KEY CONCEPTS
• An electric voltage was initially found to be generated when deionized water flowed over a superhydrophobic surface.
• Liquids with high dielectric constants replaced air in grooves in the superhydrophobic surface to increase the voltage produced. 
• The voltage was increased into the 50-100 millivolt range when a sodium chloride solution was passed over a superhydrophobic surface containing grooves filled with a fluorinated lubricant.

One resource readily available to researchers is salt water. Utilization of salt water for commercial purposes has proven difficult. An example is the large amount of energy needed to desalinate salt water to produce fresh water, which is in short supply.

The electrolytes present in salt water afford an opportunity to potentially devise a way to prepare an alternative method for power generation. In a previous TLT article, researchers developed a technique for generating electricity by building a concentration flow cell with two compartments (1). One compartment had water with a high salinity flow content flowing past one electrode while, in the second compartment, water with a low-salinity content flows past the second electrode. Power was produced from the salinity difference between the two compartments. 

In working with deionized water flowing over a surface, Prab Bandaru, professor of mechanical engineering and aerospace engineering at the University of California San Diego in San Diego, Calif., says, “We were evaluating how to minimize the amount of energy needed to move a fluid over a surface. While at rest, the amount of energy needed to move the fluid is significant because of the need to overcome friction. No slip boundary conditions exist as the fluid just sticks on the surface.”

Bandaru and his colleagues decided to reduce the friction present by modifying the surface with carbon nanotubes. 

The researchers then moved to working with a superhydrophobic surface that contained air-filled grooves. Bandaru says, “We prepared a surface with a high porosity that enabled the total surface area to consist of 75% air and only 25% solid. Tiny grooves were etched into the surface. Under these conditions, friction was minimized and the flow of deionized water continued to increase.”

During this work, Bandaru and his colleagues then decided to measure the electrical voltage generated as deionized water flowed over the superhydrophobic surface. He says, “We found that an electric voltage was generated, and the voltage was remarkably reproducible with only a 5% fluctuation in readings.”

The researchers determined that the positive ions flowing in the water past residual negative charges in the surface created the electric voltage. At this point, the researchers then decided to work with salt water, reasoning that the presence of electrolytes should lead to higher voltages. 

Lubricant-filled grooves
The researchers prepared the reaction setup shown in Figure 2 where salt water was moved through a microfluidic channel that was 11.8 centimeters in length, 0.9 centimeters in width and 250 microns in height using a syringe pump. The surface substrate consisted of silicon with a 600-nanometer thick coating of poly ortho-chloro-para-xylylene that was applied through vapor deposition. 


Figure 2. The experimental setup used to detect voltage generated when an aqueous solution was passed over a superhydrophic surface is shown. (Figure courtesy of University of California San Diego.)

Silver/silver chloride electrodes were placed on opposite ends of the channel and connected to a voltmeter used to measure the voltage. Bandaru says, “We initially started by using a 0.001M sodium chloride solution and detected a further increase in the voltage compared to using the same system with deionized water. Reducing the concentration of salt water further increased the voltage.”

The researchers then experimented with replacing the air with a charged liquid fluid that would stay in the superhydrophobic surface grooves. Bandaru says, “Our hope was that using a liquid with a high dielectric constant will further increase the voltage.”

Initially, the researchers selected a passenger car engine oil obtained from a local auto parts store and also evaluated castor oil. Both liquids were effective in raising the voltage. Bandaru says, “In selecting a fluid, we then decided to take a more scientific approach and determined that fluorinated lubricant oils should be able to produce higher voltages because they exhibit high dielectric constants.”

The researchers chose a commercially available fluorinated lubricant and achieved the highest voltages of any system studied. The figure of merit, a numerical expression used to quantify the efficiency of a device, was higher by a factor of 1.4 for the fluorinated-filled grooves compared to air-filled grooves in the superhydrophobic surface. 

Use of the fluorinated lubricant enabled the researchers to generate electricity in the 50-100-millivolt range. Bandaru says, “Our objective is to boost the efficiency of the system so that we can produce power in the one-volt range.”

Bandaru acknowledged that this objective can be realized by extending the length of the experimental system from 11.8 centimeters to one meter because the voltage generated is directly proportional to length. He says, “We would rather figure out a way to modify the existing system by increasing the number of grooves.”

The researchers hope this finding will lead to new power sources that can be used for microfluidic devices and one day may be extended to being used to power desalination plants. Additional information can be found in a recent article (2) or by contacting Bandaru at pbandaru@ucsd.edu. 

REFERENCES
1. Canter, N. (2016), “New approach to generate power from salinity difference in water,” TLT, 72 (12), pp. 12-13.
2. Fan, B., Bhattacharya, A. and Bandaru, P. (2018), “Enhanced voltage generation through electrolyte flow on liquid-filled surfaces,” Nature Communications, Article number: 4050.