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(the fundamentals of) Electret charging by electrowetting

(the fundamentals of) Electret charging by electrowetting

Electrowetting-on-dielectric (EWOD) is a technique to alter the contact angle of a conductive liquid on a dielectric by applying a voltage across the dielectric and the droplet. It is known that the electrowetting functionality may degrade due to injection of charges from the droplet into the dielectric layer [1]. While negative for EWOD, it was recently demonstrated that the same effect can be beneficial for the production of electrets [2-4]. Electrets are materials with a quasi-permanent charge (separation), thereby providing a static voltage. Electrets are widely used in  microphones (most like you carry one in your mobile phone), and they have also been shown to be suitable in nanogenerators [3,4]. Little is known, however, on the microscopic mechanism of charge trapping. For instance, it is not even clear whether the relevant charge carriers are electrons or ions.

Research objective

Your research objective will be to quantify the charge injection by electrowetting for different electrolyte solutions and substrates. You will start with bilayered silica-Teflon AF1600 substrates and quantify the trapped charges while varying the charging voltage and the ions in the electrolyte solution. Ions can be large (for example ionic surfactants) or small (for example the hydroxide ion), monovalent or multivalent, and have various electron affinities (qualitatively this is the likelihood to donate an electron). You will relate the (magnitude of) charge trapping to these ion-specific parameters. The trapped charges are be quantified using a technique as proposed in references [3,4], using a high-frequency oscilloscope. Depending on your progress, you may eventually switch to bilayer silica-Cytop substrates, that display a strong pH-dependence on charge trapping [4].

Learning objective

Apart from the standard learning objectives for a bachelor’s project (research planning, academic writing, data presenting, how to work in a lab environment, etc.), you will:

·        Learn the theory and practical aspects of drop manipulation by electrowetting;

·        Learn how to fabricate the required samples;

·        Learn how to characterize electrets by using a nanogenerator setup with high-frequency current measurements.

Contact information

·        Prof. Dr. Frieder Mugele,

·        Niels Mendel, n.mendel@(student.) /


[1]          H. J. J. Verheijen and M. W. J. Prins, “Reversible electrowetting and trapping of charge: model and experiments," Langmuir, vol. 15, no. 20, pp. 6616-6620, 1999.

[2]          H. Wu,  R. Dey,  I. Siretanu,  D. v. d. Ende,  L. Shui,  G. Zhou, and  F. Mugele, “Electrically Controlled Localized Charge Trapping at Amorphous Fluoropolymer–Electrolyte Interfaces,” Small, 1905726, 2019.

[3]          H. Wu, N. Mendel, D. v. d. Ende, L. Shui, G. Zhou, and F. Mugele, “Physics of electrical generation from drop impacting on charged surfaces," arXiv preprint, 2019.

[4]          N. Mendel, “Droplet impact energy harvesting on a charge trapping-enhanced fluoropolymer-silica electret”, Master Thesis, University of Twente, 2020. (not published; ask for the report in an email to Niels Mendel)