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Temperature-dependant attractive colloidal
systems
H. Guo, T. Narayanan, M. Sztucki, P. Schall
and G. Wegdam
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Recently, attractive colloidal systems attract
increasing interest. These systems, in which the particles exhibit a tunable
attractive potential, show a rich phase diagram with solid, liquid and gas
phases. Often, metastable- that is kinetically arrested- states are also
formed, which are of interest for investigating gel and glass formation.
We study novel attractive colloidal systems, in which
the attraction strength is controlled with temperature: By just changing
temperature, we can induce transitions from a colloidal gas (where the
particles are very dilute) to a colloidal liquid (where the particles are
condensed, but still mobile) and from a colloidal liquid to a colloidal
solid (where the particles condense into a crystal). The trick is that the
particles are suspended in a binary liquid mixture close to demixing of the
two liquids, which gives rise to a solvent mediated attraction between
particles.
| Remarkably, in our temperature-dependant system, these
tranitions are reversible: We can ‘freeze’ a colloidal liquid into a solid,
and – by changing temperature back – melt the crystal again into a liquid.
Fig. 1 shows Small Angle X-ray Scattering measurements that we performed
at the ID02 beamline of the ESRF in Grenoble. Fig1(a) shows the
intensity profile, I, as a function of wave vector q for specific
temperatures. Fig1(b) shows the structure factor S(q)=I(q)/F(q), where
we have divided by the form factor F(q). We start at 57.5°C: particles
behave like a dilute gas phase (red curves, A). When we heat to 57.7
degrees, we observe a phase that is characteristic of a liquid –like
structure (black curves, B). Further heating to 58.5 degrees, our
intensity and structure factor profiles show different order peaks –
characteristic of an fcc crystal phase (blue curve, C)!
Now – guess what happens when we lower the temperature back to 57.5oC
? The crystal melts and we observe again the initial gas phase (orange
curve, D). That’s pretty cool. But there is one more
thing: our phase behavior depends on the rate with which we heat up: we
can form a glass instead of the crystal if we choose ahigher “quench” rate, just like in
molecular systems. We have even observed that this depends on the polydispersity of the particles, but maybe that’s a bit too much for here … |
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Fig.1 Small Angle xray measurements
Scattering intensity (a) and structure factor (b) of the colloidal
suspension at T=57.5C (red and orange), 57.7C (black) and 58.5C (blue) |
By the way, Fig. 2 shows a tentative
phase diagram that we constructed based on our X-ray measurements.
f
is the particle volume fraction, and DT is the
temperature difference to the phase separation of the binary liquid.
Fig.2 Tentative phase diagram
State of the colloidal system (G=gas, L=liquid, S=solid) in a
temperature - volume fraction diagram.
DT is the temperature difference from phase separation of the binary
liquid |
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