Converting light energy into work via surface tension

As global energy demands increase, investigation of alternative energy sources is becoming more imperative. Current research in this area often intrinsically relies on multistage processes to ultimately produce useful work. For example, biofuel requires growing and then harvesting crops, converting them into fuel, transporting it, and finally turning it into work. Each step introduces inefficiencies, costs, and energy expenditures. Developing techniques that convert solar energy more directly into useful work could offer advantages.

Efforts in our laboratories have recently uncovered a mechanism for doing just that by manipulating surface tension.¹ This phenomenon typically pulls on floating objects equally in all directions, as shown in Figure 1(a), but is strongly dependent on the temperature. Placing an object in a graded change, or thermal gradient (and thus a surface tension gradient), can result in motion due to an asymmetry in forces.

We demonstrated this mechanism using nanostructured composites of carbon nanotubes (CNTs) and a clear plastic, polydimethylsiloxane or PDMS (see Figure 2). We chose CNTs because they absorb light very efficiently across the entire solar spectrum and convert it into heat, which can subsequently be transferred to the surrounding water. By focusing sunlight onto one side of the composite, as shown in Figure 1(b), we can control the thermal surface tension gradients, thus dictating the motion of the object. The resulting motion (as shown in Figure 3 and in a short video available online²) is an example of the direct conversion of solar energy into useful work. Even without optimization, we have achieved velocities of 8cm/s and controlled motion of objects traveling from submillimeters to greater than 10cm.

Figure 1. (a, b) Side view of a floating composite with CNTs thoroughly mixed into the plastic. Surface tension forces are shown with yellow arrows. When irradiated with light on the left face, the surface tension force decreases, resulting in a net propulsive force toward the right. Specifically hitting portions of a face of an object can also result in controlled motion (see Figure 3).

Figure 2. (a, b) Scanning electron micrographs of a vertically aligned CNT forest (VANT) embedded in plastic (PDMS). CNTs extend from the surface 100μm. (c) An optical image of such a composite. Alternatively, CNTs can be mixed throughout the plastic, producing a (d) solid black composite.

Figure 3. Optical image showing the laser-controlled motion of a small composite floating on water. The final position is superimposed on the initial one. The blue line traces the location of the boat over time.

Controlled rotational motion can also be achieved with a rotor that has CNTs built on one face of each fin. In this case, surface tension gradients are localized by the absorbing material, producing a concerted rotational force on the object. Rotors constructed in this fashion reached speeds up to 90rpm (see Figure 4 and a video available online³). Such devices may be extrapolated into solar-powered pumps or a means of controlling microfluidic devices.

Figure 4. Optical images of VANT/PDMS composites. Right, composite floating on water and blanket-irradiated with sunlight.

This surface tension-driven phenomenon is scale independent and can be extended into the micro and macro regimes. Importantly, it also circumvents the typical limitations associated with propulsion on small scales, such as turbulence. In general, the simplicity of this system may allow for widespread application. It is also noteworthy that though highly absorbing CNTs are optimal, a variety of less absorptive materials may be used. We are currently researching methods of controlling motion on the microscale and extracting more power out of the system.