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Molten lava and yogurt and are well-known examples of non-Newtonian fluids, also known as soft solids.
A soft solid can act like either a solid or a liquid, based upon the stress that has acted upon it. Conversely, the behavior of Newtonian fluids, like water, doesn't change based on how they have been physically affected. For instance, egg whites become stiffer after being whipped but become more liquid if left to stagnate, due to the pull of gravity over time.
The mechanisms that affect soft solids in this way are intricate and not well known, making it hard for engineers to manipulate their qualities as desired. Being capable of greater control of soft solids would open up many new opportunities in areas like food science and engineering.
The significance of soft solids was underlined by the inquiry into the April 2010 explosion and subsequent oil leak at the offshore oil rig in the Gulf of Mexico known as Deepwater Horizon. The incident killed multiple people and triggered the biggest oil spill in history.
The inquiry indicated that something went awry with the foamed concrete used in the oil well, a soft solid. The failure of this substance was one of multiple events that triggered the explosion and spill. For foamed concrete to function as desired in this application, the bubbles within it need to be properly distributed throughout, and they must be stable bubbles that don't collapse or combine to create massive air pockets.
Bubbles in Soft Solids
Bubbly soft solids are one kind of soft solid, which are commonly found in the kitchen and in nature. For instance, the properties of magma and cake batter are largely defined by the bubble these substances contain.
Researchers at the University of Cambridge recently revealed how the bubble composition of these soft solids affects their properties, an insight which may allow for more control over these substances than is currently possible. For instance, it may allow for fluffier cake batter or for safer oil drilling.
When a bubbly soft solid is infused with more bubbles, it becomes lighter. In cake batters, this results in a fluffier cake. In oil wells, it enables foamed cement to prevent oil and gas from escaping by filling gaps between the pipe and rock.
Using honey, the Cambridge researchers looked into how the amount and size of the bubbles in impacted behavior, after which looked to simulate that the behavior they observed. After that, the team repeated its process using gum solutions, which are thickening agents for sauces.
The scientists were able to use the Giesekus fluid mathematical model to represent how a base liquid responds to the addition of bubbles. With a Giesekus model and by shifting the bubble size, scientists said they are now looking to predict and fine-tune the behaviors of bubbly soft solids.
Fatigue in Soft Solids
Soft solids can lose their stiffness over time to become more like a liquid, a phenomenon attributed to “fatigue.”
In hard solids, fatigue has been thoroughly-analyzed: Repetitive strain causes microcracks to develop in the breakable internal structures. Eventually, one of these fractures reaches a critical length and the solid breaks.
Soft solids normally include a cluttered network of microscopic strands. Scientists have suspected that repetitive flexing causes strands to break, which cuts down on network connectivity, making the material less rigid. However, a recent study from Wageningen University in the Netherlands revealed lower strand plasticity could play a more fundamental part in gel fatigue than breakage.
To verify the role of plasticity, the team conducted comprehensive numerical models of a single strand of plastic gel subjected to a number of strain cycles. As a result of the strain, beads inside the strand moved along it; creating a number of thin 'necks' and significant stretching. The study scientists discovered the biggest rearrangement happened during the first strain cycle, leading to the strand being less elastic for subsequent cycles.
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