Understanding ion-induced assembly of cellulose nanofibrillar gels through shear-free mixing and in situ scanning-SAXS

During the past decade, cellulose nanofibrils (CNFs) have shown tremendous potential as a building block to fabricate new advanced materials that are both biocompatible and biodegradable. The excellent mechanical properties of the individual CNF can be transferred to macroscale fibers through careful control in hydrodynamic alignment and assembly processes. The optimization of such processes relies on the understanding of nanofibril dynamics during the process, which in turn requires in situ characterization. Here, we use a shear-free mixing experiment combined with scanning small-angle X-ray scattering (scanning-SAXS) to provide time-resolved nanoscale kinetics during the in situ assembly of dispersed cellulose nanofibrils (CNFs) upon mixing with a sodium chloride solution. The addition of monovalent ions led to the transition to a volume-spanning arrested (gel) state. The transition of CNFs is associated with segmental aggregation of the particles, leading to a connected network and reduced Brownian motion, whereby an aligned structure can be preserved. Furthermore, we find that the extensional flow seems to enhance the formation of these segmental aggregates, which in turn provides a comprehensible explanation for the superior material properties obtained in shear-free processes used for spinning filaments from CNFs. This observation clearly highlights the need for different assembly strategies depending on morphology and interactions of the dispersed nanoparticles, where this work can be used as a guide for improved nanomaterial processes.

: Illustration of the ow cell, which is identical to the one used by Rosén et al. 1 . It consists of an aluminum channel plate sandwiched between two transparent COC-lms that are all mounted together with two outer thicker aluminum plates. Reproduced from Rosén et al. 1 with permission from the Royal Society of Chemistry.
Flow cell Fig 1 shows the ow cell used for the experiments, which is identical to the one used by Rosén et al. 1 . The double-focusing channel is milled out of a 1 mm thick aluminum plate, which is sandiwiched between two COC-lms that both provide optical access to the mixing region and act as walls to the ow. The sandwich is mounted with two outer 10 mm thick aluminum plates where connections for the uid are attached at the front plate facing the X-ray beam.

Flow setup
Fig 2 shows the setup of the ow equipment, which is identical to Rosén et al. 1 . Three syringe pumps (NE-4000) are driving the main ow (with ow rates Q 1 , Q 2 and Q 3 ) with 1 mL syringes. To ll up tubing prior to the experiment and push out bubbles, two additional syringe pumps with larger 20 mL syringes are connected to the core and 2nd sheath ow, respectively. To avoid gelation in the channel while not measuring as well as ushing the channels from any gel accumulation, two additional pumps are connected to the 1st sheath   Figure 2: Illustration of the uid distribution to the experiments, which is identical to Rosén et al. 1 . To be able to run at low ow rates, small 1 mL syringes are driving the main ow. However, larger syringes are also connected to the system to be able to ll up the tubing as well as ush the ow cell with higher ow rates. To maintain a constant level in the outlet container, a peristaltic pump is continously removing water from the container. A small motor is mounted at the outlet to remove excess gel that otherwise could result in blockage. The pumps ow rates Q 1 , Q 2 , Q 2,barrier , Q 3 and Q drain are controlled remotely from outside the experimental hutch. Reproduced from Rosén et al. 1 with permission from the Royal Society of Chemistry. ow, each with two large 60 mL syringes with water. To increase the stiness of the system and avoid potential bubbles in the large syringes to store energy and act as springs in the uidic system, manual shut-o valves are placed according to the gure. During experiment all valves were shut to the syringe pumps, except for the valves to one of the 1st sheath ow pumps (with ow rate denoted as Q 2,barrier ), in order to quickly ush the system.
A peristaltic pump (with ow rate denoted as Q drain ) is used to drain the outlet container and keeping the level stationary and a small 6V motor rotating at around 5 rpm is slowly removing gel at the outlet without disturbing the ow upstream.
During the SAXS experiments, the three syringe pumps with the 1 mL syringes (ow rates Q 1 , Q 2 and Q 3 ) as well as the barrier-pump for the 1st sheath ow and the peristaltic pump are controlled remotely from outside the experimental hutch. In this experiment, all the pumps with the 1 mL syringes are running at the same ow rate Q 1 = Q 2 = Q 3 = Q, meaning that the perstaltic pump is set to Q drain = 5Q+2Q 2,barrier to keep the level constant.
The normal sequence of ow rates during the scan is as following: 1. The 1st sheath is set to Q 2,barrier = 50 mL/h, to ensure no gelation in the channel.
2. All primary ows are set to the desired ow rates Q 1 = Q 2 = Q 3 = Q, where the experiments reported in the main manuscript is running at Q = 0.5 mL/h. 3. Q 2,barrier is set to zero and the ow is allowed to stabilize for a couple of minutes prior to measurement.
4. During the scanning-SAXS measurement, the system is running continously, while data is collected from the dierent y-and z-locations in the ow. 5. To remove possible gel that might have accumulated during the scan, the channel is ushed with Q 2,barrier = 5000 mL/h for 1 s directly after the measurement and then set to Q 2,barrier = 50 mL/h again.

Supporting POM experiments
Just as in our earlier work, 1 the ow was studied with polarized optical microscopy (POM) in order to ensure that there is a gel exiting the channel at the given ow conditions as well as to determine the plug velocity prole with core ow radius R 1 and velocity V . In Given the low ow rates and low concentration of the CNF dispersion, the birefringence is not really visible at the experimental ow rate of Q = 0.5 mL/h and there is no visible dierence in the channel in the case without gelation (Fig. 4a) and with gelation (Fig. 4b).
Without gelation, we cannot observe the dispersion as it exists the channel, probably as the concentration gradient of nanobers is more smeared out as nanobers diusing into the water. With the CNF gelation at the experimental ow rate Q = 0.5 mL/h (Fig. 4b), the gel thread is observed and accumulating on the bottom of the container. At a 5X higher ow rate Q = 2.5 mL/h (Fig. 4c), some alignment is seen by the birefringence in the channel.
However, it seems that the gel is rather form outside the ow cell formed rather than inside the ow cell, similar to the dispersion with CNCs as observed by Rosén et al. 1 .
Just like with the CNCs, 1 we could also observe small impurities in the CNF dispersions.
By tracking these, it was straightforward to conrm that the ow is a plug ow where all impurities moving with a constant velocity. This allowed us also to experimentally determine the core velocity V and the projected radius R 1,y . Since we know the core ow rate Q 1 , the core radius in the viewing direction R 1,x can be calculated through Q 1 = πR 1,x R 1,y V . An illustration of the procedure is included in the supplementary video.
Estimating the ion concentration distribution in ow-focusing