text: The lower-limit size of a coffee ring depends on the time scale competition between the liquid evaporation and the movement of suspended particles.  When the liquid evaporates much faster than the particle movement near a three-phase contact line, coffee ring cannot be formed successfully.  Instead, these particles will disperse uniformly on a surface upon complete liquid evaporation.  For suspended particles of size 100 nm, the minimum diameter of the coffee ring structure is found to be 10 μm, or about 10 times smaller than the width of human hair.  The shape of particles in the liquid is responsible for coffee ring effect. On porous substrates, the competition among infiltration, particle motion and evaporation of the solvent governs the final deposition morphology.
  The pH of the solution of the drop influences the final deposit pattern. The transition between these patterns is explained by considering how DLVO interactions such as the electrostatic and Van der Waals forces modify the particle deposition process.