Recently, the research paper "Einstein's tea leaf paradox induced localized aggregation of nanoparticles and their conversion to gold aerogels" was published by the aerogel research group of the College of Physical Science and Engineering in the internationally famous comprehensive journal Science Advances, and was specially interviewed, The title of "Einstein's tea leaf paradox could help make aerogels" made headlines on the website.

Normally, stirring implies dispersion, but the Einstein tea paradox reveals the complexity of solid-liquid mixing systems. The solid substances in the fluid will aggregate in the "doughnut" shaped annular area under the secondary flow caused by stirring, and after stopping stirring, they will aggregate in the center of the cup. This work attempts to extend this effect to nanofluids by simulating the movement of nanoparticles with fluid flow rate under stirring conditions. Through laser scattering experiments and analysis of scattered light grayscale values of SiO2 nanoparticle dispersion under stirring conditions, the "tea paradox" effect in nanofluids was verified. This study also found that in addition to particle aggregation caused by secondary flow, there is also an increase in particle concentration caused by stirrer driving and localized aggregation of nanoparticles between laminar flows caused by shear action in nanofluids. The aggregation effect of various nanoparticles under stirring is very suitable for promoting the self-assembly and cross-linking of metal nanoparticles, thus accelerating the gel process.

Figure 1. Localized aggregation effect of particles in nanofluids

Based on the above research, high purity gold aerogels and other metal aerogels can be rapidly prepared by a simple continuous stirring strategy. Compared with the usual settling time of about one week, the localized condensation caused by Einstein's tea paradox can shorten the gel time to about 20 minutes. The work also found that heating can cause reversible changes in the properties of the precursor chloroauric acid solution, so that the skeleton size of the aerogel can be controlled in the range of 10-200 nm. The obtained gold aerogel has good photocatalytic degradation and surface Raman enhancement properties, and has potential applications in environmental treatment and life sciences.

Figure 2. Reversible modification of chloroauric acid solution with temperature change and photos and microstructure of gold aerogels with different size skeletons