We developed a novel method to determine the zeta potential of porous substrates saturated with a liquid. When two droplets are connected by their internal pressure, a bistable system in terms of volume distribution is established. Switching between the stable volume distributions may be realized by electroosmosis, arising near the solid/liquid boundary within the substrate when an electric potential difference is applied. Measuring the deflection of the liquid/gas interface of the droplets gives the strength of the induced electroosmotic flow. A model relates the induced electroosmotic flow to the zeta potential resulting from the combination of substrate and liquid. Measurements of a borosilicate substrate in contact with different aqueous electrolytes are performed. The results of the measurements clarify the influence of the ionic strength and pH value on the zeta potential. The results are also used to derive a new empirical correlation, reducing the number of parameters which have to be determined.

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