Correlation Spectroscopy of Microbe Movements

The movement of microbes in a fluid can be observed with a microscope. They move for several seconds a straight path, but suddenly change direction. If they are killed by chemicals added to the fluid, their characteristic movement changes and can be described by a Brownian motion if there is no external perturbation. With correlation spectroscopy (Sect. 7.12), the average mean-square velocity (n ) and the distribution f v) of the velocities of living and dead microbes can be measured. 

The sample is irradiated with a HeNe laser and the scattered light is superimposed with part of the direct laser light on the photocathode of a photomultiplier. The 

Another technique uses a stationary Moire fringe pattern produced by the superposition of two inclined beams of the same laser (Fig. 10.34b). The distance between the interference maxima is A = X sin( a), where a is the angle between the two wave vectors. If a particle moves with a velocity v across the maxima, the scattered light intensity Is(t) exhibits periodic maxima with a period At = A/(v cos P), where is the angle between v and (k -h k2). 

The sample is irradiated with a HeNe laser and the scattered light is superimposed with part of the direct laser light on the photocathode of a photomultiplier. The scattered light experiences a Doppler shift Av = v v/c) x (cos 1 1 — cos 1 2), where and 1 2 are the angles of the velocity vector v against the incident laser beam and the direction of the scattered light, respectively. The frequency distribution of this heterodyne spectrum is a measure for the velocity distribution. 

Measurements of the velocity distribution of E. coli bacteria in a temperature-stabilized liquid gave an average velocity of 15 xm/s (Fig. 15.23), where the maximum speed ranges up to 80 xm/s. Since the size of E. coli is only 

Another technique uses a stationary Moire-fringe pattern produced by the superposition of two inclined beams of the same laser (Fig. 15.17b). The distance between the interference maxima is A = Asin(ia), a being the 

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