Stockholm University study reveals experimental critical point for two-state model of supercooled water

Stockholm University researchers report experimental evidence for a liquid–liquid critical point in supercooled water, lending support to the two-state model that water can exist as high- and low-density liquids under certain conditions. The team prepared amorphous ice forms, then rapidly heated them with lasers in vacuum and adjusted pressure to probe the challenging, cold region of water's phase diagram where distinct liquid states might emerge. By watching spectral signatures as they crossed the expected critical point, the researchers observed behavior consistent with a separation between high-density and low-density liquid states, implying that everyday tap water is a form of supercooled liquid near this critical region. The finding promises to refine models of water and impact related fields from biology to climate science.

Introduction: water's oddity and the two-state model

The podcast explains that water defies ordinary liquid behavior in many ways, notably its density anomaly and high heat capacity, which are tied to its extensive hydrogen bonding network. The two-state model suggests ambient water comprises fleeting coexisting high-density and low-density liquid forms, a hypothesis that has both theoretical roots and experimental challenges. The Stockholm University study discussed in the podcast provides notable experimental backing for this model by probing the otherwise difficult supercooled region of water's phase diagram where such a distinction would be most pronounced.

"Water is weird. We know that for a fact." - Phil Robinson

Experimental route to the two-state model: reaching the critical point

To access the proposed liquid–liquid critical point, the researchers tackle the notoriously hard-to-probe conditions of low temperature and high pressure. They begin with amorphous ice forms, specifically low density amorphous ice and high density amorphous ice, and then rapidly heat these samples with laser pulses in a vacuum. The aim is to melt the ice and push the system into the phase diagram region where the two liquid states might exist side by side, allowing decompression to bring about near-critical conditions. Spectroscopic signatures observed as they moved through this regime provided evidence of a crossover between high-density and low-density liquid behavior, consistent with a nearby liquid–liquid critical point. The team also discusses the importance of extreme cold and pressures (approximately on the order of thousands of atmospheres) to access this region and the experimental ingenuity needed to prevent rapid crystallization from masking the signatures they seek.

"The experimental setup is broadly that they take samples of low density amorphous ice, high density amorphous ice, and then they rapidly heat them with lasers." - Emma Pusey

Why this matters: implications for science and life

The discussion emphasizes that confirming a liquid–liquid critical point in water would refine our understanding of water’s anomalous properties and could have wide-ranging implications, including how water behaves in biological systems and in drug solutions. A confirmed two-state picture would feed back into predictive models, enabling more accurate simulations of water’s role in physiology, biochemistry, and environmental processes. The hosts connect these fundamental insights to larger questions about life’s emergence and Earth’s habitability, noting water’s pivotal role in establishing the conditions that make Earth hospitable for life as we know it.

"It's a feedback loop where you can sort of refine things and then make better predictions perhaps about water's behavior" - Phil Robinson

Science sleuths and research integrity: PubPeer and the publishing arms race

Shifting to a different stream of science communication, the podcast surveys the landscape of research integrity and the growing community of science sleuths who help keep science honest. They discuss that the scale of misconduct is hard to quantify, partly due to culture and publication pressures, and mention phenomena like paper mills that generate fake papers for payment. PubPeer is highlighted as a post-publication review platform where researchers can raise questions about a paper and point out potential issues such as duplicated panels or suspicious methodology. The discussion also covers tortured phrases used to evade plagiarism detectors and stresses the importance of reporting through proper channels to journals’ ethics teams rather than public accusations, given the reputational and legal risks involved.

"Paper mills are run by dodgy organizations or collectives, shall we say, who will produce a piece of research, often based on plagiarism or entirely fabricating results, and then they sell the authorship on that paper." - Emma Pusey

Chemistry in history: sodium silicate and the soap revolution

The episode closes with a brief journey into chemistry history, recounting how sodium silicate transformed soap production in the 19th century. William Gossage’s experimentation with sodium silicate added to soda ash produced harder, longer-lasting soap at lower cost, effectively revolutionizing the soap industry. Although newer detergents have largely supplanted this older approach, sodium silicate’s legacy persists in some modern cleaners, illustrating how a single chemical insight can reshape everyday consumer products and industrial practices.

"Sodium silicate persists today in some modern detergents, but it's largely been replaced by newer, more efficient detergents like phosphates and synthetic surfactants." - Mariana Knippers