Aqueous dispersions of core/shell CdSe/CdS quantum dots as nanofluids for electrowetting

This paper illustrates the electrowetting behavior of aqueous dispersions of quantum dots (QDs) functionalized with 3-mercaptopropionic acid. The pH, QD concentration, type of QDs (core/shell CdSe/CdS versus CdTe/CdS) and the polarity of the voltage have been varied. The major parameter that controls the electrowetting properties is the polarity of the applied potential. This is explained by the electrophoretic transport of the negatively charged QDs to the in-liquid electrode (air/water interface) or to the fluoropolymer surface depending on the polarity of the electric field. In the negative range of applied voltage, the reversibility is excellent and contact angle hysteresis is minimal (1–2). The electric field favors the accumulation of the QDs at the air/water interface which reduces the liquid/vapor surface tension. The contact angle values depend mainly on the QD concentration regardless of pH and QD composition. For positive applied voltages, the electrowetting behavior becomes different. The adsorption of the QDs onto the fluoropolymer surface is observed when the electric field is applied. The adsorption increases with both the pH (negative zeta potential of the QDs) and the QD concentration. The surface roughness in the presence of adsorbed QDs leads to non-reversible electrowetting and significant contact angle hysteresis. The large contact angle change versus voltage at basic pH and high QD concentration is attributed to the enhancement of the capacitance of the fluoropolymer dielectric layer due to the presence of highly insulating QDs adsorbed onto the fluoropolymer surface.

The electrowetting behavior of aqueous dispersions of quantum dots (QDs) has been investigated by varying the pH, QD concentration, QD composition (core/shell CdSe/CdS versus CdTe/CdS) and the polarity of the applied voltage.Figure optionsDownload as PowerPoint slideResearch highlights
► Electrowetting properties are affected by the polarity of the applied potential.
► Negative voltage: highly reversible electrowetting with very low hysteresis.
► Negative voltage: contact angle values depend mainly on the QD concentration.
► Positive voltage: non-reversible electrowetting and large contact angle hysteresis.
► Positive voltage: large contact angle change at basic pH and high QD concentration.