Physics usually tackles big questions, but smaller everyday ones can also be interesting. Sangwoo Shin of the University of Hawaii at Manoa and colleagues have solved a long-standing problem in laundry detergency called the stagnant core problem – figuring out how detergent gets stain particles out from between tightly wound fibres, where water flow is restricted. Shin et al. explain this by the migration of particles along a concentration gradient, a process that is far more effective than naive solute diffusion. This manifests as stain particles being removed from cloth far better by rinsing soapy fabric in fresh water than in detergent-filled water – an insight with potentially wide industrial applications, and perhaps something to think about the next time you do laundry.

Further reading

S Shin et al. 2018 Phys. Rev. Appl. 9 034102.