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Soft matters maintain their structures via weak interactions, such as Van der Waals, capillary, hydrogen bonds etc. The entropy plays a key role in the ordering of this kind of materials. They behave as a solid until a sufficiently large stress is applied, and then behave as a viscoelastic liquid. Due to the unique structures and rheological properties, soft matter such as concentrated suspensions, emulsions, pastes and gels often exhibit unusual slow relaxation and aging effect. Studying the relaxation dynamics may gain more insight into the microstructure of the material and may also shed light on the understanding of the physical origin of glass transition.
Nanoparticle can be trapped at the air-water interface. The formed layer is a typical 2D soft matter, which plays essential role in foams, emulsification and pharmacy. However, owing to the low dimension and absence of theory, the study of 2D relaxation is quite a challenge. By using the Langmuir troughs technique and oscillating bubbles or drops methods, a deformation can be exerted on the layer rapidly. The relaxation then can be studied by monitoring the time viaration of the surface pressure. However, these techniques can not obtain the anisotropy of the layers and hence are limited.
Dr. ZANG Duyang et al from the Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, School of Science, Northwestern Polytechnical University, has developed a novel approach in which two orthogonal Wilhelmy plates have been utilized to measure the surface pressure in the two directions. By comparing the relaxation dynamics under static and oscillation state, the relaxation mechanisms have been elucidated.
Unlike the surfactant of low molecular weight, the adsorption of nanoparticles at the interfaces is irreversible. This leads to the different Π-Γ isotherms. With the increase of surface density, as well as the textual change, the surface pressure present remarkable anisotropic effect. The anisotropy suggests that the layer is under non-equilibrium state. The relaxation towards equilibrium occurs by means of particle rearrangement. When the layer is kept static, the particle rearrangement is driven by the inner stress stored in the layer. Thus, the relaxation is slow. While under barrier oscillation, additional driving force is exerted by the barriers. Consequently, the relaxation is accelerated significantly.
This work is supported by the Northwestern Polytechnical University Foundation for Fundamental Research (NPU-FFR-JC20100242).
See the article: Zang D Y, Zhang Y J. Surface pressure anisotropy and complex relaxation of silica nanoparticle monolayer at the air-water interface. Sci China Phys Mech Astro, 2011, 41(9)
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