Orientational ordering benefits nanorod sonication

While ultrasonication is a well established method to disperse nanorods, insufficient attention has been given to the impact of the particle mass fraction <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>W</a:mi> <a:mi>s</a:mi> </a:msub> </a:math> at which sonication takes place. Its importance goes far beyond viscosity tuning, since a dilute isotropic nanorod suspension transitions to a liquid crystal (LC) phase if <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>W</b:mi> <b:mi>s</b:mi> </b:msub> </b:math> surpasses a threshold, via a wide biphasic window of isotropic–LC equilibrium phase coexistence. By systematically varying <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>W</c:mi> <c:mi>s</c:mi> </c:msub> </c:math> of suspensions of cellulose nanocrystals (CNCs) across and beyond the biphasic range we show that <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msub> <d:mi>W</d:mi> <d:mi>s</d:mi> </d:msub> </d:math> strongly influences the sonication process. Near the original suspension's stability limit of the isotropic phase, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msubsup> <e:mi>W</e:mi> <e:mn>0</e:mn> <e:mo>*</e:mo> </e:msubsup> </e:math> , the sonication shear flow induces a temporarily ordered paranematic state which assists particle individualization and minimizes damage. Our atomic force microscopy study supports this, finding increasing aspect ratio nanorods as <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>W</f:mi> <f:mi>s</f:mi> </f:msub> </f:math> is increased from the fully isotropic range past the onset of the equilibrium biphasic window. <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msub> <g:mi>W</g:mi> <g:mi>s</g:mi> </g:msub> </g:math> impacts also the macroscopic properties like the phase diagram and the pitch of the cholesteric LC helix into which CNCs self-assemble. A further key finding is that CNC suspensions concentrated beyond the threshold for gelation should be sonicated to remove end-to-end chains formed in the gel, since otherwise these remain even after dilution. Our conclusions apply also to many other nanorod, nanotube and nanoplatelet suspensions, where reproducibility problems may be due to the lack of control of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msub> <h:mi>W</h:mi> <h:mi>s</h:mi> </h:msub> </h:math> and to agglomerates remaining from high-concentration precursor suspensions.