Effects of Shearing and Extensional Flows on the Alignment of Colloidal Rods

Abstract

Cellulose nanocrystals (CNC) can be considered as model colloidal rods and have practical applications in the formation of soft materials with tailored anisotropy. Here, two contrasting microfluidic devices are employed to perform an experimental quantification of the role of shearing and planar extensional flows on the alignment of a dilute CNC dispersion. Characterization of the flow field by microparticle image velocimetry is coupled to flow-induced birefringence analysis to quantify the deformation rate–alignment relationship. The deformation rate required for CNC alignment is 4× smaller in extension than in shear. The birefringence signal rising from the CNC alignment in shear and extension can be scaled on a single master curve using a Péclet number that accounts for the shear and extensional viscosity of the solvent fluid, respectively. Based on this simple scaling relationship, it is possible to anticipate the alignment of rigid colloidal rods under purely extensional deformation by knowing the respective alignment profile in a shearing flow that is more accessible via multiple rheo-optical techniques. Quantification of the differences between shearing and extensional kinematics at aligning colloidal rods establishes coherent guidelines for the manufacture of structured soft materials.

Birefringent Properties of Aqueous Cellulose Nanocrystal Suspensions

“…As expected, increasing the amount of CNC increases the birefringence response Calabrese et al(2021). studied the birefringence of a 0.1 wt% CNC dispersion that had been similarly prepared (CNC purchased from CelluForce…”

“…Above, cholesteric tactoids are formed. The suspensions in this study are above 0.5 wt% and we hence expect interparticle interactions which could explain our different exponent Calabrese et al(2021). report that extensional forces, described by the extensional rate \dot {\varepsilon }, are four times more effective in aligning CNCs than shear forces, stating \Delta n\sim {\dot {\gamma }^{0.9}}\sim {(4 \cdot \dot {\varepsilon })^{0.9}}.…”

Alignment of colloidal rods in crowded environments

“…The microfluidic channel has a rectangular cross section with height H = 2 mm along the z-axis, corresponding to the optical path, and width W = 0.4 mm, along the y-axis, thus providing an approximation to a two-dimensional (2D) Poiseuille flow. 18,44 The flow in the microfluidic channel is driven along the channel length (x -axis) by a syringe pump (Cetoni Nemesys) and Hamilton Gastight syringes infusing and withdrawing at an equal and opposite volumetric flow rate (Q) from the inlet and outlet, respectively. The average flow velocity in the channel is…”

“…The CNC has an average length l = 260±180 nm, a maximum length l max = 700 nm, an average diameter d = 4.8 ± 1.8 nm as detected from atomic force microscopy. 18 The effective hydrodynamic diameter is computed as d = δ + d = 27.4 nm, considering the estimated contribution of the electric double layer δ = 22.6 nm in deionised water. 41 Assuming a cylindrical shape, the number density of the CNC was calculated as ν = (4 φ volume )/( d 2 l π) where l is the hydrodynamic length of the CNC obtained experimentally through Dr 0 (specified in the main text), and φ volume is the volume fraction of the suspended CNC (calculated using a CNC density of 1560 Kg/m 3 ).…”

“…Existing literature has shown that the most important control parameter for the onset of hydrodynamic alignment of rigid colloidal rods is the Péclet number P e = γ/Dr, a dimensionless number quantifying the relative strength between the imposed deformation rate (e.g., the shear rate, γ) and the rotational diffusion coefficient of the rods (Dr). [14][15][16][17][18] In dilute suspensions, the Péclet number can be defined as P e 0 = γ/Dr 0 , with the rotational diffusion coefficient of the rods given as…”