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Direct Conversion of Light into Work

David Okawa, Stefan Pastine, Alex Zettl, Jean M. J. Fréchet

College of Chemistry and Department of Physics, University of California Berkeley, Berkeley, California 94720 and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Current technology and research on solar energy conversion often intrinsically relies on the production of energy storage and distribution systems to facilitate the production of useful work.  Research in our laboratories has uncovered a mechanism for converting solar energy into work in a more direct fashion.  Nanostructured composites, consisting of vertically aligned forests of carbon nanotubes embedded in a plastic, polydimethylsiloxane (PDMS), efficiently absorb light and convert it into heat.  When floating on a liquid, this heat locally decreases the surface tension of the liquid.  When an object is asymmetrically heated, a surface tension gradient is created and the forces on the object are unbalanced, resulting in a net propulsive force on the object.  By spatially defining irradiation, focused light can be used to controllably move an absorbing object on a variety of liquids. This method of propulsion circumvents the typical limitations associated with propulsion on small scales such as turbulence.  Rotational motion can also be obtained by carefully designing the locations of the light absorbing materials as shown below.

The simplicity of this surface tension based mechanism for producing motion may allow for widespread application.  It is important to note that though highly absorbing carbon nanotube forests produce excellent responses, a variety of less absorptive materials may also be used. 



The following media files are intended for public access and may be reproduced with credit:  “Courtesy Frechet and Zettl Research Groups, Lawrence Berkeley National Laboratory and University of California at Berkeley.”  All rights reserved ©2009.

Quicktime video (Sun_Boat.mov) showing the directionally controlled motion of a multiwall carbon nanotube – PDMS composite using a simple glass lens. 

Quicktime video (Sun_rotor_compiliation.mov) showing the sunlight controlled rotational motion of a small nanotube-plastic composite. 

Link to a Youtube rotor video:  http://www.youtube.com/watch?v=gaHdnmEpxsg


Boat on water blender.avi  showing an aminated version of the principle behind the surface tension mediated conversion of light into controlled motion.  Red dot is the focal point of the illuminating beam.

Controlled motion.mov showing the laser controlled motion of a small carbon nanotube-plastic composite on the surface of water. 


The following media files are intended for public access and may be reproduced with credit:  “Courtesy Frechet and Zettl Research Groups, Lawrence Berkeley National Laboratory and University of California at Berkeley.”  All rights reserved ©2009.

Expos_MeOH_H2O.tif (nanotube_pdms.tif)

Figure 1. Vertically aligned Carbon nanotube Forest grown by chemical vapor deposition.

Nanotube_PDMS_Composite_SEM.tif (Nanotube_PDMS_Composite_SEM.tif)

Figure 2. A vertically aligned carbon nanotube forest embedded in PDMS.  The composite is substantially more robust then as grown nanotube forests, allowing for more widespread use. 



Figure 3.  Schematic depicting surface tension forces upon optothermal heating.  (a, b) show side view images of a homogenously mixed PDMS composite.  Arrows depict surface tension forces on the object.  As the left face is heated the surface tension force locally decreases and the object feels a net force and responds by moving to the right.  This phenomenon can be further controlled by spatially selecting the heating location (c, d).   

Rotor_Sun_Square.JPG Rotor_Sun_Square.jpg

Figure 4.  A small VANT-PDMS rotor floating on water and strongly illuminated by sunlight.  The shadow of the rotor is evident on the lower left.  Controlled motion can also be obtained with blanket irradiation if directionality is incorporated into the device design. As shown in Figure 5, incorporating the absorbing material onto only one face of each fin of a rotor, allows for rotational motion when irradiated.  Such solar powered rotors could be useful for solar powered water pumps or optically controlled microfluidics.  Speeds greater then 70rpm have been obtained.   

Boat_hand.JPG Boat_Glove.JPG

Rotor_Hand.JPG Rotor_glove.JPG

Figure 5.  Above is a VANT-PDMS composite boat.  Below is a VANT-PDMS composite rotor.


Figure 6. Nanotube-PDMS composite with picturesque background. 

CNT-PDMS Composite.jpg

Figure 7. Carbon nanotube – PDMS composite boat.


Acknowledgment. The authors acknowledge financial support from the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The project described (S.J.P.) was also supported by Award Number F32GM078780 from the National Institute of General Medical Sciences. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of General Medical Sciences or the National Institutes of Health. D.O. and A.Z. received financial support from the Sea Change Foundation. We thank Mark Llorente for the production of VANTs, Brian Kessler for helpful discussions, and the Miller Institute (Professorship for AZ).