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Semi-flexible polymers in crowded environments exhibit complex dynamics that play a crucial role in various biological and material design processes. Based on the classic reptation theory, it is generally believed that semiflexible polymers are trapped within static confinements. Here we demonstrate that semi-flexible polymers are indeed trapped in short-lived kinetic cages. We have developed a novel scaling law for rotational diffusion through the examination of the polymer network at the level of an individual nanofiber and quantify the onset of entanglement and entanglement itself by introducing an effective tube diameter. We also show that understanding these dynamics is critical for interpreting the macroscopic behavior of such systems, as evidenced by our micro-rheology analysis. The insight from our study provides a rectified microscopic understanding of the dynamics of semi-flexible polymers in complex environments, yielding new principles to better understand the macroscopic behavior of crowded systems, from biophysics to materials science.

Nanofibrous networks are the foundation and natural building strategy for all life forms on our planet. Apart from providing structural integrity to cells and tissues, they also provide a porous scaffold allowing transport of substances, where the resulting properties rely on the nanoscale network structure. Recently, there has been a great deal of interest in extracting and reassembling biobased nanofibers to create sustainable, advanced materials with applications ranging from high-performance textiles to artificial tissues. However, achieving structural control of the extracted nanofibers is challenging as it is strongly dependent on the extraction methods and source materials. Furthermore, the small nanofiber cross-sections and fast Brownian dynamics make them notoriously difficult to characterize in dispersions. In this work, we study the diffusive motion of spherical gold nanoparticles in semi-dilute networks of cellulose nanofibers (CNFs) using X-ray Photon Correlation Spectroscopy (XPCS). We find that the motion becomes increasingly subdiffusive with higher CNF concentration, where the dynamics can be decomposed into several superdiffusive relaxation modes in reciprocal space. Using simulations of confined Brownian dynamics in combination with simulated XPCS-experiments, we observe that the dynamic modes can be connected to pore sizes and inter-pore transport properties in the network. The demonstrated analytical strategy by combining experiments using tracer particles with a digital twin may be the key to understand nanoscale properties of nanofibrous networks., Comment: 57 pages, 17 figures, supplementary material

When two miscible fluids are brought into contact with each other, the concentration gradients induce stresses. These are referred to as Korteweg stresses and are analogous to interfacial tension between two immiscible fluids, thereby acting as an effective interfacial tension (EIT) in inhomogeneous miscible systems. EIT governs the formation of a viscous thread in flow-focusing of two miscible fluids. To further investigate its significance, we have studied thread formation of a colloidal dispersion focused by its own solvent. Experiments are combined with three-dimensional numerical models to systematically expand previous knowledge utilising different flow-focusing channel setups. In the reference setup, the sheath flows impinge the core flow orthogonally while in four other channel setups, the sheath flows impinge the core flow at an oblique angle that is both positive and negative with respect to the reference sheath direction. As an initial estimate of the EIT, we fit the experimentally determined thread shape in the reference setup to a master curve that depends on EIT through an effective capillary number. By numerically reproducing these experimental results, it is concluded that the estimated EIT is within 25% of the optimal EIT value that can be deduced by iteratively fitting the numerical results to the experimental measurements. Regardless of channel setups, further numerical calculations performed using the optimal EIT evaluated from the reference setup show good agreement with the experimental findings in terms of thread shapes, wetted region morphologies, and velocity flow fields. The one-to-one comparison of numerical and experimental findings unveil the crucial role of EIT on the thread detachment from the top and bottom walls of the channel, bringing useful insights to understand the physical phenomenons involved in miscible systems with a high-viscosity contrast., Comment: 23 pages, 12 figures

An interface between two miscible fluids is transient, existing as a non-equilibrium state before complete molecular mixing is reached. However, during the existence of such an interface, which typically be at short timescales, composition gradients at the boundary between the two liquids cause stresses effectively mimicking an interfacial tension. Here, we combine numerical modelling and experiments to study the influence of an effective interfacial tension between a colloidal fibre dispersion and its own solvent on the flow in a microfluidic system. In a flow-focusing channel, the dispersion is injected as core flow that is hydrodynamically focused by its solvent as sheath flows. This leads to the formation of a long fluid thread, which is characterised in 3D using Optical Coherence Tomography and simulated using a volume of fluid method. The simulated flow and thread geometries very closely reproduce the experimental results in terms of thread topology and velocity flow fields. By varying the effective interfacial tension numerically, we show that it clearly influences threading dynamics and that it can be described by an effective capillary number. Furthermore, we demonstrate that the applied methodology provide means to measure the ultra-low but dynamically highly significant effective interfacial tension., Comment: 23 pages, 14 figures, under consideration for publication in Journal of Fluid Mechanics

Successful assembly of suspended nanoscale rod-like particles depends on fundamental phenomena controlling rotational and translational diffusion. Despite the significant developments in fluidic fabrication of nanostructured materials, the ability to quantify the dynamics in processing systems remains challenging. Here we demonstrate an experimental method for characterization of the orientation dynamics of nanorod suspensions in assembly flows using birefringence relaxation. The methodology is illustrated using nanocelluloses (cellulose nanocrystals and nanofibrils) as model systems, where the coupling of rotational diffusion coefficients to particle size distributions as well as flow-induced orientation mechanisms are elucidated. Our observations advance the knowledge on key fundamental nanoscale mechanisms governing the dynamics of nanotubes and nanorods allowing bottom-up assembly into hierarchical superstructures., Comment: 6 pages, 4 figures, supplemental information

New materials made through controlled assembly of dispersed cellulose nanofibrils (CNF) has the potential to develop into biobased competitors to some of the highest performing materials today. The performance of these new cellulose materials depends on how easily CNF alignment can be controlled with hydrodynamic forces, which are always in competition with a different process driving the system towards isotropy, called rotary diffusion. In this work, we present a flow-stop experiment using polarized optical microscopy (POM) to study the rotary diffusion of CNF dispersions in process relevant flows and concentrations. This is combined with small angle X-ray scattering (SAXS) experiments to analyze the true orientation distribution function (ODF) of the flowing fibrils. It is found that the rotary diffusion process of CNF occurs at multiple time scales, where the fastest scale seems to be dependent on the deformation history of the dispersion before the stop. At the same time, the hypothesis that rotary diffusion is dependent on the initial ODF does not hold as the same distribution can result in different diffusion time scales. The rotary diffusion is found to be faster in flows dominated by shear compared to pure extensional flows. Furthermore, the experimental setup can be used to quickly characterize the dynamic properties of flowing CNF and thus aid in determining the quality of the dispersion and its usability in material processes., Comment: 45 pages, 13 figures

The increased availability and brilliance of new X-ray facilities have in the recent years opened up the possibility to characterize the motion of dispersed nanoparticles in various microfluidic applications. One of these applications is the process of making strong continuous filaments through hydrodynamic alignment and assembly of cellulose nanofibrils (CNF) demonstrated by H{\aa}kansson et al. [Nature communications 5, 2014]. In this process it is vital to study the alignment of the nanofibrils in the flow, as this in turn affects the final material properties of the dried filament. Small angle X-ray scattering (SAXS) is a well-suited characterization technique for this, which typically provides the alignment in a projected plane perpendicular to the beam direction. In this work, we demonstrate a simple method to reconstruct the full three-dimensional (3D) orientation distribution function (ODF) from a SAXS-experiment through the assumption that the azimuthal angle of the nanofibril around the flow direction is distributed uniformly; an assumption that is approximately valid in the flow-focusing process. For demonstrational purposes, the experimental results from H{\aa}kansson et al. (2014) have been revised, resulting in a small correction to the presented order parameters. The results are then directly compared with simple numerical models to describe the increased alignment of CNF both in the flowing system and during the drying process. The proposed reconstruction method will allow for further improvements of theoretical or numerical simulations and consequently open up new possibilities for optimizing assembly processes, which include flow-alignment of elongated nanoparticles., Comment: 37 pages, 10 figures, supplementary material