7 research outputs found
Ice Particles Sink below the Water Surface Due to a Balance of Salt, van der Waals, and Buoyancy Forces
According to the
classical Archimedes’ principle, ice floats
in water and has a fraction of its volume above the water surface.
However, for very small ice particles, other competing forces such
as van der Waals forces due to fluctuating charge distributions and
ionic forces due to salt ions and charge on the ice surface also contribute
to the force balance. The latter crucially depends on both the pH
of the water and the salt concentration. We show that a bulge in the
air–water interface due to interaction of surface tension with
the rising ice particle becomes significant when the particle radius
is greater than 50–100 μm. The role of these forces in
governing the initial stages of ice condensation has never been considered.
Here, we show that small ice particles can only form below an exclusion
zone, from 2 nm (in high salt concentrations) up to 1 μm (in
pure water at pH 7) thick, under the water surface. This distance
is defined by an equilibrium of upward buoyancy forces and repulsive
van der Waals forces. Ionic forces due to salt and ice surface charge
push this zone further down. Only after growing to a radius larger
than 10 μm, will the ice particles eventually float toward the
water surface in agreement with the simple intuition based on Archimedes’
principle. Our result is the first prediction of observable repulsive
van der Waals forces between ice particles and the water surface outside
a laboratory setting. We posit that it has consequences on the biology
of ice water as we predict an exclusion zone free of ice particles
near the water surface which is sufficient to support the presence
of bacteria