The relative positions of floating objects can remain stable for days, despite some disturbance in water or waves. This phenomenon was illustrated and verified by a team led by Professor Zheng Quanshui from the Department of Engineering Mechanics.

They found that the formation rigidity of floating particles is a result of the existence of lateral liquid bridges between contacting particles. Acting as capillary springs between particles, the liquid bridges with contact angle hysteresis can keep the particles stable or even drag them back to their previous positions under conditions of minor disturbance. This mechanism may make assembly in water easier or more accessible.

Their findings also resolve a longexisting riddle in capillarity. Ideally, rafts of identical floating objects should assemble closely in liquid. However, the assemblies always exhibit large and various defects. The conjunction of lateral liquid bridges and contact angle hysteresis freezes the rotation of particles around their neighbors, which results in imperfect assembles.

The team published their findings in the interdisciplinary journal Langmuir. The paper “Strongly Metastable Assemblies of Particles at Liquid Interfaces” was chosen by the American Chemical Society as an Editor’s Choice paper. The ACS introduced its Editor’s Choice in 2014 to feature new scientific articles of broad public interests to be made open to anyone at no charge. Third-year undergraduate at Tsinghua,
Xue Nan, was the first author.

                                                                                                                                                                                              [update:2015-04]