinders with small aspect ratio

TL;DR

This study explores the phase behavior and dynamics of short, soft colloidal nanocylinders with small aspect ratios, revealing complex transitions from liquid to jammed states and insights into their unique viscoelastic properties.

Contribution

It provides the first detailed investigation of soft colloidal nanocylinders with small aspect ratios, highlighting their rich phase and dynamic behavior distinct from long rods.

Findings

Transition from isotropic liquid to multi-domain structures with increasing concentration

Observation of jamming and shell interpenetration at higher concentrations

Detection of hexagonally packed order in densely jammed states

Abstract

The yet virtually unexplored class of soft colloidal rods with small aspect ratio is investigated and shown to exhibit a very rich phase and dynamic behavior, spanning from liquid to nearly melt state. Instead of nematic order, these short and soft nanocylinders alter their organization with increasing concentration from isotropic liquid with random orientation to one with preferred local orientation and eventually a multi-domain arrangement with local orientational order. The latter gives rise to a kinetically suppressed state akin to structural glass with detectable terminal relaxation, which, on increasing concentration reveals features of hexagonally packed order as in ordered block copolymers. The respective dynamic response comprises four regimes, all above the overlapping concentration of 0.02 g/ml: I) from 0.03 to 0.1 g/mol the system undergoes a liquid-to-solid like transition with a structural relaxation time that grows by four orders of magnitude. II) from 0.1 to 0.2 g/ml a dramatic slowing-down is observed and is accompanied by an evolution from isotropic to multi-domain structure. III) between 0.2 and 0.6 g/mol the suspensions exhibit signatures of shell interpenetration and jamming, with the colloidal plateau modulus depending linearly on concentration. IV) at 0.74 g/ml in the densely jammed state, the viscoelastic signature of hexagonally packed cylinders from microphase-separated block copolymers is detected. These properties set short and soft nanocylinders apart from long colloidal rods (with large aspect ratio) and provide insights for fundamentally understanding the physics in this intermediate soft colloidal regime, as well as and for tailoring the flow properties of non-spherical soft colloids.