62 S. Patel and J. Notbohm NFC gels were next placed in the loading device but not subjected to compression, with the expectation that the gel would remain undeformed. Surprisingly, εxx increased over a time period of a couple of hours (Fig. 2b). This may suggest that the deformation was caused by interaction between the sample and the loading device. To verify this, we placed gels on a glass coverslip outside of the loading device. When quantifying strains with DIC, εxx was near zero for all time points (Fig. 2c). This indicates that the sample interacted with the loading device, causing time-dependent deformation. We reasoned that the interaction could be through differences in surface tension of the NFC gel in the different conditions. This suggests that NFC gels may exhibit elasto- or visco-capillary behavior. Fig. 2 Strains in experiments with NFC gels. (a) NFC gels were placed in the loading device and were subjected to uniaxial compression in thexdirection. Timet = 0 denotes when the deformation was applied. (b) NFC gels were placed in the loading device but not subjected to any loading. εxx shows that despite no loading, the strain increased over a period of a couple of hours. (c) NFC gels were not placed in the loading device. The strains in these gels were near zero for the duration of the experiment. All images show cartoons of NFC gels (green) and the location imaged by the microscope (squares). Panels a and b show the gel in the loading device (black). In all images, εxx is quantified through DIC and averaged at each time point. Dots in each image denote the mean strain, and error bars denote standard deviations over space. The dashed line denotes the macroscale strain applied by the loading device on the gel. As a simple initial quantification of capillary behavior, we placed NFC gels of various aspect ratios on glass coverslips and imaged them for a few hours. Some gels had length-to-width ratios of 3:1, while other gels were circular. The average strains are shown in Fig. 3. For the elongated gels, the normal strains along the short axis (y direction) increased over an hour (Fig. 3b). This indicates that the gels deformed to become more circular. Deformations of the gels were apparently permanent, as the gels did not return to their initial size and shape. By contrast, the circular gels remained essentially undeformed with strains being approximately zero for all time points (Fig. 3c). Because we did not apply a load on the gels during these experiments and elongated gels became circular, the gels’ deformation was likely caused by surface tension, consistent with capillary behavior. We next quantified the response of NFC gels to introduction of a void space containing water. This experiment was intended to give a sense of poroelastic swelling at a length scale close to that of the fibers, which is a smaller length scale than the experiments in Figs. 2–3. To create void space in the NFC gels, we used microspheres of PNIPAAm, a hydrogel that contracts and expels water when heated. Microspheres were added to NFC gels embedded with fluorescent particles. An image was collected at 29◦C followed by increasing the temperature to 39◦C, which caused the microspheres to contract and expel water. A second image was collected for subsequent quantification of displacements by DIC. Fig. 4a shows a schematic of the described experiment, wherein a PNIPAAm microsphere (black) contracts and pushes water out, indicated by the white region. For most engineering materials, this experiment would cause no deformations, because the PNIPAAm microspheres were not attached to the surrounding material, but the NFC gel could potentially swell in response to the free water. Fig. 4b displays a representative image in the reference state, with the PNIPAAm microsphere being the black circle in the center of the image and the contrast provided by the fluorescent particles embedded in the NFC gel. After contraction, the fluorescent particles appeared to move inward (Fig. 4c), indicating inward deformation of the NFC gel. The inward deformation was confirmed in the displacements quantified by DIC, with a maximum magnitude of displacement of ≈5 µm (Fig. 4d). This inward motion was likely due to swelling of the NFC into the region containing water expelled by the contracting microsphere.
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