1. Surface Tension Mediated Conversion of Light to Work
(J. Am. Chem. Soc. 2009, 131, 5396–5398)
Surface Tension Mediated Conversion of Light to Work
David Okawa,†,‡ Stefan J. Pastine,† Alex Zettl,‡,§ and Jean M. J. Fréchet*,†,§
Today, I’d like to introduce a paper concerning light control of object transportation.
Title is “Surface tension mediated conversion of light to work”, published at Journal of the American Chemical Society in this year reported by these members.
College of Chemistry and Department of Physics, University of California Berkley, Berkley, California 94720, and Materials Sciences Division, Lawrence Berkley National Laboratory, Berkley, California 94720

2. Introduction Light-driven Molecular Motor
(Linear Motion)
(Rotational Motion)
+ E44 +
50 mm
Cholesteric liquid-crystal
Recently, several attempts to control linear or rotational motion of micro- or nano-scale objects on surfaces by light have been reported.
Because Kamei sensei already explained detail about the molecular motors last meeting, I skip the detail explanation and I’ll just show you only two typical works of light driven linear and rotational motion.
Very recently, Kurihara sensei reported the light-controlled linear motion of a micro-scale polystryrene beads by a photochromic azobenzene doped in a liquid crystalline host.
Feringa and co-workers reported the light-driven rotational motion of a micro-scale object by a photoresponsive molecule embedded in a liquid crystalline matrix.
These researches are very interesting and challenging, but several controversial points still remain. For example, low speed of the motion, size dependence, low durability, complicated system, and so on.
Actually, this authors have negative opinions for these works. They claim that the light energy is not effectively used in these systems and the efficiency of light conversion is low for all cases.
In this paper, they proposed a simple and robust solid/liquid interfacial system can directly and efficiently convert light energy into useful work.
(S. Kurihara et al., Angew. Chem. Int. Ed. 2009, 48, 2144.)
(B. L. Feringa et al., Nature. 2006, 440, 163.)
Average Speed; ~ 100 mm/min (50 ~ 100 mW/cm2)
Average Speed; 0.2 ~ 0.7 rpm (Light Power; unknown)
Controversial Points: Low-speed, Size-dependence,
Low-durability, Complicated-system,・・・
Author’s Purpose
Development of a simple and robust solid/liquid interfacial system
Light energy (fuel) directly converts to useful work

3. Object Transfer by Using Surface Tension
Camphor boat （樟脳船）
move to this direction
camphor
As the driving force of object transportation, authors focused on the surface tension. Surface tension is an attractive property of the surface of a liquid, as you know.
One example of object transportation by using surface tension is champhor boat. Champhor boat, Japanese name is しょうのうせん, is the old-time toy in Japan. Do you know this toy? Is there in India?
Anyway, Champhor is attached to back side of the boat. When we launched this boat on water, champhor melts gradually, and then the surface tension decrease around here, as a result, generate the surface tension gradient. And then, the boats moves to higher surface tension side spontaneously. This is the mechanism of the motion of this toy.
decrease of surface tension
Propulsion force : Surface tension gradient
When the surface tension gradient is generated by some reason, the object floated on liquid moves to higher surface tension side spontaneously.

4. Object Transfer by Using Surface Tension
Surfactant-driven boat
Similar behavior is observed by adding the surfactant, like this. Sorry Japanese, Before adding the surfactant, the surface tensions equal between the forward and the backward.
When adding the surfactant, the surface tension decrease around here, and generate the surface tension gradient. And then, the boat moves to higher surface tension side spontaneously.
Authors tried to apply this mechanism to their work. But, now, the fuel to produce the surface tension gradient should be light.
Propulsion force : Surface tension gradient
When the surface tension gradient is generated by some reason, the object floated on liquid moves to higher surface tension side spontaneously. 4

5. Light-based Control of Objects on Water
Temperature dependence of the surface tension
Increase of temperature
Decrease of surface tension
(i) hn
object
Supporting
polymer
water
So, to produce the surface tension gradient by light, they focused the temperature dependence of the surface tension.
Generally, when the temperature of the surface increases, the surface tension decreases.
Therefore, if the light energy converts to heat efficiently, thermally induced surface tension gradient can be produced by light.
Specifically, they thought up to make like this object. This object is composed of light absorbing material and supporting polymer.
The irradiating light is absorbed by this material.
The absorbed light energy converts to heat and the interface temperature increases, and the surface tension decrease at this part and the surface tension gradients are produced. After generating the surface tension gradients, the object moves to higher surface tension side as described above.
light absorbing
(ii) hn
Light energy converts to heat
(Local heating)
converting light energy into heat
→ generate the surface tension gradient
Generate the surface tension gradient
(iii) hn
Move to higher surface tension side
work (propulsion)

6. VANT–PDMS Composite VANT (Vertically Aligned NanoTube)
On the basis of this idea, they design the VANT-PDMS composite as the light controllable object.
Vertically aligned nanotube, call VANT, was selected as the light absorbing material. VANT is known as the extremely black material. VANT almost completely absorb the incident light and the light energy convert to heat during the relaxation process. Therefore, VANT is ideal light-activated thermal source, which can heat the surrounding liquid and maximize surface tension gradient.
In order to increase their structural quality, VANT was embedded in supporting polymer. Authors was selected polydimethylsiloxane, call PDMS, as the supporting polymer by these reasons. PDMS is optically transparent for visible and near-infrared light and its density is similar to water. And, PDMS strongly adheres to the VANT.
This composite is stable to water and other liquid, has superhydrophobicity and can be created with various shapes and orientations.
Authors tried to demonstrate the light-induced transportation of this composite. They launched the composite on water and then irradiated with the near-infrared laser light or focused sunlight. When irradiated with the light, this composite heats dramatically. This dramatic heating produces the surface tension gradients.
VANT (Vertically Aligned NanoTube)
　・Extremely black material
(absorbing > 99.9 % of incident light)
PDMS (Polydimethylsiloxane)
　・ Optically transparent
　・ Density is similar to water
　・ Strongly adhesion to VANT
Z. P. Jang et al., Nano Lett. 2008, 8,
VANT-PDMS composite
　　・High stability for water and solvent
　　・Superhydrophobicity (contact angle: > 155º)
　　・Good formability (various shapes and orientations)

7. Linear Propulsion of VANT-PDMS Object
(see Movie) hn
First, I’ll show you the movie of the linear motion of VANT-PDMS composite upon light irradiation.
As you can see here, the object moves from left to right side under irradiation with the light.
The speed depends on the object scale, light power and the length of the heated interface. But, the motion itself (move or not) did not depend on the object scale. The linear motion was observed for the size from mm (mg) to cm (~ 10g) under laser or sunlight irradiation. The speed of the motion is estimated from the image and the value raised to 8 cm/sec for mm scale object. This value is very very high compared to Kurihara sensei’s system.
Light source; Near Infrared light (450 mW, 785 nm Diode laser)
Scale independent; from millimeters (milligrams) to tens of centimeters (tens of grams)
Speed raised to 8 cm/sec for millimeter object
Propulsion force depends on the size of the object, the contact length of the heated (absorber) interface, and the power of irradiation light

8. Control Experiments (Absorber Effect) (Surfactant Effect) VANT-PDMS
Some control experiments were demonstrated to confirm that the motion was produced by light and the driving force was attributed to the surface tension gradients.
First, authors checked the effect of absorbing material. This figure shows the motion curve of objects under irradiation with constant collimated laser light. In the case of VANT-PDMS composite, clear response (motion) was observed. On the other hand, in the case of PDMS object lacking absorbing material, no response was observed like this. Second, authors checked the surfactant effect. Generally, surface tension was suppressed by adding the surfactant such as sodium dodecyl sulfate (SDS). As shown here, the motion was quenched on water contains SDS. As the supplemental experiment, authors also checked the effect of light irradiation and the result is also shown here. The light irradiation was started at this time. As you can see here, the motion started after light irradiation. From these results, author concluded that the observed motion was produced by light and the optically induced surface tension gradients are the cause of the motion.
irradiation
Pristine PDMS
* SDS: Sodium Dodecyl Sulfate

9. Light Control of Linear Motion
(Linear propulsion)
(Right-turning propulsion)
Irradiation to
the center part
Irradiation to
the left part
By spatially controlling the light irradiation, the controlled linear motion can be achieved. Controlled linear motion was obtained by focusing light on different parts of the object. When irradiated the light to the center part of object, the object moves straightly. On the other hand, when irradiated the light to the left part, the object turns to the right direction. Similarly, left-turning propulsion can be achieved by irradiating the light to right part of object. Here, shows the image of the light-controlled linear motion of a VANT-PDMS composite floating on water. The white line shows the path of the composite. The composite is directed to the right, turned around in a circle, and sent back to the left. I’ll show you two movie concerning the light controlled linear motion. This first movie exhibits the motion of object when used the laser light as the light source. Next is sunlight version. The motion of the object was observed for both light source conditions. From these demonstrations, authors insisted to achieve the controlled linear motion of objects.
The scale independence and versatility of light sources presents an advantage over other light-controlled transportation systems as mentioned above.
(see Movie)

10. Light Control of Rotational Motion
VANT
(Object)
VANT
Sunlight
or nIR
VANT
VANT
By devising the object shape, rotational motion can be also achieved. For example, by placing light absorbing VANTs on the one face of each fin of a rotor, that is, here, here, here and here, localized heating by light produces surface tension forces. The object rotates in the counterclockwise direction, as shown here. I’ll show you the Movie of this rotational motion. Sunlight was used as the light source. As shown here, the object continuously rotates upon light irradiation. The estimated rotational speed is 70 rpm, which is very fast compared to the Feringa’s system. This rotor can also mounted an axle or a metal wire, which may increase the utility of the rotors with potential application for simple solar powered pomp.
Near Infrared Laser (450 mW, 785 nm)
(see Movie)
Rotation rate; 70 rpm

11. Conclusions
Authors presented a simple and versatile method for the direct conversion of light energy into useful linear and rotational work.
Authors demonstrated the remotely control the linear and rotational motion of small objects on water.
The motion speeds are very fast compared to other systems and the motion can be produced by even sunlight.
Conclusions of this work are summarized here.
書いてあることを読む？
That’s all for my presentation. Thank you for your attention.
Now, authors are trying to extend this system into micro- and macro-scale. 11