Colloidal rod dynamics under large amplitude oscillatory extensional flow

TL;DR

This study combines experiments and simulations to analyze how rod-like colloidal particles, specifically cellulose nanocrystals, orient under oscillatory elongational flow, revealing nonlinear dynamics and the effects of flow parameters.

Contribution

It provides a detailed experimental and theoretical analysis of CNC orientation dynamics under oscillatory flow, including nonlinear responses and the influence of flow parameters.

Findings

Birefringence response is sinusoidal and linear at low Pe and De.

Response becomes nonlinear with increasing Pe, saturating at high strain rates.

Asymmetry and residual effects appear at higher De, indicating limited particle response.

Abstract

We perform a combined experimental and theoretical investigation of the orientational dynamics of rod-like colloidal particles in dilute suspension as they are subjected to a time-dependent homogeneous planar elongational flow. Our experimental approach involves the flow of dilute suspensions of cellulose nanocrystals (CNC) within a cross-slot-type stagnation point microfluidic device through which the extension rate is modulated sinusoidally over a wide range of P\'{e}clet number amplitudes ($Pe_0$) and Deborah numbers ($De$). The time-dependent orientation of the CNC is assessed via quantitative flow-induced birefringence measurements. For small $Pe_0 \lesssim 1$ and small $De \lesssim 0.03$, the birefringence response is sinusoidal and in phase with the strain rate, i.e., the response is linear. With increasing $Pe_0$, the response becomes non-sinusoidal (i.e., nonlinear) as the birefringence saturates due to the high degree of particle alignment at higher strain rates during the cycle. With increasing $De$, the CNC rods have insufficient time to respond to the rapidly changing strain rate, leading to asymmetry in the birefringence response around the minima and a residual effect as the strain rate passes through zero. These varied dynamical responses of the rod-like CNC are captured in a detailed series of Lissajous plots of the birefringence versus the strain rate. Experimental measurements are compared with simulations performed on both monodisperse and polydisperse systems, with rotational diffusion coefficients $D_r$ matched to the CNC. A semiquantitative agreement is found for simulations of a polydisperse system with $D_r$ heavily weighted to the longest rods in the measured CNC distribution. The results will be valuable for understanding, predicting, and optimizing the orientation of rod-like colloids during transient processing flows such as fiber spinning and film casting.