Getting ever closer to the critical temperature, we continue with our slow, cooling temperature changes to the sample. Today we passed the 1-milliKelvin mark, working our way into the 100-microKelvin decade.

Our goal is to get to Tc + 100 microKelvin and take good decay rate measurements there. To do that, we need to know the location of Tc--on our thermostat's thermometry scale--with a resolution better than 100 microKelvin. Starting to refine our knowledge of Tc's location has been the focus of the last 48 hours.

The 100-microKelvin decade is relatively unfamiliar territory. We did get some data there on the first Zeno flight, but it hasn't been seen like this since then. Simply put, the critical point in earth's gravity seems ill defined within the 1-milliKelvin decade: density gradients in the fluid smear out the diverging thermodynamic effects and the critical point becomes a critical blob.

So, we move carefully, taking data at each step, seeing how it supports our current thinking about where Tc is, refining that location and then using that knowledge to move to the next temperature and start the iterative process over again. As I write, we have just settled in to a temperature 560 microKelvin above Tc, good to at least 200 microKelvin.

To give some idea of just how close we get, how lightly we tread, and how some of the properties of the fluid are diverging on the approach to Tc, here's a graph of forward-scattering intensity data that we have collected so far during flight:

The horizontal scale, showing distance from Tc in milliKelvin, is a linear scale in this graph, to give a feeling for the divergence of the forward-scattered intensity. The vertical scale, however, is logarithmic, because the changes are so large, and these data stop further than 1 mK from Tc!

This next graph is a record of some of the decay-rate data we've collected. These are the fluctuation decay rates measured in our forward-scattering direction at an angle near 12 degrees; both scales are logarithmic. The measurements are shown as symbols connected by a line; the line is a theoretical curve. One thing evident in this graph is the critical slowing down: in going from 100 mK to 10 mK above Tc, the decay rate has dropped by nearly a factor of 10.

Studying the data in this curve, plus corroboration from forward-scattering intensity plots and the behavior of the turbidity measurements is what we use to refine our knowledge of Tc. A neat trick when we do it well. So far so good!