As part of my continuing series of images of everyday objects observed under high magnification the images in this post are of a piece of paper towel as observed via scanning electron microscopy. When we look at a paper towel with the unaided eye it appears to be a thin solid sheet. However as we can see under higher magnification, as in the first image above at 500x magnification, paper towels are actually composed of strands or threads of paper fibers that are woven and pressed into a matted sheet. Additionally as we can see in the image above even though paper towels appear to be solid sheets to the human eye they can contain small gaps and holes. In the image below, which is of the same area of the paper towel only now magnified 1500 times, we can see one of these individual strands that has come loose and is now sticking up slightly away from the bulk of the rest of the paper towel.
The first image below is an SEM image of a human hair as seen at approximately 1500 times magnification. A average human hair is approximately 100 microns in diameter and is a favorite yard stick for scientists, myself included, when describing very small objects. The hair in this particular image is slightly more than 70 microns in diameter.
The second image below is of the same human hair now magnified 7000 times. At this magnification you can now clearly see the outer layer of hair called the cuticle. The cuticle is composed of dead cells, overlapping in layers, which form scales and give the hair shaft strength. Measurements of the image below show that the scales stick up from the main body of the hair shaft about 1 micron high.
Scanning electron microscopy (SEM) is a powerful tool that scientists use to look at and understand the structure of tiny materials such as nanoparticles. However, one of the things I have always found fascinating is using SEM as a tool to examine common everyday objects in a new and unique way. One example I found fairly interesting, and the topic of this post, is common table salt and crushed pepper. These are two materials we see almost every day on our kitchen table, and at least to the naked eye don’t appear to be very interesting. However, when you look more closely (specifically 1000 times normal size as in the SEM image below) you can begin to see that these materials have very interesting and very different shapes and structure which of course leads one to wander, what are salt and pepper really made of and why do they look so different?
So what exactly are salt and pepper made of and why do they look so different? Well let’s first start with salt. Salt is actually a crystalline mineral comprised primarily of sodium chloride (NaCl). Because it is a crystal it has a very regular and well defined atomic structure which leads to its tendency to form cubic structures like the ones seen in the SEM image above. In fact, a newly formed salt crystal will typically have very sharp edges that get worn down by friction during packaging and handling to form a somewhat dulled edge like the ones seen in the image below. Another interesting fact about salt is that although it is a very safe and inert material (and is even essential to life) it is composed of two extremely reactive and toxic elements, Sodium and Chlorine. Elemental sodium, which is a metal, burns violently upon contact with water and is typically stored in oil to prevent its reaction with humid air. Elemental Chlorine, under normal conditions exists as a gas, and is extremely toxic and was even used as chemical weapon during World War I.
Pepper on the other hand (or more specifically the black pepper shown in the SEM Image below) is not a mineral like salt but is actually a fruit. Black pepper is produced by first boiling and then drying the unripe fruit of a pepper plant. This causes the fruit to shrink and the fruit’s skin to harden and wrinkle in a fashion similar to that of a raisin, which is a dried grape. Once the pepper has fully dried and hardened it is then crushed into the powder that is found in your typical pepper shaker. It is this drying and wrinkling process that causes the unique shape and structure you see in the SEM image above on the right, and this grain of pepper is just one of many shattered pieces of pepper that once comprised a whole individual peppercorn.