Abstract
The isotropic and nematic (I + N) coexistence for rod-like colloids is a signature of the first-order thermodynamics nature of this phase transition. However, in the case of amyloid fibrils, the biphasic region is too small to be experimentally detected, due to their extremely high aspect ratio. Herein, we study the thermophoretic behaviour of fluorescently labelled β-lactoglobulin amyloid fibrils by inducing a temperature gradient across a microfluidic channel. We discover that fibrils accumulate towards the hot side of the channel at the temperature range studied, thus presenting a negative Soret coefficient. By exploiting this thermophoretic behaviour, we show that it becomes possible to induce a continuous I-N transition with the I and N phases at the extremities of the channel, starting from an initially single N phase, by generating an appropriate concentration gradient along the width of the microchannel. Accordingly, we introduce a new methodology to control liquid crystal phase transitions in anisotropic colloidal suspensions. Because the induced order-order transitions are achieved under stationary conditions, this may have important implications in both applied colloidal science, such as in separation and fractionation of colloids, as well as in fundamental soft condensed matter, by widening the accessibility of target regions in the phase diagrams.
Highlights
Strength and high temperature[23]
This is because the gain in free volume is higher than the loss in orientational entropy at sufficiently high concentration[17]. This phase transition is expected to be discontinuous from a thermodynamic point of view, so an intermediate concentration window is expected where the fibril suspension undergoes a macro-phase separation with an obvious interface, exhibiting a diluted I phase coexisting with a concentrated N phase at equilibrium, with the width of the region being inversely proportional to the aspect ratio[17, 26,27,28,29,30]
When comparing the present findings to the evolution of I-N phase separation in dispersions of anisotropic particles under gravity[27,28,29], the current kinetic process is significantly faster due to the presence of thermophoresis
Summary
Strength and high temperature[23]. By increasing the volume fraction, a suspension composed of amyloid fibrils undergoes a first-order phase transition from a randomly oriented isotropic (I) phase to an ordered nematic (N) phase, where fibrils are preferentially oriented along a common direction[24, 25]. We employ a thermophoretic force to allow the simultaneous observation of the I and N phases, by accumulating β-lactoglobulin fibrils to one side of a channel (and depleting them from the opposite side), taking full advantage of the small size of microfluidic channels to maximize the magnitude of the temperature gradient (∇T, as high as tens of degrees per mm) This is of fundamental importance as it affords the capability of inducing a liquid crystalline phase transition on any rod-like particle system without having to change the composition, the physico-chemical properties or the interaction potential of the fibrils suspension. It is essential to first characterise the Soret coefficient of β-lactoglobulin fibrils to guide a careful estimation of the required conditions for phase separation
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