Flow-induced voltage generation over monolayer graphene in the presence of herringbone grooves
© Lee et al.; licensee Springer. 2013
Received: 16 October 2013
Accepted: 2 November 2013
Published: 20 November 2013
While flow-induced voltage over a graphene layer has been reported, its origin remains unclear. In our previous study, we suggested different mechanisms for different experimental configurations: phonon dragging effect for the parallel alignment and an enhanced out-of-plane phonon mode for the perpendicular alignment (Appl. Phys. Lett. 102:063116, 2011). In order to further examine the origin of flow-induced voltage, we introduced a transverse flow component by integrating staggered herringbone grooves in the microchannel. We found that the flow-induced voltage decreased significantly in the presence of herringbone grooves in both parallel and perpendicular alignments. These results support our previous interpretation.
Over the past decade, theoretical and experimental studies have demonstrated that a voltage is generated when carbon nanotubes (CNT) and graphene surfaces are exposed to fluid flows [1–8]. Kral and Shapiro first proposed theoretical mechanisms for flow-induced current generation within metallic single-walled carbon nanotubes (m-SWCNTs) . This flow-induced voltage was then experimentally demonstrated for the first time by Sood et al., who used a SWCNT film deposited between electrodes immersed in a flowing liquid . Similar experiments were conducted with multiwalled carbon nanotubes (MWCNTs) . The aligned MWCNTs were found to generate voltages 15 times higher than SWCNTs. We also reported that semiconducting single-walled carbon nanotubes (s-SWCNTs) can produce voltages three times higher than m-SWCNTs in flowing liquids . Similar phenomena were observed on graphene surfaces on exposure to fluid flows. Dhiman et al. reported that a graphene surface could generate a peak voltage of approximately 25 mV in fluid flows . They proposed surface ion hopping as the major mechanism for the flow-induced voltage generation.
However, the precise mechanism of flow-induced voltage generation over graphene and CNT surfaces remains unclear. To understand the origin of the flow-induced voltage, we previously conducted experiments with two different electrode-flow configurations: electrodes aligned parallel and perpendicular to the fluid flow. These experimental results suggested that the main mechanism for parallel alignment was the ‘phonon dragging model’ , while that for perpendicular alignment was the ‘enhanced out-of-plane phonon mode’ .
A monolayer of graphene was grown separately on Cu foil in a chemical vapor deposition chamber, as reported previously [12, 13]. It was verified that the graphene was a monolayer using Raman spectroscopy (the ratio of G and 2D peaks was 2 as shown in Figure 1c) .
The fabrication process for the device is shown in Figure 1d. To make the herringbone grooves in a silicon wafer, we used deep reactive ion etching (DRIE) [15, 16]. There were 20 grooves with a width of 350 μm and a depth of 100 μm (see the scanning electron microscopy (SEM) image in Figure 1e). Each groove has staggered lengths of 865 μm and 1,000 μm. The grooves were designed to be at an angle of 45° to the channel wall and were spaced with an interval of 840 μm (center to center) along the length of the channel. The electrodes were then fabricated on the Si wafer with grooves using a lift-off technique . A 10-nm-thick Cr layer and a 40-nm-thick Au layer were deposited sequentially on a predefined photoresist layer on the Si wafer to form the electrode patterns. After defining the electrodes, the wafer was diced into smaller substrates (15 mm × 20 mm). The graphene monolayer was then transferred onto the Si wafer and placed between the electrodes. The resistance of the graphene was about 1 kΩ.
Finally, the Si wafer with grooves, electrodes, and graphene was bonded to a polydimethylsiloxane (PDMS) layer, which had a fluidic channel of 100 μm in height, 1.5 mm in width, and 20 mm in length defined by replica molding. The PDMS layer was sealed to the Si surface by oxygen plasma treatment. Four types of samples were prepared in Figure 1f:
Type 1: the electrodes aligned parallel to the flow in the absence of grooves
Type 2: the electrodes aligned perpendicular to the flow in the absence of grooves
Type 3: the electrodes aligned parallel to the flow in the presence of grooves
Type 4: the electrodes aligned perpendicular to the flow in the presence of grooves
A syringe pump (Legato 180; KD Scientific, Holliston, MA, USA) was used to inject fluid through the PDMS microchannel. The flow-induced voltage over the graphene was measured using a digital multimeter (DM 2002; Keithley Instruments, Cleveland, OH, USA). All experiments were carried out at room temperature (25°C).
Results and discussion
Now, let us consider the effects of the herringbone grooves in both parallel and perpendicular alignments (type 3 and type 4 in Figure 3a). In the case of the parallel alignment, a significant decrease in the induced voltage was observed with the herringbone grooves. At a flow rate of 1,000 μL/min, the voltage decreased by almost tenfold, from 0.17 mV (type 1) to 0.018 mV (type 3). At a flow rate of 10,000 μL/min, the induced voltage dropped from 0.49 mV (type 1) to 0.11 mV (type 3). To understand why the presence of herringbone grooves significantly decreased the induced voltage, we performed simulation studies on flow velocity and vorticity. Figure 3b shows the flow velocity in the x-direction (longitudinal, flow direction) over the graphene surface as a function of flow rate. While the volumetric flow rate was kept constant for both type 1 and type 3, the flow velocity in the x-direction decreased when herringbone grooves were added. At a flow rate of 1,000 μL/min, the flow velocity in the x-direction decreased from 169.36 to 122.27 mm/s. This was due to the presence of transverse flow generated by the grooves in the microfluidic channel. The decrease in flow velocity (x-direction) resulted in a reduced electron dragging effect, and as a result, the flow-induced voltage decreased. Moreover, vorticity increased in the presence of groove as shown in Figure 3c. At a flow rate of 1,000 μL/min, the vorticity in the channel with herringbone grooves was 38% higher than that in the channel without grooves. Vorticity, the curl of the velocity vector, indicates local spinning or rotational motion of a fluid. It seems that the increased vorticity of fluid disturbed the directional electron dragging, resulting in a further decrease in voltage generation. Therefore, the significant decrease in the induced voltage in the presence of herringbone grooves is due to the combined effects of reduced flow velocity and increased vorticity.
In the case of perpendicular alignment, a significant decrease in the induced voltage was observed as well when herringbone grooves were included. At a flow rate of 1,000 μL/min, the voltage decreased by fourfold, from 0.057 mV (type 2) to 0.013 mV (type 4). At a flow rate of 10,000 μL/min, the induced voltage dropped from 0.15 mV (type 2) to 0.03 mV (type 4). At a glance, this result may be surprising because one may think that the increased transverse flow along the y-direction would induce a stronger phonon dragging effect. However, simulation data revealed that the flow velocity in the y-direction along the electron path between the electrodes was appreciably small compared to that in the x-direction. As a result, any added electron dragging effect due to the increase in transverse flow was buried in the effect of the overall flow momentum decrease due to the decrease in x-directional flow velocity in Figure 3b. Moreover, the increased vorticity seems to interfere with the out-of-plane phonon mode, minimizing the momentum transfer from the fluid flow in Figure 3c. In summary, the significant decrease in the induced voltage in the presence of herringbone grooves is because of the overall flow momentum decrease due to the decrease in x-directional flow and increased vorticity.
In conclusion, we investigated flow-induced voltage generation over a graphene monolayer in the presence of staggered herringbone grooves to better understand the origin of the voltage generated. The flow-induced voltage decreased significantly in the presence of herringbone grooves in both parallel and perpendicular alignments. The numerical simulation study revealed that the presence of herringbone grooves decreased longitudinal flow velocity while increasing transverse flow and vorticity. As a result, the directional charge dragging effect was significantly reduced in the parallel alignment, resulting in decreased voltage generation. In the case of the perpendicular alignment, the momentum transfer from the fluid flow to the graphene (out-of-plane phonon mode) was affected by the decreased flow velocity and increased vorticity, causing the voltage generation to drop. We also found that the voltage signal with the perpendicular alignment showed a bigger oscillation than that of the parallel type and that the signal oscillation was amplified by the herringbone groove. These data support that the mechanism for flow-induced voltage in perpendicular alignment is different from the parallel alignment case and that it is related to momentum transfer from the fluid flow. Taken together, the experimental data presented here support our previous proposal regarding the distinct flow-induced voltage generation mechanisms for parallel and perpendicular alignments.
This work was supported by the National Research Foundation of Korea (NRF) via grant no. 2010–0017795.
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