Our sample cell appears schematically in Fig. 1.
The suspension of volume fraction 
is confined to a thin horizontal layer between smooth clean glass walls
separated by 2.5 
m.
This sample volume is hermetically sealed at the edges to minimize
the invasion of air-borne contaminants.
Reservoirs of ion exchange resin in
diffusive contact with the observation volume further
help to maintain chemical purity.
Under these conditions the screening length is limited by the
intrinsic concentration of counterions to roughly 
nm.
Clean glass surfaces develop large negative surface charges in contact with
water [24]
which repel the negatively charged colloid and provide a smooth
and featureless confining
potential [25].
They also prevent colloid from aggregating onto the walls through
van der Waals interactions.
The glass walls and their supports provide a transparent
area of about 1 cm
for observation.
We image the colloidal spheres using a conventional
reflected light microscope with a 140
N.A. 1.3 oil
immersion objective and
a Xe arc illuminator
filtered to avoid sample heating and degradation.
The microscope's depth of focus is 
0.2 
m, which is
comparable to a sphere diameter.
Images are captured at a rate of 30 frames per second with
a CCD video camera and recorded on video tape
before being digitized
and analyzed on frame-by-frame basis.
The total system magnification is 68.2 nm/pixel at the CCD camera.
Although the spheres are too small to form diffraction-limited images,
they appear as distinct bright features on an otherwise dark background,
as can be seen in Fig. 2.
The upper glass surface of the sample volume
has a pair of parallel plate
thin film electrodes patterned onto it in transparent chemically inert
-InO
.
A potential applied across the 3 mm wide gap induces flow in the
suspension through electrophoresis and electro-osmosis [26].
This motion is resisted by the rigidity of the
colloidal lattice and so the observed direction of flow depends also on
the lattice's orientation, at least at low flow rates.
Higher flow rates shear-align the lattice.
Because the flowing lattice is damaged in an uncontrolled fashion
at the edges of the cell,
we drive the flow with a small amplitude (1Vpp)
sinusoidal signal to prevent propagation of this
damage into the observation region.
The peak-to-peak amplitude of the response at 1 Hz is roughly 100 
m.
The lattice deforms both elastically and plastically
as it flows around obstructions such as the columnar
feature shown schematically in Fig. 1(b).
Measurements of the lattice's deformation can be used to estimate
its elastic moduli [27].
We will describe such measurements on
geometrically confined suspensions elsewhere.
Sufficiently vigorous shearing around obstructions introduces a sufficient
density of defects to melt the lattice as can be
seen in Figs. 2(c) and (d).
The melting point is gauged by the disappearance of shear rigidity,
at which point the driving signal is abruptly turned off.
There is a brief coherent flow of particles as the pressure in the
system equilibrates after the driving stops.
Because imaging is difficult during this transient, we start taking
data at 
sec after cessation of shearing.
Although shear melting releases the lattice's latent heat of crystallization, the heat capacity of the surrounding water is so large that the system's overall temperature does not change measurably. This isothermal isobaric process therefore produces in a supercooled fluid whose degree of supercooling is exactly the binding free energy of the initial crystal's unit cell. Following our earlier work [28], we use this observation to define an effective supercooling parameter:
where 
is the volume fraction at melting
for this
colloid at the experimental temperature and ionic environment.
Eqn. (4) corresponds to the usual supercooling parameter
for conventional materials
under the assumption that
and after linearizing about the typical separation
.
This is at best a rough estimate of the degree of supercooling
for the present study
since 
is measured for bulk unconfined colloid in a different
sample cell.
The sample in this study has 
while the melting point
for the bulk fcc crystal is about 
.
The value 
corresponds
to a supercooling of roughly 
C in water.
