Carbon Nanotube-Based Fluid Flow/Shear Sensors
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0963-Q23-03
Carbon Nanotube-Based Fluid Flow/Shear Sensors Chi-Nung Ni, Christian P. Deck, Kenneth S. Vecchio, and Prabhakar R. Bandaru Materials Science and Engineering, U. of California, San Diego, La Jolla, CA, 92093
ABSTRACT The response of vertically oriented multi-walled nanotubes (MWNTs) to fluid flows forms a basis for monitoring shear forces at the nanoscale. We report on the modulation of polarized light transmission through a MWNT mat, as a function of the nanotube axis relative to the laser polarization. Nanotubes are deflected under fluid flow, with fluid pressures corresponding to pico-Newton range forces. This deflection is measured as a function of transmitted laser light intensity. While the response of the CNTs to the flows could be modeled through standard elastic beam deflection theory, their recovery, after the force removal, invokes viscoelastic behavior due to CNT disentanglement. The fluid flow direction, as well as its pressure, can also be determined by monitoring the change in transmitted laser polarization. INTRODUCTION Carbon nanotubes (CNTs) have been shown to have a remarkable combination of mechanical properties, which include exceptionally high elastic moduli (in TPa range).[1] Along with the high stiffness[2] of the CNTs, the ability[3] to undergo reversible bending and buckling, withstanding strains[4] of up to 30% has been reported. It has been observed[5] that these deformations can occur with minimal defect formation. In this work, we propose that the strength and flexibility of MWNTs can be used for shear force sensing, as for example in fluid flows. The optical properties of these tubes were also studied and showed that the mats of CNTs polarize light. By monitoring the polarization and intensity of the transmitted light through a CNT ensemble subject to lateral forces, we introduce the feasibility of CNT based force sensors for a wide range of engineering applications. EXPERIMENTAL DETAILS Vertically aligned mats of MWNTs were grown on quartz substrates, through thermal chemical vapor deposition (CVD) at 900°C, using a benzene-ammonia-hydrogen gas mixture, along with Ar as the carrier gas. A 6 nm thick nickel layer on the transparent quartz substrate serves as a
supporting catalyst layer for CNT growth. The grown mats are seen to consist of well-aligned MWNTs, with average lengths in the range of 30 µm and diameters around 50 nm. Other details of the synthesis are reported elsewhere.[6] For optical polarization measurements, a polarized He-Ne laser (with wavelength of 632.8 nm; power of 10 mW) was used as laser source. Nanotube substrates were positioned in the beam using a gimbal optical mount with two degrees of freedom, allowing y- and z-axis rotation. A light polarizer, followed by a silicon photo-detector, was set behind the substrate to monitor the intensity of laser transmission as a function of polarizer angle; a schematic of the experiment is shown in Figure 1. The sensitivity of MWNTs to shear forces was probed, using an apparatus similar to that used for polari
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