Why is blood negatively charged colloid

Colloidal crystals and spherical packings

Colloids are ubiquitous in everyday life. Researchers are investigating how the small particles can be arranged in the most space-saving way possible and what symmetries the resulting structures are based on.

Physicists understand colloids as particles just under a micrometer in size that move in a finely distributed manner in a carrier medium. The carrier medium can be water, for example, and the colloids can be small fat droplets or cellular components - as is the case with milk or blood. The particle sizes and distances are in the range of a few hundred nanometers and are therefore accessible for diffraction with visible light. Electron and light microscopy also allow a direct view of the arrangement of the colloids.

A particularly popular model system are so-called suspensions of electrostatically negatively charged spheres in low-salt or distilled water. As the concentration of the beads increases, the water initially becomes very milky and cloudy. As with fog, you can see the light scattered back evenly for all colors and in all directions. With a sufficiently high concentration of the colloids, however, you can see small crystals that shimmer in certain colors. A look through the microscope at the arrangement of the particles in these artificial crystals shows that the suspended matter particles have moved to regularly arranged grid positions. These grid positions take up the greatest possible distance from one another with the highest packing density at the same time. The packing density indicates how much volume the particles make up compared to the total volume.

How do you pack balls as space-saving as possible?

An interesting question is which crystal structures occur in these very simple and highly symmetrical particles. Since the repulsion of the negatively charged particles is the same in all directions, structures with high regularity - and thus great symmetry - have lower energy than irregular structures and random packings.

Crystal stacking

A very simple possibility for a symmetrical arrangement would be a cubic grid, which is composed of many small cubes, with the particles having their places on the cube corners. In addition to symmetry, efficient packing also plays an important role. Already Johannes Kepler argued at the beginning of the 17th century that the best way to pack spheres in a space-saving way is through a so-called face-centered cubic grid. Such a grid can be imagined as a cube in which the balls are not only located on the corners of the cube, but are also placed in the middle of the individual cube surfaces.

Crystal structures like stacks of oranges

To see in a practical way how the balls are arranged as optimally as possible, you can stroll through a large weekly market, for example. There you will often find artfully arranged stacks of fresh oranges or melons. If you take a closer look at these stacks, you can see that there is always an orange in the gap of three other oranges in the layer below. In the third layer, you have the choice of either choosing a gap that has not yet been used, or placing the additional orange exactly over one of the first layer. In the first case there is a so-called ABC stacking, in the second case an ABA stacking. Each crystal layer is assigned a letter, starting with layer A and depending on how the following layers are arranged in relation to the A layer, they are provided with the letter B or C. The ABC stacking corresponds to the face-centered cubic lattice already mentioned, the ABA stacking a hexagonal, i.e. hexagonal arrangement.

ABA stacking

The ABA stacking of a crystal.

Both packings actually have the largest possible space filling of 74 percent that can be achieved with balls when the balls are packed in abutment. Such arrangements, which take up space as much as possible, are found, for example, with noble gas crystals and many metals. In the case of colloidal crystals, these densely packed spheres regularly occur for proportions by volume greater than about thirty percent. Here, however, the external shape is given by the sample container. In the case of lower concentrations, the packing does not play such an important role; rather, because of the extensive repulsion, an arrangement is preferred in which the particles can move as far out of the way as possible, but in which there is still a high degree of symmetry. Here you can mostly find body-centered cubic structures, i.e. cube-shaped lattices in which the particles sit not only in the cube corners but also in the center of the cube itself.

Crystals in boxes

Researchers at the University of Utrecht even managed to stabilize spheres one micrometer in size in a grid with typical spacings of twenty micrometers. However, due to the large spacing, these crystals are extremely soft and delicate. They can be destroyed by simply shaking them, but will re-form immediately if the suspension is left alone. This offers great advantages if you want to learn the basics about crystallization processes, because here you can observe the "atoms" very comfortably with the microscope.

In an in-depth article, you will learn how the structure and properties of the crystals change when they are not grown in a large glass container, but in a very small space - that is, locked between two walls.