Applying Einstein’s Tea Leaf Paradox to Nanofluids: How Can Localized Aggregation of Nanoparticles Be Used in Forming Gold Aerogels?

Substances can be dispersed evenly in liquid by stirring. However, Einstein's tea leaf paradox (ETLP) shows how tea leaves concentrate in a doughnut shape through a secondary flow effect while stirring. Researchers in China demonstrated ETLP-induced concentration in nanofluids and applied the localized concentration to form gold aerogel.

Einstein's Tea Leaf Paradox

In 1926, Albert Einstein explained an experimental observation describing how the leaves followed a spiral trajectory toward the center of a stirred teacup. As the tea leaves follow the motion of the fluid, they move in a circular motion while also moving toward the center of the bottom of the cup. This phenomenon is explained by friction between the rotating fluid and the walls and bottom of the cup, leading to decreased fluid velocity and a resulting spiral inward.

This behavior of tea leaves, where they gather under stirring due to the secondary flow, can be used in collecting microscale particles in dispersion systems. During ETLP, the flow velocity paradox induces laminar flows since nanoparticles with better stability normally move together with the fluid due to Brownian motion. This drives the localized concentration of colloidal nanoparticles inside the thin flow.

In China, a group of physics and engineering experts tried obtaining a grayscale nanofluids analysis under stirring and standing processes. Led by Zehui Zhang, the researchers simulated the nanoparticle trajectory under stirring and used the localized concentration to attain ultrafast aggregation of gold nanoparticles.

The relationship between nanoparticle distribution and flow velocity in nanofluids was investigated using COMSOL Multiphysics software to recreate the movement of nanoparticles in laminar flow under stirring. Zhang and his colleagues monitored the nanoparticle trajectory after stirring for 500 seconds, where the nanoparticles in the middle were shown to move faster with a longer trajectory.

It was observed that the high motion frequency and amplitude of the nanoparticles in the high-velocity areas promote the encounters of nanoparticles, making them more concentrated or cross-linked. Based on these findings, the team theorized that the motion of the nanoparticles in nanofluids would follow the law of ETLP.

Development of Gold Aerogels

The experts prepared a locally aggregated gold gel using the ETLP process to reduce gold ion clusters. They created a chloroauric acid solution with gold clusters and dried the components either at room temperature or by using a heating source of light for transmission electron microscopy observations. The skeleton microstructure of the aerogels was analyzed using small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy.

Zhang's team constructed gold ion clusters of various sizes by regulating the temperature of chloroauric acid. They completed the experiments with ETLP-driven aggregation effects and carbon dioxide drying as they tried to create aerogels with different skeleton sizes, including the capacity for future aerogels to be prepared similarly. From this research, scientists confirmed that ETLP can be applied to nanofluids with unexpectedly localized aggregation effects to develop gold aerogel from simple stirring.