This was a test produced in order to achieve a similar effect to a watercolor ink being applied to paper.
The example below was created with Particle Flow in 3dsmax 6.0 by animating an emitter along a spline and animating the speed (spread) of the particles. The slower the emission, the thinner the line and vice-versa. The example above used 100,000 particles.
Created by Richard Rosenman.