Colloids and Experiment

Electrical charges interact strongly with colloidal particles whose diameters are about 25 Å in diameter, typical of silver particles in a photographic image. Here we see a charge passing close to an ordered colloid. The image charges induced in the colloid are left behind and exert a retarding force on the particle causing it to lose energy. An alternative view is that electromagnetic energy tunnels from the particle into the colloid where it is dissipated in the complex mode structure.

6eV loss colloid
image Howie and Walsh (Micros. Microanal. Microstruc., 2, 171, 1991) have studied aluminium colloids with an electron microscope fitted with energy analysis capabilities. They can choose to create an image using only electrons with a certain energy loss. On the right is an image of a colloid created from electrons with a 6eV loss. scale: 1cm = 100 Å

Stopping
power vs. energy graph

Using our theory we can calculate the probability of a given energy loss by electrons in colloids. We assume that it is the density of the colloid that determines the loss properties (this sample has a 12% filling fraction), and replace the disordered sample of the experiment with the ordered colloid of our theory. Here we see the dominant 6eV loss feature reproduced. The nature of modes absorbing energy is explored in the other pages.


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