1. Photomanipulation of a Droplet by the chromocapillary Effect
Angew. Chem. Int. Ed. 2009,
Antoine Diguet,1 Reine-Marie Guillermic,2 Nobuyuki Magome,3 Arnaud Saint-Jalmes,2 Yong Chen,1 Kenichi Yoshikawa,4 and Damien Baigl1,*
1.Department of Chemistry, Ecole Normale Superieure, 24 rue Lhomond, Paris (France)
2.Institut de Physique de Rennes, Université Rennes 1, 263 avenue du Général Leclerc, Rennes (France)
3.Institute for Integrated Cell-Materials Science, Kyoto University, Kyoto (Japan)
4.Department of Physics, Graduate School of Science, Kyoto University, Kyoto (Japan)

2. Introduction
Normally, when a liquid drop is placed on a solid surface, it will stay where it is introduced and will assume a certain shape dictated by the contact angle and the relative importance of the gravitational force and the surface tension force on the fluid
contact angle is the angle between a tangent to the liquid surface at the contact line and the solid surface, measured in the liquid phase
In many application it is useful to be able to move a drop that is located on a solid surface or liquid surface

3. Fluid motion induced by an interfacial tension gradient can be obtained by thermal, electrical, electrochemical, chemical, or physicochemical methods.
various research group studied the spontaneous motion of a drop on a solid surface or the motion of a drop on a horizontal solid surface by applying a temperature gradient on the surface known as thermocapillary effect
One of the challenging alternative to achieve droplet motion is by light using solid surfaces with specific photosensitive surface properties (studied by K.Ichimura and co workers)

4. This paper discuss a new phenomenon known as Chromocapillary effect
which is the phenomenon in which light creates a wavelength-dependent interfacial tension gradient at a liquid/liquid interface, thus inducing an interfacial flow that is able to generate droplet motion in the opposite direction to the gradient .
This can be obtained by illuminating an oil droplet that floats on a water bath containing a surfactant whose polarity depends on the illumination wavelength.
This can be used for the controlled manipulation of liquid droplets using light, without any surface chemical modification, and is applicable to various types of liquids.

5. Experimental
To obtain a light-sensitive liquid/liquid interface, a cationic photosensitive azobenzene trimethyl- ammonium bromide surfactant (AzoTAB) was dissolved in the aqueous phase. The apolar tail of AzoTAB contains a photosensitive azobenzene group, which photoisomerizes into trans (less polar) and cis (more polar) configurations upon visible and UV illumination, respectively

6. Principle-chromocapillary effect
The photoinduced geometry change of azobenzene between trans and cis isomers results in a change in the molecular shape and dipole moment, which in turn gives rise to reversible switching of surface wettability through intermolecular interactions.
Thus it is possible to create an oil/water interfacial tension gradient by partial illumination of the droplet with 365 nm light or a combination of 365 nm and 475 nm light. This gradient results in a flow at the droplet interface from low r to high r values with respect to the droplet center, and droplet motion in the opposite direction with respect to the liquid bulk.

7. When partially illuminate the droplet at 365 nm it spontaneously moved away from the illuminated area, that is, from high r (UV illuminated area, mainly cis-AzoTAB) to low r values (nonilluminated area, mainly trans-AzoTAB)
By following the droplet to maintain the partial illumination, the droplet motion was maintained along a distance of a few centimeters at a speed of about 300 micrometer per second

8. After UV illumination, when the same droplet was illuminated in the same manner at 475 nm, motion was in the opposite direction, that is, the droplet moved toward the illuminated area.

9. Since the oil droplet spontaneously moved toward areas illuminated with visible light and was repelled from zones illuminated with UV light, this also allows one to build a “chromocapillary trap” by a two-color concentric illumination, which is used to manipulate the droplet along trajectories of
any desired shapes, with good precision.

10. Movie shows that a droplet “thrown” by a pipette tip is efficiently stopped
by the trap and tends to localize at the center of the visible light zone.

11. Then, by moving the Petri dish at a constant speed it is also possible to make desired shape( as shown in figure) by keeping droplet on the fixed trap.

12. Photomanipulation of a droplet along a trajectory reproducing the letter ‘N’.

13. Advantages………
Chromocapillary effect exploit for easy, fast, and precise manipulation of a floating oil droplet.
It offers new possibilities for the use of light as an external stimulus for the motion of liquids and the conversion of light.
This new strategy, which is applicable to various organic/aqueous systems, makes the controlled manipulation of millimeter-sized liquid solutions by light.
This method can be integrated to micro- or millifluidic systems and will open new perspectives for light-driven fluidics, safe handling of dangerous liquid samples, and smart photoactive materials.