My friend and scientific collaborator Kostya Trachenko, who has died aged 54 of cancer, was a theoretical physicist. He helped to reshape our understanding of liquids and revealed surprising connections between fundamental physical constants and the properties of everyday materials.

Kostya’s research findings, often in cooperation with his longtime collaborator Vadim Brazhkin, brought new clarity to the elusive behaviour of liquids. He developed a mathematical framework explaining how, under certain conditions, liquids behave like solids – such as when falling into water from a height. This insight led to a deeper understanding of how transverse sound waves propagate through liquids, and how these behaviours change with temperature. His work also explained the longstanding mystery of why the heat capacity of liquids often decreases with temperature, unlike in solids.