NOTE: This post is the second in a series of posts trying to explain what it is that I research. I will make every effort to make it accessible to the layman (my family). The first post about Brownian motion can be found here.
Today we will discuss what are liquid crystals and how we can use depletion interactions to form them. To start out, a liquid crystal is a phase of matter that can be hard to understand. Essentially, it is kind of like a crystal in that the molecules that make up the material show a definite order. It is this order that gives such remarkably perfect geometric shapes to the crystals we put in our jewelry. However, those crystals are solid, whereas what sets a liquid crystal apart is that it is able to change shape to fill the container it is in, just like a liquid. This gives it tremendously useful properties. Pretty much anybody that reads this post will be reading it from a screen that uses liquid crystals to change the color of the light at each pixel on the screen, making a picture that we understand. That is why flat panel televisions and computer monitors are called LCD’s, it stands for Liquid Crystal Displays.
There are two main types of liquid crystals and they are called nematic and smectic. The difference between them can be subtle to explain, but pictures help show it fairly clearly. The nematic phase is when the molecules making up the liquid crystal all are oriented in the same direction, but they are not in any order in relation to one another. For example:
As you can see, all of the rod shaped molecules are roughly aligned vertically, but they do not form any kind of pattern otherwise. The smectic phase on the other hand has what we call “spatial order” in addition to the alignment of the nematic phase. For example:
See how the molecules are neatly arranged into a regular pattern of rows? That kind of regularity is what the smectic phase has that the nematic phase lacks. Something to keep in mind when thinking about this is that the particles making up these pictures are free to move, that is what makes them liquid. But what makes them a liquid crystal is that even when they move around, they keep their ordered nature. I hope that helps you understand a little bit more about what liquid crystals are. They have really interesting properties, but I won’t get into those now, maybe in a much later post. Now we need to move on to depletion interactions.
A depletion interaction only happens when we mix two different types of molecules together, one big and one small. So, let’s imagine that we have a bunch of people floating in our zero-g room that is filled with super balls from last time. As the balls bounce off of each person and they slowly drift around the room, they will eventually run into one another. When they get close enough to one another, they will block any super balls from coming into the space between them. This means that the super balls will be hitting the side of their body that is facing away from the other person, but not hitting the side of their body that is facing towards the other person. The same is true for the other person. That means that the super balls are pushing the people together. For example, imagine the people are rods, and the super balls are ball shaped molecules, we would have a situation like this:
The interaction caused by this, the attractive force between the rods, is called a depletion interaction because in the volume between the two rods, the concentration of the balls is depleted to zero. You might be able to see where I am going already, but if not, we are almost there. The final step is to imagine instead of just two rods, lets put in many many rods. What is going to end up happening is that these rods will bunch together. As new rods get close to a bundle, they will join it and as bundles get close to each other, they will merge, making a bigger bundle. Eventually what we end up with is pancakes of these rods that float around.
Does that remind you of anything we talked about already? Bingo! It is a nematic liquid crystal! It is nematic because they are all aligned in the same direction, but they do not arrange themselves in any kind of order with respect to one another. In other words, if we were to look at the pancake from the top down, there would not be a regular pattern of rods, it would be random. We can make a smectic liquid crystal from this system though. All we need to do is add more little balls to the solution. The higher the concentration of balls, the stronger the force pushing the rods together is. Once we get a strong enough force, we get smectic liquid crystals that look like this:
Basically, these nematic pancakes form regular layers. This regularity makes this liquid crystal smectic rather than nematic.
In the Laboratory:
What is really cool is that we can see these phases of liquid crystal with a microscope. To get rods, we use a virus that is shaped like a rod naturally. For balls, we use a chemical called PEG that is just a long strand that wraps around on itself into a ball shape, like the picture above. The viruses are too small to see individually, but when many of them bunch together into these pancakes, they grow to a size that can be seen under a microscope. Here is a sample with many of these pancakes:
The reason these membranes aren’t all merging even though it looks like they are touching each other is that they are at different depths in the sample, so some are in the foreground and some are in the background. Finally, we also are able to see the smectic phase under a microscope:
The bright bands in the image are the different pancake layers. I know I covered a lot in this post, so I do not blame you if you got lost or think I can explain something better. Leave me a comment and I will be sure to answer it.
That does it for this post! Tune in next time to learn about optical tweezers and how we use them to control samples under a microscope.