A charged surface immersed in a viscous electrolyte solution attracts ions of opposite charge. Yet the ions are also subject to thermal diffusion and hydrodynamic drag in the viscous medium. This creates a "cloud" of counter-ions above the surface known as the diffuse layer, electrical double layer, or Debye layer. The charge in the Debye layer is equal and opposite to the charge on the surface, effectively screening the charge on the surface. The thickness of the "cloud" is given by the Debye length , which is a function of ion concentration, ion valence, relative permittivity, and temperature of the fluid; you can vary these parameters using the controls. Interestingly, the Debye layer depends only on properties of the solution, not on the surface which it screens.
The Debye length is an important parameter in colloid science, particularly for electrophoresis (used for DNA sequencing and electrophoretic display technology) and electroosmotic pumping in microfluidics. The Debye length is also a measure of stability in a colloidal dispersion.