It implies you have two different species of particles each with a different zeta potential. Without knowing anything else about your material, I can't be more specific. Likewise, without seeing the distributions, knowing how you measured the sample etc, it's hard to postulate.

Sometimes, something as simple as a dust in your sample could appear as a second type of particle.

Please share more information.

Hi Anna Shevchenko ,

this could be two populations of particles with different charge, so I agree with John. Typically, this could be either two positive or two negative peaks. In most practical situations, the mean zeta is of primary interest.

If the data are from a Zetasizer it would be great if you could attach a data file (.dts). To extract just a selection of records see https://www.materials-talks.com/blog/2014/08/19/faq-how-can-i-submit-a-data-file-to-the-help-desk/