Flow fields control nanostructural organization in semiflexible networks.

Tomas Rosén,Nitesh Mittal,Stephan V Roth,L Daniel Söderberg,Fredrik Lundell,Peng Zhang

doi:10.1039/c9sm01975h

Abstract

Hydrodynamic alignment of proteinaceous or polymeric nanofibrillar building blocks can be utilized for subsequent assembly into intricate three-dimensional macrostructures. The non-equilibrium structure of flowing nanofibrils relies on a complex balance between the imposed flow-field, colloidal interactions and Brownian motion. The understanding of the impact of non-equilibrium dynamics is not only weak, but is also required for structural control. Investigation of underlying dynamics imposed by the flow requires in situ dynamic characterization and is limited by the time-resolution of existing characterization methods, specifically on the nanoscale. Here, we present and demonstrate a flow-stop technique, using polarized optical microscopy (POM) to quantify the anisotropic orientation and diffusivity of nanofibrils in shear and extensional flows. Microscopy results are combined with small-angle X-ray scattering (SAXS) measurements to estimate the orientation of nanofibrils in motion and simultaneous structural changes in a loose network. Diffusivity of polydisperse systems is observed to act on multiple timescales, which is interpreted as an effect of apparent fibril lengths that also include nanoscale entanglements. The origin of the fastest diffusivity is correlated to the strength of velocity gradients, independent of type of deformation (shear or extension). Fibrils in extensional flow results in highly anisotropic systems enhancing interfibrillar contacts, which is evident through a slowing down of diffusive timescales. Our results strongly emphasize the need for careful design of fluidic microsystems for assembling fibrillar building blocks into high-performance macrostructures relying on improved understanding of nanoscale physics.

Highlights

Teknikringen 56-58, SE-100 44 Stockholm, Sweden † Electronic supplementary information (ESI) available
Microscopy results are combined with smallangle X-ray scattering (SAXS) measurements to estimate the orientation of nanofibrils in motion and simultaneous structural changes in a loose network
We demonstrate a flow-stop technique using polarized optical microscopy (POM) that is easy to operate and is able to obtain a thorough understanding on the nanoscale in situ dynamics of birefringent cellulose nanofibrils (CNFs), which are used as model nanofibrillar systems due to their bio-based origin and abundant availability

Summary

Introduction

Teknikringen 56-58, SE-100 44 Stockholm, Sweden † Electronic supplementary information (ESI) available. § Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Dynamic characterization of nanofibrils under flow is required for a thorough fundamental understanding of the nanoscale physics. The knowledge will contribute towards the highly uniform spatial organization of building blocks in the three-dimensional macrostructures. The motion of dispersed nanofibrils in flows is due to the translational/rotational advection (the externally forced motion of the nanofibrils due to the deformation of the surrounding medium) and translational/ rotational diffusion (the internally forced motion of a collection of nanofibrils together with its solvent towards thermodynamic equilibrium). The translational diffusive motion leads to a uniform spatial distribution of nanofibrils in dispersion that typically remains unaffected by the flow. Translational diffusion will affect spatial variations of orientation distributions, the timescales of rotational diffusion is usually much shorter.[4]

Results

Discussion

Conclusion