Phase change involves the collective movement of bulk
molecules. Such bulk molecular re-organisation can therefore be a relatively
slow process compared to the time scales that can be monitored with modern
lasers. Despite this the dynamics of phase change are not entirely understood
in terms of molecular motion and changes in molecular contacts such as
We have attempted to examine the dynamics of phase
change using probes of both morphology and molecular interactions.
The main trick that we use is to generate a very rapid
(8ns) temperature jump (T-jump) in aqueous solutions by Raman shifting
1064nm light from an Nd-YAG laser to 1900nm in a high pressure cell
containing 35 atmospheres of hydrogen. 1900nm light can be absorbed by
water combination bands vibrationally heating the solution without needing
to dissolve an extra species in the system to absorb light.
|In this way we can rapidly induce phase change in a neat water solution.
For example we can boil water in under a microsecond (as shown on
the right). 1900nm light is absorbed by a liquid jet raising its temperature
far above its boiling point and after 25ns we see a dark patch form on
the liquid surface. This dark patch swells and grows and eventually explodes
into a plume of ejected material re-condensing into droplets by about 20
microseconds after the initiation of the explosive boiling.
These images were taken using the method of shadowgraphy, in which
we use a fast strobe light produced by photo-excited fluorescent dyes to
capture images of a fast process on a CCD camera. The timing of the strobe
is regulated by timing the T-jump laser pulse with a second laser that
excites the strobe producing dye solution.
Phase Separation In Binary Liquid Mixtures.
Some binary liquid mixtures have a lower critical consolute temperature
(an LCST) meaning that they mix at low temperature BUT de-mix at higher
temperatures. We have used our T-jump to very rapidly send such mixtures
(a)triethylamine/water J. Phys.Chem. B, (2003) 107, (41), 11411
-11418.) and b) 2-butoxyethanol/water Phys. Rev. E, rapid commun.
(2003), 68, 020501-1-4) from the mixed state to the un-mixed region of
the phase diagram and monitored the changes in these systems using shadowgraphy
and Raman spectroscopic probes.
In this way we have shown that molecular level changes finish after
about 1-2 microseconds (left) at a time when phase domains are themselves
extremely small (~50-100nm). Phase domain growth was shown to be self similar
and isotropic forming bi-continuous phases that grew with persistent growth
exponents for several hundred microseconds.
This kind of domain growth is characteristic of spinodal phase change.
From our work it seems that the early and intermediate stages of phase
separation are over within 2 microseconds and after this time we simply
observe the later stages of spinodal phase change when the domains merge
and coarsen to reduce surface energy.