Friends Like Salad Dressing

Yohei Fujitani (Professor, Department of Applied Physics and Physico-Informatics)

For the past two years or so, I have been spending my time with the uniform phase of a binary fluid near its phase separation critical point.

For example, if you mix a tablespoon each of aniline and hexane, you get a binary fluid. At room temperature and pressure, this fluid separates into two phases. Some salad dressings are like that, right? But of course, you shouldn't pour this on your salad. One phase is almost entirely aniline, and the other is the opposite. Shaking the bottle won't make it uniform. However, if you warm it up a bit, above a certain temperature, the two phases will mix on their own to a uniform concentration. This is the uniform phase. In other words, as you raise the temperature, the two phases undergo a phase transition into a single phase. This probably doesn't happen with salad dressing.

In the example above, we used a tablespoon of each, but if you adjust the component ratio just right and raise the temperature, it undergoes a phase transition through a special state called the phase separation critical point. Near this point, the fluctuations in component concentration are large, and the fluid becomes sensitive to stimuli. It's like how people's emotions become more intense when their minds are unsettled, isn't it? In that sense, it's very human-like. Now, the uniform phase of a binary fluid near its phase separation critical point is starting to feel like a friend, don't you think? It's a shy one. When it doesn't pass through the critical point, it undergoes a sudden phase transition and remains insensitive. That's not a friend. So as not to offend my other friends besides the aniline-hexane system, I'll call them component A and component B.

Let's put a single micron-sized spherical particle into this fluid. Being a friend, it will probably allow that. The surface of the particle is picky and selfishly says it prefers component A over component B. Since the fluid is at a sensitive age and susceptible to stimuli, the favored component A forms a thick layer around the particle. As a result, the force the particle experiences from the fluid when it moves changes compared to when there is no such thick layer. It's like how you struggle to get off a crowded train when you're wearing bulky clothes, right? It's the same thing. Well, actually, it's different. The osmotic pressure originating from the concentration gradient comes into play.

I consider various similar situations and calculate by hand, as much as possible, what happens to the force experienced by the object when it moves. There are some results that contradict the "crowded train" analogy, which makes it quite enjoyable. The reason I do calculations by hand is, after all, because I've always loved doing it. I've been calculating by hand whatever caught my eye. Yes, it's as if I've grown older while chasing dragonflies I thought I could catch, sometimes succeeding, sometimes failing. The sun is setting now, and I feel a bit uneasy, but I've reinterpreted the saying "youth is fleeting..." to mean "the heart of a youth ages easily," and I continue to chase dragonflies as always. Besides, I feel my skills have improved, even if just a little.

Shhh. This next one seems a bit bigger.