Japanese

Tears of wine

The thin film of water-ethanol mixture spontaneously climbs up glass surface when the ethanolevaporates from the film. Since the evaporation of ethanol is slower near the bulk of the mixture, there occurs the gradient of ethanol concentration on the film. Ethanol is surface active chemical; therefore, surface tension of the mixture is higher at the top of film. This unbalanced surface tension results in the Marangoni flow. Finally, the liquid is spontaneously soaked up to the higher glass surface, and by the effect of gravity, the mixture forms droplet on the glass surface.

Further details are found in
Sumino's website

Self propelled particle

Following movies are numerical simulation of probably the most simple model for self-propelled particle, so called Vicsek model.
We see many arrows moving around, which represent self propelled particle. They try to align themselves with their neighbor, but they changes their direction due to noise. In this movie, noise is weak enough to have ordered collective motion.
Above calculation is only a part of larger simulation, and if we show the calculation in full area the result becomes as follows.
Here, as we show 16 times 16 larger area, local density of particles are shown, where brighter region corresponds to higher local density.

When the noise is much larger, all particle move randomly. Interestingly, between random and ordered phase, this model develops density wave as follows.

Artificial amoeba

Placing an oil droplet with fatty acid on an aqueous phase with cationic surfactant, the oil droplet spontaneously deforms its interface in the absence of any biological components. Accompanied with the spontaneous deformation, surfactant aggregation is formed in the aqueous phase close to the oil-water interface. The energy source of the oil droplet deformation is the gain of chemical potential energy by the transport of the fatty acid from the oil droplet to the aqueous phase.
Amoeaba like motion of an oil droplet accelerated by 10 times. The droplet is floating on water and the volume is 1000 micro litter.

Spontaneous blebbing of an oil droplet accelerated by 6 times. The droplet is floating on water and the volume is 500 micro litter.

A walking oil droplet whose volume was 3 micro litter, where the droplet was made havier than water by adding 12 vol.% of 1,1,2,2,tetrabromoethane. Movie is accelerated by 10 times.


Initically we speculated the formation of passive aggregate is the mechanism of droplet motion, but later we found the required pressure to produce interfacial deformation is much higher than theoreticaly estimated pressure induced by aggregate formaion.
Recently we found transiton of the aggregate structure in the proximity of oil-water interface. Such transition of aggregate structure might contribute interfacial motion, which should require additional driving force.

Furthe detail:
Sumino's website
Concentration dependence of blebbing size Phys. Rev. E 76, 055202 (2007),
Surface tension, details of dynamics  J. Phys. Chem. B 113, 15709-15714 (2009),
Size dependence of blebbing size Soft Matter 7, 3204-3212 (2011),
SAXS analysis of aggregate Langmuir 28,3378-3384 (2012),
Walking droplet experiment Euro. Phys. J. ST 223, 1345-1352 (2014).,
SANS analysis of aggregate showing structural transition Langmuir 32, 2891-2899 (2016).

Free running droplet

Following is real time movie. The diameter of petri dish is 70mm, the droplet size is 30 micro litter.
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. The oil droplet is nitrobenzene containing iodine. The oil droplet is set on a glass surface inside of an aqueous phase containing cationic surfactant.
If we tune the shape of a glass substrate, the droplet shows shuttling motion on the glass substrate. The following movie is sideview. The thickness of glass substrate is 1 mm, length is 26 mm and the width is 3.4 mm. The volume of oil droplet is 30 micro litter.

An oil droplet shows acrobatic motion too. Following is real time movie. The diameter of the ring-shaped substrate is 25 mm, and the volume of oil droplet is 20 micro litter.
Though the relative density of oil is 1.2, larger than water, oil droplet shows roller-coaster motion.

If you tame the droplet, it can climb up stairs. Following is half-speed movie. The height of a step is 1 mm, the size of the oil droplet is 60 micro litter.The volume of the oil droplet is around threshold volume, such that the droplet try its best to climb up stairs.

For further details:
Sumino's website
Various droplet behavior, Phys. Rev. Lett.94, 068301 (2005),
Chemical sensing, Phys. Rev. E72, 041603 (2005),
Stair climbing, Prog. Theor. Phys. Suppl.161, 348-351 (2006)
Mathematical model, Chaos18, 026106 (2008)