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Do Thermal Effects Cause the Propulsion of Bulk Graphene Material?

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Do Thermal Effects Cause the Propulsion of Bulk Graphene Material? correspondence Do thermal effects cause the propulsion of bulk graphene material? To the Editor — In a recent article, graphene sponge. So collisions between bottom of the glass tube and the graphene Zhang et al.1 observed the direct propulsion air molecules and the solid surfaces sponge is 10 cm, and the characteristic time of a bulk graphene sponge when exposed of both the graphene sponge and the for the gas molecules travelling between to laser light. They attributed this to the tube outnumber collisions between air the glass tube and the graphene sponge is momentum of the light-induced ejected molecules. According to the kinetic theory about 0.5 ms). The time required for the electrons. However, the force provided by of gases3, the radiometric force is: establishment of the radiometric force is the ejected electrons is about 2.7 × 10–11 N dozens of this characteristic time, which is P σ T + (1 − σ )T (supposing that the average current and the F = A M h M ∞ − about 10–20 ms. kinetic energy of the ejected electrons are 2 T∞ If the radiometric force is the cause of 9 × 10–7 A and 70 eV, respectively), which the motion of the graphene sponge, then it is far smaller than the gravitational force σMTc + (1 − σM)T∞ is impossible for the solar sail proposed by of a 0.86 mg graphene sponge. From our Zhang et al. to be driven directly by sunlight T∞ knowledge of rarefied gas dynamics, the with propulsion orders of magnitude greater horizontal, vertical and rotational motion where P is the air pressure before laser than the radiation pressure, since the of the laser-illuminated graphene sponge illumination, A is the cross-sectional area radiometric force vanishes in the vacuum of could be due to the radiometric force. of the graphene sponge, Th and Tc are the deep space. The effect of the radiometric temperatures at the bottom and the top of Finally, it is interesting to note force is most commonly seen in the the illuminated graphene sponge, T∞ is the that Sir William Crookes invented his Crookes radiometer — an airtight glass temperature of the glass tube, and σM is the radiometer in 1873 as a spin-off from bulb in which there is a partial vacuum. tangential momentum accommodation his chemistry research. He was weighing Inside the bulb is a set of thin vanes coefficient of the surface. Assuming σM = 1, samples in a partially evacuated chamber to mounted on a spindle. Each vane has a and Tc = T∞ = 300 K, the radiometric reduce the effect of air currents, but noticed blackened side and a silvered side. The force is larger than the gravitational force his measurements were disturbed when vanes rotate when exposed to light, with of a 0.86 mg graphene sponge when sunlight shone on the balance. Zhang et al. the silvered surfaces leading the motion. Th = 3,000 K. also placed their graphene sponge into The reason for this rotation was much Such a high temperature difference a (near) vacuum to avoid the probable debated following the invention of the (2,700 K) between the bottom and the top interference of air — so history may have device, but in 1879 the currently accepted of the graphene sponge may be possible, repeated itself. ❐ explanation for the rotation was proposed as Zhang et al. estimated that with a laser by Osborne Reynolds2, who attributed it pulse width of 2 ms and a laser power of References to thermal transpiration. In brief, when 3 W the average temperature increase of 1. Zhang, T. et al. Nature Photon. 9, 471–476 (2015). 2. Reynolds, O. Phil. Trans. R. Soc. Lond. a gas molecule collides with a surface, it the graphene sponge is 822 K if all the laser 170, 727–845 (1879). is diffusely reflected; for a surface with a energy is converted to thermal energy. Since 3. Gimelshein, S. F., Gimelshein, N. E., Ketsdever, A. D. temperature gradient, a molecule from the the absorbance of a single layer of graphene & Selden, N. P. AIP Conf. Proc. 1333, 693–700 (2011). hotter side loses more momentum than a is 2.3%, the temperature at the bottom of molecule from the colder side. This is the the graphene sponge will be over 7,000 K if Lei Wu1*, Yonghao Zhang 1, Yian Lei 2 and source of a ‘radiometric’ force that is applied radiation and heat conduction are ignored. Jason M. Reese3 on the surface in the direction from the Even if we consider heat conduction for a 1James Weir Fluids Laboratory, Department hotter side to the colder side. graphene sponge of length 11 mm, density of Mechanical and Aerospace Engineering, The graphene sponge propulsion 1 mg ml–1, and a conductivity of 0.5 S m–1, University of Strathclyde, Glasgow G1 1XJ, experiment was conducted in a glass tube a uniform temperature will only be reached UK. 2School of Physics, Peking University, that contained air at a pressure of about after a characteristic time of 17 ms. This Beijing 100871, China. 3School of Engineering, 0.1 Pa. The mean-free path of the air time interval is comparable to the time for University of Edinburgh, Edinburgh molecules in this tube is 65 mm, which the establishment of the radiometric force EH9 3FB, UK. is much larger than the diameter of the (supposing that the distance between the *e-mail: [email protected] Reply to ‘Do thermal effects cause the propulsion of bulk graphene material?’ Zhang et al. reply — In our work1, we graphene, which is the origin of many of its individual graphene sheets are retained experimentally synthesized a bulk 3D remarkable electronic and optoelectronic in the bulk state is probably the most crosslinked graphene material that exhibits properties (partially demonstrated in this important implication of our work because the intrinsic properties of individual work1), as well as its mechanical properties “it has been a great challenge to realize graphene sheets. This includes the unique (shown in our earlier paper2). In our view, the intrinsic properties of single-layer Dirac-type electronic band structure of the fact that the intrinsic properties of graphene in the bulk state, because stacking NATURE PHOTONICS | VOL 10 | MARCH 2016 | www.nature.com/naturephotonics 139 © 2016 Macmillan Publishers Limited. All rights reserved.
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