The images just below are both coated by two particle systems. The particle object is Suzanne monkey. One particle system bears responsibility for covering surface. The other do inhomogeneous treatment on top of that like moss.
The base object has two particle system like below.
ParticleSystem has smaller size; 0.008. And its children is “interpolated” to cover entire object.
ParticleSystem1 has bigger size; 0.02. And children is “simple” to surround each parent particle ,which makes non-uniform surface.
The trick here is radius=0.3, which makes surrounding radius of children wider to be more flat distribution. Otherwise, the convergence will be more dense and unnatural.
3D view in the progress is below.
I think post-processing with filter > shadow in compositing node is suit for microscopic image like below
The particle setting came from my modeling project for fried shrimp that is still working in progress.