Unexpected Time Dependence in Nanofibrillated Cellulose Gels 63 Fig. 3 Mean strains in free NFC gels of two different aspect ratios. (a) Imaging setup for quantifying strains in elongated or circular gels. (b) Elongated gels, where the length of the gel (≈0.75 in) was much larger than its width (≈0.25 in). Here, εyy increased over time, indicating elongation of short axis so the gel became more circular. (c) Circular gels, with radii close to the width of the long gels. Here, εyy ≈0 for all time points. In subfigures (b-c), colors denote results from different samples. In all experiments between both shapes, εxx was approximately zero for all time points. Fig. 4 Local displacements in an NFC gel. (a) Schematic of contracting PNIPAAm microspheres in NFC gel. The microspheres are not physically or chemically adhered to the NFC. The white space around the contracted microsphere denotes water that NFC can expand into. (b-c) Images of NFC gel before (b, 29◦C) and after (c, 39◦C) contraction of a representative PNIPAAm microsphere. (d) Local displacements quantified through DIC. The color indicates magnitude of displacements, and arrows show direction and magnitude. White regions in the displacement field are erroneous regions that are removed from the analysis as in ref. [17]. The black circle indicates the location of the microsphere; it is larger than the size of the microsphere, as DIC can not get proper data at boundaries in the image. All scale bars denote 50 µm. Conclusions In this report we have shown an unexpected time dependence, with deformations occurring on the order of hours in gels made of NFC. Subsequent investigation showed that elongated gels on hydrophilic substrates deformed to become more circular, suggesting capillary behavior may be the underlying cause for the long time dependence. The experiments with PNIPAAm microspheres revealed that NFC gels swelled into the water expelled by the PNIPAAm. These observations, combined with prior studies on poroelastic effects in NFC gels, indicate that the mechanics of NFC gels results from a combination of swelling, poroelasticity, and capillary behavior. Future work will design experiments to test this combination of properties in more detail. A challenge to consider in future work is drying of the gels, which occurs on time scales of hours, similar to the time scales of deformation observed in this study. In the experiments shown in Fig. 2, it would not be possible to wait for the sample to equilibrate in the loading device before testing, because the hours of equilibration time would cause the sample to dry excessively. Experiments are being designed to use samples having dimensions that minimize the surface area exposed to air, which will enable rigorous quantification of mechanical properties.
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