Self-motion of an oil droplet on a surface
Following is schematic representaiton of the mechanism of oil droplet motion.
For an aqueous phase 1-5mM Stearyl trimethyl ammonium chloride, generally used cationic surfactant, is used, whereas for an organic phase potassium iodide saturated 3-10 mM iodine solution of nitrobenzene is used. As a solid substrate, we have used glass substrate, which has negative charge at natural pH.
Because glass surface has negative charge at natural pH, the contained cationic surfactant aggregate on the surface, resulting in the surface to be hydrophobic. When an oil droplet, which contains iodine that makes hydrophobic ion pair with the surfactant, passes through the glass surface, the aggregated surfactant is absorbed into the oil droplet from the surface. As a result, the surface of the glass is hydrophobic (water hater) in its front and hydrophilic (water lover) in its rear. Hence, the oil droplet is driven by the surface tension difference.
The novel point of this system is that glass surface is, again, becomes hydrophobic since in the aqueous phase there is rich amount of surfactant. Therefore, the trace of the oil droplet motion intersects each other. At the same time, the regular, repetitive motion is generated under appropriate boundary condition.
Following is the various motion of an oil droplet, together with the short description of the experimental setup.
Oil droplet moving irregularly
Following is real time movie. The diameter of petri dish is 70mm, the droplet size is 30 micro litter.
Following is schematic representation of the above movie,
The motion of an oil droplet is irregular in isotropic boundary condition. Once the droplet collides into the wall of petri dish, it cannot leave the wall.
Oil droplet showing shuttling motion
Following is real time movie. The height of a step of glass plate is 1mm, the length is 26mm, the width is 3.4mm, and the droplet size is 30 micro litter.
Maybe, this is too quick to follow the movie, the next movie is the half speed of the above.
Following is schematic representation of the above movie.
The motion of an oil droplet is regular and repetitive on a narrow glass substrate. This shuttling motion cannot be reproduced unless glass surface recovers to be hydrophobic.
Oil droplet moving up slope
Following is half speed movie. The height of a step of glass stair is 1mm, the droplet size is 20 micro litter.
The relative mass density of an il droplet is about 1.2, which is heavier than the aqueous phase whose relative mass density is almost 1.0. Nontheless, an oil droplet spontaneously moves up slope.
Oil droplet showing rotational motion like a roller coaster
Following is real time movie. The diameter of a loop is 25mm, the droplet size is 20 micro litter.
Following is schematic representaiton of the above movie,
The relative mass density of an il droplet is about 1.2, which is heavier than the aqueous phase whose relative mass density is almost 1.0. Nontheless, an oil droplet spontaneously moves in a vertically set loop.
Oil droplet climbing stairs
Following is real time movie. The height of a step of glass stair is 1mm, the droplet size is 30 micro litter.
The relative mass density of an il droplet is about 1.2, which is heavier than the aqueous phase whose relative mass density is almost 1.0. Nontheless, an oil droplet spontaneously moves up stair-like substrate.
Oil droplet climbing stairs with one's best effort
Following is half speed of real time movie. The height of a step of glass stair is 1mm, the droplet size is 60 micro litter. Since the size of droplet is just possible to climb the stair, it tries many times to climb it up.
The relative mass density of an il droplet is about 1.2, which is heavier than the aqueous phase whose relative mass density is almost 1.0. Nontheless, an oil droplet spontaneously moves up stair-like substrate.
Further detail is in
Phys. Rev. Lett.94, 068301 (2005),
Phys. Rev. E72, 041603 (2005),