Supporting Information Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate M. Bulut Coskun a, Abozar Akbari b, Daniel T. H. Lai c, *, Adrian Neild a, Mainak Majumder b, Tuncay Alan a, * a. Laboratory for Micro Systems, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, 3800, Australia b. Nanoscale Science and Engineering Laboratory, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, 3800, Australia c. College of Engineering and Science, Victoria University, Melbourne, 3011, Australia * Corresponding Authors: Daniel.Lai@vu.edu.au, Tuncay.Alan@monash.edu.au S-1
1. Formation of liquid crystals of graphene oxide Liquid crystallinity defines a state between a solid and a fluid, in which the matter is strongly anisotropic but can still flow and respond to shear and be aligned in the shear direction. When shear forces are imposed on the ordered liquid crystalline phases, these phases align along the direction of shear as the shear forces overcome the rotational Brownian motion. Given the disc-like nature of the mesogens (GO sheets), the degree of order should be higher than rod-like mesogens (such as carbon nanotubes) because the disc-like mesogens (in GO liquid crystals) cannot tumble on its axis (unlike rod-like mesogens), they can only lie down along the direction of shear. 1-2 As shown in Figure S1a, at low concentrations (0.25 mg/ml) because of high free volume and high orientation entropy, the dispersion is in full isotropic phase 3 and does not form a liquid crystal. However, as the GO concentration increases to 35mg/mL, the excluded volume increases and the GO sheets orients in parallel to each other and forming a nematic liquid crystal 4 (Figure S1b). Figure S1: LC-PolScope processed images demonstrate transition of 2D disc like dispersion of GO from a disordered isotropic (0.25 mg/ml) to ordered nematic phase (35 mg/ml). The images displaying the azimuth and retardance, where the hue and brightness represents the azimuth and retardance, respectively, as depicted by the colored legend. (Scale bar denotes 50 micrometers) 2. Experimental results for rgo on Polyamide 66 Substrate S-2
The casting method employed in this work is also applicable to other polymers. To demonstrate this capability, GO films were cast on Polyamide 66 substrates, were subjected to 3 and 8 hour reduction times, and their strain response was characterized. Figure S1a demonstrates the voltage output of the polyamide 66 based devices under 0.2, 0.6 and 0.8% dynamic strain inputs. Figure S1b shows the repeatable and consistent resistance change of the devices for 50 cycles under 0.6% strain. As shown in Figure S1c, the sensitivity of the device increases significantly by modifying the reduction time, which in turn changes the overall conductivity of the film. The gage factor (GF) is 106 after an 8-hour reduction step (Figure S1d). An optical micrograph of a deformed rgo-on-polyamide sample is presented at Figure S1e. Figure S2 (a) Response of the rgo coated polyamide 66 to the cyclic strain cycles (0.2, 0.6, and 0.8%) recorded with a 100Hz sampling frequency. (b) The response of the film after 50 cycles of strains under 0.6%. Voltage output here is converted to the rate of change of resistance via eq 1. (c) Percentage change in the resistance of rgo after different reduction times. (d) The slope of the linear fits define the gauge factor as 106 after 8 hour reduction, (e) An optical micrograph of the tested rgo on polyamide 66 sample. S-3
3. Gage factors of the previous graphene based strain sensors Table S1 Gauge factors of the previous works in the literature Reference Device Gauge Factor Strain Measurement Range (%) 5 Nanographene films on mica 300-0.30 to 0.35 6 7 Transfer of graphene ribbons on prestrained PDMS Graphene woven fabrics with high crack density on PDMS 20 0 to 25 ~10 6 0 to 10 8 Layered Percolative graphene film ~15 0 to ~1.7 9 Monolayer graphene transferred on PDMS 151 3 to 5 10 Graphene/epoxy composite 11.4 0 to 0.1 11 Graphene transferred on rubber 6.1 0 to 1 12 Few layer graphene on PDMS 2.4 0 to 1.5 13 Epoxy/graphene nanocomposite 56.7 ± 0.69 0 to 2 14 Graphene nanoplatelets coated on stretchable yarns 0.1 (woolen yarn), 1.4 (nylon covered), 1800 (rubber yarn) 0 to 50 (woolen yarn), 0 to 150 (nylon covered), 0 to 100 (rubber yarn) 15 Screen printing graphene nanoplatelets and carbon nanotube composition on PDMS 2 to 100 0 to 40 16 Spray coating of graphene suspension 200 0 to 0.14 17 Assembled rgo sheets on human hair 4.46 0 to 30 18 rgo mesh fabrics This work Casting high concentration GO dispersion on masked PDMS 20 (ε 5%), ~1000 (ε > 7.5%) 137.2 (ε 0.8), 261.2 (ε > 0.8%) 0 to 7.5 0 to 2 S-4
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