X-Message-Number: 17324
From: "Jan Coetzee" <>
Subject: Water turns to jelly under pressure
Date: Sun, 19 Aug 2001 17:40:37 -0400
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Water turns to jelly under pressureSyrupy squeezed water could affect proteins
and plate tectonics.
17 August 2001
PHILIP BALL
Jelly, jelly everywhere and not a drop to eat.
Photodisc
Water squeezed between two surfaces turns to jelly, US physicists have found1.
This behaviour could affect proteins interacting in cells, sediments aggregating
in rivers and rocks moving deep in the earth.
Confined between two mineral layers, a water film just a few molecules thick can
have a viscosity many times greater than normal, say Yingxi Zhu and Steve
Granick of the University of Illinois at Urbana-Champaign. This stickiness
depends on the orientation of the surfaces relative to one another - rotating
one alters the water's viscosity significantly.
This dependence on the surfaces' twist caught the researchers unawares when
their initial experiments with oscillating slivers of mica - a clay-like mineral
that can be cleaved to extremely flat sheets - generated seemingly
irreproducible results. The more viscous the intervening layer of water, the
greater the 'shear force' on the sheets.
The orderly arrangement of atoms at the mica surfaces affects the character of
the film trapped between them, suggest Zhu and Granick. If the rows of atoms in
one mica sheet are aligned with those in the other, this may encourage the water
molecules to take on a similar configuration, like eggs in an egg carton.
Otherwise, the water molecules may remain disordered.
Theorists have predicted this ordering effect of a crystal surface on the
structure of a liquid layer adjacent to it. But contrary to these predictions,
the water doesn't seem to adopt a strictly regular molecular structure, as they
would when freezing to form a thin layer of ice. Instead, the molecules remain
more mobile, like those in a gel.
Zhu and Granick reach this conclusion rather nervously. They are mindful of the
'polywater' scandal of the late 1960s, in which Soviet scientists claimed to
have found a new, gel-like, polymerized form of water in thin glass capillaries.
The claims ended in ignominy when the 'new' state was found to be the result of
impurities.
However, the viscous films cooked up in Illinois are much thinner than those in
'polywater' samples, and have ample support from theory.
The films' influence on the interactions of biomolecules and minerals is not
going to be easy to predict, given that the lateral atomic structure of the
surfaces seems to influence how ordered the water film is.
References
a.. Zhu, Y. & Granick, S.Viscosity of interfacial water. Physical Review
Letters, 87, 096104, (2001).
Nature News Service / Macmillan Magazines Ltd 2001
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