Amoeaba-like motion of an oil droplet
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.
The mechanism of the droplet deformation can be considered as follows. The internal pressure of an oil droplet mechanically increases due to the generation of elastic aggregation only at the oil-water interface. If the increase of the internal pressure exceed a threshold value, the deformation of the inteface occurs accompanied with the destruction of the aggregation. Based on this theoretical model, the shape of the interfacial deformation becomes circular and/or spherical.
The actual deformation is shown in the following movie.
Amoeaba like motion of an oil droplet accelerated by 10 times
As it is seen from the movie, the characteristic shape of the deformations is circular and/or spherical. The other dynamical character of the deformations is slow extension and quick retreat of the deformations. These characters are well observed in the following movie,
Spontaneous blebbing of an oil droplet accelerated by 6 times
Cellular systems utilize various mechanism to generate its motion. One of the mechanisms is generation and collapse of elatstic actomyosin gel. This spontaneous deformation of oil droplet also induced by generation and collapse of elastic aggregation. We believe that this droplet motion can be simple physico-chemical model system of cellular motility.
Furthe detail is in
Phys. Rev. E 76, 055202 (2007).
J. Phys. Chem. B 113, 15709-15714 (2009).