From left: Dankook University Professor Kim Se-young, master's students Kim Sang-yun and Jin Xin-zhe. [Dankook University]
From left: Dankook University Professor Kim Se-young, master's students Kim Sang-yun and Jin Xin-zhe. [Dankook University]

A research team led by Professor Kim Se-young of the Department of Polymer Engineering at Dankook University in Yongin has for the first time experimentally confirmed that nanoparticles at a water surface deform under the water's surface tension.

The team measured in real time how nanoparticle structure changes depending on particle stiffness and surface tension, finding that the two factors can deform nanoparticles by as much as 75%. The findings were published in Nano Letters (impact factor 9.1), a leading international journal in nanoscience published by the American Chemical Society. The paper is titled "Elastocapillary Deformation of 'Glassy' Polymer Micelles at an Air–Water Interface."

"Surfactant nanoparticles" — particles that lower surface tension at interfaces between immiscible substances such as oil and water — are considered a next-generation material capable of replacing conventional surfactants. Their strong emulsifying effect stabilizes interfaces in cosmetics, pharmaceuticals and eco-friendly coating processes, helping protect active ingredients.

The shape of a nanoparticle is a particularly critical factor in determining how well it stabilizes an interface and performs as an emulsifier. Emulsification performance depends on how flat or deformed a nanoparticle becomes at the interface. Knowing precisely under what conditions and to what degree nanoparticles deform is therefore a key challenge in designing surfactant nanoparticles.

Because nanoparticles are extremely small, directly observing and quantitatively measuring their deformation at a water surface has been difficult. Researchers had long predicted that water's surface tension would act strongly enough at the nanoscale to deform particles, but no study had experimentally confirmed the deformation process in real time.

The team used spherical polymer nanoparticles with tunable stiffness and conducted neutron reflectivity experiments to measure structural changes in the particles at the water surface in real time and without damaging the samples.

Using a Langmuir trough to control surface tension and measure deformation, the researchers confirmed that deformation rates reached as high as 75% depending on the level of surface tension and particle stiffness.

The experiments provided direct evidence of the "elastocapillary" phenomenon, in which an elastic material deforms under surface tension. The team showed through experiment that structural changes in nanoparticles — which had been difficult to explain using existing theories based solely on rigid bodies or liquids — arise from the interplay between a particle's elasticity and surface tension.

"This study is the first experimental demonstration that the elastocapillary phenomenon has a far greater influence on the nanoscale world than previously expected," Kim said. "We anticipate it will serve as a foundational technology for designing next-generation surfactant nanoparticles, including high-surface-area chemical materials and drugs that modulate biological interfaces."

Master's students Kim Sang-yun and Jin Xin-zhe, both from the Department of Polymer Engineering, served as co-first authors.


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