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Phase Change

Solidification processes in casting and freezing

Drop freezingDrop freezing and forming cusp

Credit: Dan Anderson

These two photographs show a droplet of water before and after it is frozen from below, having been placed on a cold base.  The ruler on the right-hand side is graduated in millimetres.  Prediction of the final shape of the droplet involves an understanding and quantification of the freezing front, which is a dynamically evolving free boundary.  The mathematical modelling and methods of solution of this problem are relevant to various types of containerless casting. See: Anderson, Worster & Davis (1996)Schultz, Worster & Anderson (2001).

 

Ammonium chloride freezing and forming chimneys Credit: Tim Schulze

An aqueous solution of ammonium chloride is cooled by contact with a cold base. Crystals of ammonium chloride form a porous matrix through which residual, buoyant, un-solidified fluid can flow under the action of gravity.  Where the convection flow is upward, the residual fluid, which is relatively dilute of ammonium chloride, re-dissolves the crystals to form almost vertical channels. The layer of crystals in this image is a few centimetres tall, and the channels are a few millimetres wide.  This laboratory experiment models processes that occur during casting of metallic alloys and during freezing of the ocean to form sea ice. See: Schulze & Worster (1998)Chung & Worster (2002).


 

Frost heave

 clay freezing 160311

Credit: Stephen Peppin

A layer of bentonite clay particles in water is cooled from below to form a polygonal pattern of ice crystals.  The overall expansion of the medium, called 'frost heave', is accompanied by very large pressures, which causes enormous damage to roads and buildings and is responsible for many geomorphological features in regions of permafrost.  Frost heave is not caused at all by expansion of water as it turns into ice (as is commonly reported in the press) but by additional water being sucked from surrounding areas towards the freezing fronts. See: Peppin, Elliott & Worster (2006)Anderson & Worster (2012).