While scientists have known that pumice can float because of pockets of gas in its pores, it was unknown how those gases remain trapped inside the pumice for prolonged periods. If you soak up enough water in a sponge, for example, it will sink. They then used an X-ray imaging technique at the ALS known as microtomography to study concentrations of water and gas — in detail measured in microns, or thousandths of a millimeter — within preheated and room-temperature pumice samples.
To tackle this problem, Zihan Wei, a visiting undergraduate researcher from Peking University, used a data-analysis software tool that incorporates machine learning to automatically identify the gas and water components in the images. So surface tension really dominates. The team also found that a mathematical formulation known as percolation theory, which helps to understand how a liquid enters a porous material, provides a good fit for the gas-trapping process in pumice.
And gas diffusion — which describes how gas molecules seek areas of lower concentration — explains the eventual loss of these gases that causes the stones to sink. The study showed that previous estimates for flotation time were in some cases off by several orders of magnitude. Surface tension serves to keep these bubbles locked inside for prolonged periods. Recent X-ray studies carried out at Berkeley Lab have finally produced some answers. But pumice pores are actually largely open and connected - more like an uncorked bottle.
What is even more interesting is that in the laboratory the pumice stones would bob up and down, sinking during the evening and resurfacing during the day. To find out what was going on inside the pumice stones, the team used wax to coat bits of water-exposed pumice samples. They then used an X-ray imaging technique known as microtomography to study concentrations of water and gas in the pores. X-ray microtomography uses x-rays to create a cross-sections of a physical object which can be used to make 3D models.
Researchers found that the gas-trapping processes that are in play in the pumice stones relate to "surface tension," a chemical interaction between the water's surface and the air above it that acts like a thin skin.
This is the same effect that allows insects and very small animals to walk on water. Many pores in the pumice stone are very small, about the width of a human hair. Because there are many of them the surface tension is high. The scientists were able to predict how long a pumice stone would stay afloat by calculations based on the rock size and the diffusion of trapped gas. Michael Manga, a staff scientist in Berkeley Lab's Energy Geosciences Division and a professor in the Department of Earth and Planetary Science at UC Berkeley who participated in the study, explained, "There are two different processes: one that lets pumice float and one that makes it sink".
The X-ray studies helped to quantify these processes for the first time. The study showed that previous estimates for flotation time were in some cases off by several orders of magnitude.
Surface tension serves to keep these bubbles locked inside for prolonged periods. But there is also a slow but continuous diffusion of gas from the pores which allows more water to enter. The bobbing observed in laboratory experiments of pumice floatation is explained by trapped gas expanding during the heat of day. Just because something has more mass or is bigger does not mean it is has a higher density.
A quick way of describing density is to describe an object as heavy or light for its size. Pumice stone, unlike regular rock, does not sink in water because it has a low density. Pumice stone is igneous rock formed when lava cools quickly above ground lava froth. You can clearly see where little pockets of air have formed. Ironwood is a common name for a large number of species of wood that are very hard. This very hard wood is resistant to compression and is valued for making tool handles and fence posts.
An ironwood branch is very dense and sinks in water. Demonstrate how the distribution of particles in a substance determines its density. How does the ironwood stick compare to the regular wood stick?
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