It’s Friday, so let’s do a weird one. This new paper describes a strange thing that happens with very small particles of sulfur, but it has implications for nanoscience research as well as illustrating the sorts of phenomena that we are able to detect these days. I’ve mentioned before that one of the biggest changes in the chemical literature from my 1980s grad school days is the detail about nanoscale behavior (catalysts, surfaces, small atomic or molecular clusters, etc.) It’s interesting to realize that these things have always been going on right under our noses, from an instrumentation perspective, but it’s just that we haven’t had the ability to see any of it.
The authors were studying how particles of sulfur sublime off as the particle size gets smaller, and (as is so often the case) below a certain size limit bizarre things start to happen that you wouldn’t have been able to predict from our experiences up here in the macro-scale. Sublimation is of course when a substance goes directly from the solid phase to the vapor phase, without passing through a liquid one. That might sound a bit weird, but we’ve all seen it, because water does that sort of thing pretty easily. Consider the ice crystals that form on the inside of a plastic freezer bag if you have some air gaps in there. That’s water that has sublimed off from the ice crystals in the food and settled out on the surface of the bag. There’s certainly been no liquid water phase involved deep in the freezer; it’s all solid/vapor transitions. Loss of that water from whatever you froze in the bag will cause its exposed surfaces to gradually dry out, giving you what’s commonly called “freezer burn”. As a child in Arkansas, I used to wonder why, when I would try to save a snowball in the freezer for later use, it would became smaller and smoother, and there’s the reason. In the lab that phenomenon is exploited on purpose – you freeze aqueous solutions and put a hard vacuum on them with a cold trap in the system, and eventually all the water sublimes off and leaves you with your desired dry powder. That’s lyophilization, or freeze-drying.
Dry ice (solid carbon dioxide) is a familiar example in the lab of a substance that sublimes away and disappears without a liquid phase, since it has no such phase anywhere around room pressure. Sublimation can be a beautiful way to purify some substances – the crystals that result can be very light, feathery, and well-formed and are often very pure. Compounds with a lot of symmetry tend to be able to fly off into vapor phase a bit easier from the solid state, and you can often see sublimed crystals of things like iodine, ferrocene, and white phosphorus (assuming you work with that!) even when you weren’t deliberately trying to sublime them. Here’s a good video of ferrocene subliming. Sulfur sublimes very well, too, since as a solid (it can be very crystalline indeed) it’s mostly in the form of packed symmetrical rings of S8 (octasulfur), and those are what you see at ambient temperatures in the vapor phase, too (but as you heat things up the picture gets considerably more complicated).
What this new paper observes (using a home-built dark-field microscope rig that can track what’s going on) is that as you sublime off smaller and smaller solid sulfur particles, below a certain threshold you start seeing a dramatic Leidenfrost effect. That’s most familiar as droplets of liquid skittering across a hot surface – their evaporation allows them to hover just above the surface (and such droplets will last much longer than if they were actually in contact). This works with sublimation, too – small pieces of dry ice will go zipping across a room-temperature flat surface, riding on a thin layer of their own vapor.
But Leidenfrost behavior on the nanoscale has not been well investigated, and you would expect odd things to happen down there with such a phenomenon, depending as it does on vapor pressure, surface tension, heat transfer, and other physical processes that are going to skew oddly at those levels. Simulations suggest, for example, that water droplets in the nm size range would show Leidenfrost behavior at far lower temperatures than usual. The sulfur particles in this work get gradually smaller as they sublime away, but once their size gets below about 360 nm, they suddenly start to levitate and glide randomly around the surface. And this can happen even when there is no difference in temperature between the solid substrate and the particles, which is not something you’re going to see much on the macro scale. Comparing this with single particles of iodine and ferrocene (as mentioned, classic sublimation examples) suggests that the critical size is largely a function of the substance’s vapor pressure and density, which certainly makes sense.
This spontaneous takeoff stuff is either going to complicate nanoscale fabrication or provide new opportunities for it, depending on your general outlook on life (and of course what you might have gotten funding to try!). Subliming solids in general (at small particle sizes) have apparently been launching themselves off surfaces and levitating away the whole time without us knowing about it. Anyone working with substances at this scale when they can have any sort of appreciable vapor pressure will have to take this into consideration.
