You can fill objects with particles, or cover their surface with a regular particle matrix:

• Add a "Fill" emitter and drag the object meant to be covered with particles, to the emitter's "Body" slot.
• Set "Particle Distribution" to "Particle Layer".
• The emitter's "Emission" tab collapses and shows a reduced parameter set.
• In order to get a more or less dense particle distribution increase/decrease the fluid container's "Resolution".
• If the particle layer is not updated automatically, reset the scene: go to the timeline's last frame, and then back to frame 0.

With the help of the "Texture" option it is possible to restrict the particle layer to the patterns of a shader or bitmap:

• Create a standard material and add a bitmap or a shader like noise to one of the channels, e.g. "Color".
• Drag the material's texture thumbnail to the "Texture" slot of the parameter.
• White parts of the image/shader will be used to create the particles.
• Reset, if necessary.