Ever heard of „surface freezing“ and its romantic background? This phenomenon,
which arguably constitutes one of the most simple molecular self-assembly processes in nature, refers to the spontaneous formation of a microscopically thin, crystalline layer floating on top of the melt close to, but above the freezing temperature of the bulk liquid. Capillary filling experiments on molten wax in tiny pores (only a few nanometers across), published today in Physical Review Letters, testify that this collective orientational molecular ordering is also observable at advancing, highly curved liquid menisci self-propelled by capillarity in silica pores. There you may call it “meniscus freezing” and it significantly affects the imbibition dynamics in the mesoporous matrix.

Patrick wins the best talk award at the Nano- and Microfluidics workshop in Bad Honnef, Germany. He presented work of Simon Gruener on spontaneous imbibition dynamics of linear hydrocarbons (n-alkanes) in mesoporous silica. Congrats to both !

Surprisingly, spatially nano-confined liquid n-hexane is still an isotropic liquid: We did not find any hints of collective orientational order as a function of filling of glass capillaries with 7 nm mean diameter and 400 micrometer length. A combined optical birefringence/vapour sorption isotherm study reporting this message was published in Physical Review B today. The absence of any orientational order in the confined liquid state highlights the importance of an anisotropic crystallization process named after Percy W. Bridgman (a Harvard scholar and Nobel Laureate) for the occurence of preferred orientations upon solidification of n-hexane and medium-length n-alkanes in nanochannels of mesoporous silicon.

Study on capillary rise dynamics of water in hydrophilic nanopores published in Physical Review E. A ray-tracing movie which illustrates this spontaneous imbibition process in mesoporous silica glass (monolithic Vycor) can be downloaded here. It employs the Liquid-Surface-Simulation Macro package (LSSM) for POV-Ray by Tim Nik Wenclawiak.