science: a quick tangent
In the comments, someone mentioned the LCD. As it turns out, they got the explanation for how it works somewhat right. Somewhat.
It’s not his fault, he’s not an engineer or a device physicist, but I thought I’d take the time to bang out a quick tangential post about how liquid crystals actually do that thing that they do in your LCDs. It’s not horribly complicated, though it will require talking about things like “indices of refraction,” and “wave retardation,” or “polarization rotation.” I’ll try to make it as straightforward and easy to understand as possible.
The first thing to keep in mind about your neat LCD in your handheld device is that the foundation, engineering-wise, for that neat piece of technology is the twisted-nematic effect. That’s part of what the acronym TNTFT stands for - Twisted Nematic Thin Film Transistor. So what does “twisted-nematic” mean? Well, first you need to understand that the liquid crystal stuff that is in your LCD exists in the nematic phase. In the previous post I mentioned that one particular phase, referred to as the nematic phase, is a phase where all of the molecules are completely disordered in the bulk (like a fluid) but every molecule is oriented in a common direction (not at all like an isotropic fluid). Going back to the toothpicks analogy, this is exactly what you see in a giant box of toothpicks. Every toothpick in the box is lined up length-wise so they are all parallel, but they are all free to move in the box whichever way as long as they remain parallel. If you can imagine thermotropic liquid crystals as being shaped similar to toothpicks (rod-like, i.e. - calamitic), then you have a whole ensemble of rod-like molecules all pointing in a common direction, their long axis parallel to their neighbors throughout the sample.
Secondly, what the twisted-nematic effect depends on is that liquid crystals will respond to an applied field. If you fill a space between transparent electrodes (like indium-tin oxide coated glass) with some kind of mesogen (making a crude liquid crystal cell), and then apply some kind of electric field across the liquid crystal, you get all sorts of neat electrooptic responses from the liquid crystal. The response is dependent on all sorts of things such as how the liquid crystal likes to sit on the surfaces of the electrodes as opposed to the middle of the filled space, how much the molecules themselves affect light as it passes through the sample, and how sensitive the molecules are to an applied field. The simplified view is that the molecules will simply reorient themselves to minimize the amount of energy required to maintain an orientation in the presence of the applied field. In the case of the stuff in your LCD, that happens to be aligning their long axis along the same direction as the electric field. This, in effect, makes the molecules all “stand up” with respect to the applied field. The third thing the twisted-nematic effect exploits is that light interacts with the nematic molecules differently if you have the molecules’ long axis parallel or perpendicular to the plane of polarization of light. In other words, if you have polarized light where the polarization is in the xy-plane, having the long axis of the molecule along the z-axis versus in the xy-plane yields totally different results from the point of view of the polarized light. The twisted-nematic effect works the way it does because polarized light “ignores” the molecules when they “stand up” perpendicular to the plane of polarization.
See where this is going?
The construction of the twisted-nematic cell is as follows: first take your electrodes and coat them in stuff that forces the molecules to lay flat on the surface of the electrodes. Turn one of the electrodes to a right angle compared to the first, so that the long axes of the molecules on the surfaces of the electrodes must be at right angles to each other. Why is this important? Well, if you take linearly polarizing films and put them on the outside of the cell so that the planes of polarization are rotated ninety degrees to each other, light will be polarized in a direction perpendicular to the resulting polarizer - in the case where light is “unmolested” by anything in the cell, no light will pass through the system since the second polarizer blocks the polarized light (this is also how your polarized sunglasses work). If you fill the space between the electrodes with a carefully chosen nematic mesogen, you can arrange the system so that the two electrodes will force the individual molecules in the bulk to twist in order to meet the alignment requirements on the surfaces of the electrodes (remember the whole treating the electrode surfaces bit?). This is important because if you chose that nematic mesogen right, then the long axis of the molecules will twist throughout the bulk of the material; and the twist in the long axis will guide the polarized light - rotating the direction of polarization, so that at the other electrode the polarized light is rotated ninety degrees and able to escape through the second polarizer. In the “off” state where no field is being applied, the liquid crystal guides the polarized light so that it can pass through the second polarizer. If you apply an electric field, suddenly the molecules “stand up” along the applied field, interacting with the polarized light differently, and the polarized light “ignores” the nematic mesogen. This fails to rotate the polarized light and it can’t escape through the second polarizer. No light passes through and you get a dark state.
See how it all comes together?
By carefully picking your polarizing conditions, you can create an initial bright or dark state by having the molecules rotate linearly polarized light so that it can or can’t escape through a second polarizer. Application of the electric field creates a system where the linearly polarized light does not rotate and subsequently can’t or can escape through the second polarizer, respectively.
Now imagine a giant array of these twisted-nematic cells where you can individually address each little cell so that light can or can’t pass through a filter. Call each of these little cells a pixel and now you have an array or addressable pixels that turn on and off with the application of a field. Unfortunately, in order for the molecules to “stand up” along the field the field must continuously be on, but you can always create a drive scheme to balance out the applied field and choose nematic mesogens that operate independently of whether or not the molecules are “right side up” or “upside down.” Still, the continuous application of field means you’re always pulling current for the entire system to operate, and even minimizing the voltage necessary to cause the molecules to all switch orientation can’t get around a continuous draw of current. That problem is currently being addressed by the use of ferroelectric liquid crystals, but that is an entirely different system that requires another lengthy explanation, and I’ve already reached the word limit for this post!
It’s not his fault, he’s not an engineer or a device physicist, but I thought I’d take the time to bang out a quick tangential post about how liquid crystals actually do that thing that they do in your LCDs. It’s not horribly complicated, though it will require talking about things like “indices of refraction,” and “wave retardation,” or “polarization rotation.” I’ll try to make it as straightforward and easy to understand as possible.
The first thing to keep in mind about your neat LCD in your handheld device is that the foundation, engineering-wise, for that neat piece of technology is the twisted-nematic effect. That’s part of what the acronym TNTFT stands for - Twisted Nematic Thin Film Transistor. So what does “twisted-nematic” mean? Well, first you need to understand that the liquid crystal stuff that is in your LCD exists in the nematic phase. In the previous post I mentioned that one particular phase, referred to as the nematic phase, is a phase where all of the molecules are completely disordered in the bulk (like a fluid) but every molecule is oriented in a common direction (not at all like an isotropic fluid). Going back to the toothpicks analogy, this is exactly what you see in a giant box of toothpicks. Every toothpick in the box is lined up length-wise so they are all parallel, but they are all free to move in the box whichever way as long as they remain parallel. If you can imagine thermotropic liquid crystals as being shaped similar to toothpicks (rod-like, i.e. - calamitic), then you have a whole ensemble of rod-like molecules all pointing in a common direction, their long axis parallel to their neighbors throughout the sample.
Secondly, what the twisted-nematic effect depends on is that liquid crystals will respond to an applied field. If you fill a space between transparent electrodes (like indium-tin oxide coated glass) with some kind of mesogen (making a crude liquid crystal cell), and then apply some kind of electric field across the liquid crystal, you get all sorts of neat electrooptic responses from the liquid crystal. The response is dependent on all sorts of things such as how the liquid crystal likes to sit on the surfaces of the electrodes as opposed to the middle of the filled space, how much the molecules themselves affect light as it passes through the sample, and how sensitive the molecules are to an applied field. The simplified view is that the molecules will simply reorient themselves to minimize the amount of energy required to maintain an orientation in the presence of the applied field. In the case of the stuff in your LCD, that happens to be aligning their long axis along the same direction as the electric field. This, in effect, makes the molecules all “stand up” with respect to the applied field. The third thing the twisted-nematic effect exploits is that light interacts with the nematic molecules differently if you have the molecules’ long axis parallel or perpendicular to the plane of polarization of light. In other words, if you have polarized light where the polarization is in the xy-plane, having the long axis of the molecule along the z-axis versus in the xy-plane yields totally different results from the point of view of the polarized light. The twisted-nematic effect works the way it does because polarized light “ignores” the molecules when they “stand up” perpendicular to the plane of polarization.
See where this is going?
The construction of the twisted-nematic cell is as follows: first take your electrodes and coat them in stuff that forces the molecules to lay flat on the surface of the electrodes. Turn one of the electrodes to a right angle compared to the first, so that the long axes of the molecules on the surfaces of the electrodes must be at right angles to each other. Why is this important? Well, if you take linearly polarizing films and put them on the outside of the cell so that the planes of polarization are rotated ninety degrees to each other, light will be polarized in a direction perpendicular to the resulting polarizer - in the case where light is “unmolested” by anything in the cell, no light will pass through the system since the second polarizer blocks the polarized light (this is also how your polarized sunglasses work). If you fill the space between the electrodes with a carefully chosen nematic mesogen, you can arrange the system so that the two electrodes will force the individual molecules in the bulk to twist in order to meet the alignment requirements on the surfaces of the electrodes (remember the whole treating the electrode surfaces bit?). This is important because if you chose that nematic mesogen right, then the long axis of the molecules will twist throughout the bulk of the material; and the twist in the long axis will guide the polarized light - rotating the direction of polarization, so that at the other electrode the polarized light is rotated ninety degrees and able to escape through the second polarizer. In the “off” state where no field is being applied, the liquid crystal guides the polarized light so that it can pass through the second polarizer. If you apply an electric field, suddenly the molecules “stand up” along the applied field, interacting with the polarized light differently, and the polarized light “ignores” the nematic mesogen. This fails to rotate the polarized light and it can’t escape through the second polarizer. No light passes through and you get a dark state.
See how it all comes together?
By carefully picking your polarizing conditions, you can create an initial bright or dark state by having the molecules rotate linearly polarized light so that it can or can’t escape through a second polarizer. Application of the electric field creates a system where the linearly polarized light does not rotate and subsequently can’t or can escape through the second polarizer, respectively.
Now imagine a giant array of these twisted-nematic cells where you can individually address each little cell so that light can or can’t pass through a filter. Call each of these little cells a pixel and now you have an array or addressable pixels that turn on and off with the application of a field. Unfortunately, in order for the molecules to “stand up” along the field the field must continuously be on, but you can always create a drive scheme to balance out the applied field and choose nematic mesogens that operate independently of whether or not the molecules are “right side up” or “upside down.” Still, the continuous application of field means you’re always pulling current for the entire system to operate, and even minimizing the voltage necessary to cause the molecules to all switch orientation can’t get around a continuous draw of current. That problem is currently being addressed by the use of ferroelectric liquid crystals, but that is an entirely different system that requires another lengthy explanation, and I’ve already reached the word limit for this post!