Spontaneous deformation of oil-water interface induced by
the generation of a surfactant aggregate.
Yutaka Sumino1, Hiroyuki Kitahata2, Hideki Seto3
Department of Applied Physics, Graduate School of Engineering, The
University of Tokyo, Tokyo 113-8656, 2 Department of Physics, Graduate
School of Science, Chiba University, Chiba, 263-8522 and PRESTO, JST,Saitama 332-0012, 3 KENS and CMRC, Institute of Materials Structure Science,High Energy Accelerator Research Organization, Ibaraki 305-0801.
1

Generation of elastic aggregates is one of mechanisms ofbiological motility; generation of F-actin gel fromG-actin is essential for the formation of pseudopod inamoeboid motion. Genetic information often gives acontrol on such biological process, but also physicalmechanism itself, known as dissipative structure, should
give relevant contribution for the control. We can havebetter understanding on such physical mechanisms
through the construction and the analysis of aphysico-chemical model system that mimics certain
biological process. For this purpose, we have previously
constructed an oil-water system with two surfactants[1,2].
An organic phase containing palmitic acid (PA), where asolvent was tetradecane, was placed on an aqueous phase
containing stearyltrimethylammonium chloride (STAC).
In this setup, the interface between the aqueous phaseand the organic phase spontaneously deforms in acontinuous manner. The deformations are spherical and Fig: (a) Experimental
their extension and retraction typically takes 10 s. This setup. (b) Typical mannerprocess (blebbing) occurs in an arbitrary area at the of pillar-like aggregate
formation.
oil-water interface and continues for typically 1 hour.
Blebbing of oil-water interface can be understood in terms of the generation of anaggregate composed of STAC and PA in the aqueous phase according to ourmathematical model [1]. Appearance of aggregates in the aqueous phase was confirmedwith small angle X-ray scattering (SAXS) at BL-15A in PF, KEK, we measured themicroscopic structure of the aggregates, and it was suggested the aggregates havelamellar structures with an interlayer distance of around 40 nm [2].
In the above measurement, we put the organic phase on the aqueous phase in acrylic
cell whose thickness was 2 mm（Fig. (a)）. While observing the aggregate, we found thatthe pillar-like structure of aggregates was generated at the oil-water interface
accompanied with the interfacial motion (Fig. (b)). The pillar-like structure was about afew submilimeters in width. It was observed that the pillar was pushed downward, i.e.
into the aqueous phase from the oil-water interface. Despite that the microscopic
structure of aggregates was determined through the previous SAXS measurement, therelation between the interfacial motion and pillar-like structure has not been understood.
This is due to the fact that the precise position of X-ray beam has not been controlled.
In this paper, we executed SAXS measurements using a microbeam of X-ray at BL-4A
in PF, KEK, where the precise control of beam position in a sample is possible. In themeasurement, the beam was scanned horizontally 1 mm below from the oil-waterinterface. Through the measurement, it was found that the direction of lamellar in theaggregates with in a pillar like structure strongly ordered in parallel to the oil-waterinterface.
[Reference][1] Y. Sumino, H. Kitahata, H. Seto and K. Yoshikawa, Phys. Rev. E 76, 055201 (2007).
[2] Y. Sumino, H. Kitahata, H. Seto, S. Nakata and K. Yoshikawa, J. Phys. Chem. B 113,15709 (2009).