Application of Aligned Carbon Nanotubes in Micro Shear Stress Sensing

In recent years, shear stress sensors based on MEMS technology, capable of measuring shear stress in the micro-regime with excellent spatial and temporal resolution without disturbing natural flow have been developed. A fluid flowing over a solid surface results in a normal and tangential force on the surface. The tangential stress is called the surface or wall shear stress. Measurement of shear stress is significant in applications such as drag reduction on moving surfaces [1], flow rate measurements in micro- and nano-fluidic systems, as well as determining the relationship between blood circulation and vascular cell behavior in biological systems [2]. Techniques for wall shear stress measurement can be classified as direct and indirect [3]. MEMS shear stress sensors based on the direct and indirect sensing principle have been developed by [4-6]. Currently, one of the most successful micro-flow sensors is the micro thermal shear stress sensor. This particular sensor determines local surface shear stress by relating the rate of convective heat transfer from an ohmic heated, highly-doped polysilicon sensing element to the surrounding flow. The usage of highly-doped polysilicon as a flow sensing material has several drawbacks including; i) polysilicon requires high processing temperature and ii) it is difficult to shrink the polysilicon sensing element down to the nanometer scale required for future nanofluidic applications.

This work was performed to explore the possibility of fabricating a micro-scale thermal shear stress sensor using a sensing material capable of being shrunk down to the nanometer scale. For this, a recently discovered form of carbon, carbon nanotube (CNT) , was used as a replacement to polysilicon as a sensing element. The primary advantage of CNT compared to polysilicon lies in the size effect. CNTs are ~ 3 orders of magnitude smaller in the axial and radial directions than the smallest existing polysilicon sensor. Additionally, CNTs exhibit a large aspect ratio (several nanometers in diameter to several micrometers in length), which is highly desirable for directional sensitivity in shear stress sensing applications. Aligned CNTs possess excellent thermal material characteristics for use in sensing applications, while also exhibiting the potential to form features with sizes of a few nanometers without the need for photolithography. Here, the results of a successfully fabricated aligned MWCNT based surface shear stress sensor tested in a two-dimensional wind tunnel are presented.

Electrical resistivity of a perfect SWCNT has been experimentally found to be around 0.34 x 10^-4 Ohm-cm , making them the highest known conducting carbon fiber. CNTs have also been found to have exceptional thermal conductivity in the axial direction ~1800 – 6000 W/m K for a rope of CNTs at room temperature compared to the thermal conductivity of diamond, which is in the range of 900 – 1600 W/m K. Experiments have also revealed that CNTs exhibit thermal coefficient of resistitvity (TCR) behavior, a fundamental requirement for thermal sensing MWCNTs were used for this study. However, the natural state in which CNTs exist makes their use very difficult. Typically both forms of CNTs exist as agglomerated bundles in the form of CNT ropes entangled with each other. Each CNT bundle or rope typically consists of a few tens to a few hundreds of CNTs. For use as a sensing material/element the bundles of CNTs have to be disentangled and the longitudinal axis of the CNT has to be oriented between electrodes to complete the fabrication of the sensor.

Fabrication of the Sensor

Fabrication of the sensor began with the mixing of 1mg of multi-walled carbon nanotubes fabricated by a CVD method with 4mL of DI water-Nanosperese® solution as the suspending medium. The mixture was subsequently sonicated for 8 to 10 minutes at 44KHz to disperse the CNTs. Nanosperese® is a Poly (Oxy-1,2-Ethandiyl, Alpha-(Nonylphenyl)-Omega-Hydroxy based surfactant, which was added to the mixture to speed up the dispersion process. After the CNTs become well dispersed, a single drop of the CNT mixture was placed in a 360-µm gap between two micro fabricated Au electrodes on a glass substrate. A 1 mm thick glass wafer was used as the substrate for the electrode chip. A 2000°A layer of Au was sputtered on top of a 200°A Cr adhesion layer to glass. The Au and the Cr layers were patterned and wet etched to yield the electrodes. Using dielectrophoresis, the CNTs in the mixture were aligned by an AC electric field applied through the electrodes. For shear stress sensing, it is necessary to achieve a long and slender line of aligned CNTs. When an electric field was applied, the CNTs typically aligned themselves along the direction of the electric field, forming a thin conduction path at the end of the alignment process.