Microdroplet Manipulation at Ultra-Low Voltages on Conjugated Polymer Using Electrochemical Tunable Wetting

Manipulation of microdroplets has broad applications in microfluidic systems, lab-on-a-chip technologies, biotechnologies, and sensor devices. While the electrowetting on dielectric (EWOD) technique has been successfully used for the manipulation of microdroplets among other technologies, the actuation voltages of 12-80 V of EWOD-based techniques result in less portable power sources and potential interference with biological applications. Therefore, an alternative technique to achieve low-voltage manipulation of liquid droplets is highly desirable.

This study demonstrated a manipulation of organic microdroplets at 0.9 V by utilizing the electrochemical tunable wetting and adhesion properties of the dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)) surface. The PPy(DBS) switched between an oxidized and a reduced state upon application of  low voltages. The PPy(DBS) surface reduced at -0.9 V is more oleophobic and less adhesive to an organic droplet than the PPy(DBS) oxidized at 0.6 V. We first demonstrated a lateral transportation of a dichloromethane (DCM) microdroplet from one PPy(DBS) electrode to another in 0.1 M NaNO₃ solution at 0.9 V. The PPy(DBS) electrodes were fabricated using an electropolymerization method on Au/Cr patterned silicon dioxide substrates. The PPy(DBS) electrodes were oxidized first by applying a voltage of 0.6 V. When a voltage of -0.9 V was applied to reduce a PPy(DBS) electrode, a wettability gradient was induced between activated and inactivated electrodes. Consequently, a DCM droplet placed across the boundary of these two electrodes moved from the reduced electrode, which was more oleophobic, to the inactivated electrode. 

We also demonstrated a vertical capture-release of a DCM microdroplet at 0.9 V by utilizing the tunable adhesion of PPy(DBS) surface. To achieve a transition from super sticky to super slippery surface, a microstructured PPy(DBS) surface was used to amplify the change in adhesion force during the reduction and oxidization of PPy(DBS) surface. The microstructured PPy(DBS) surface was fabricated through an electrodeposition process on micropatterned Au/Cr coated silicon substrate. The oxidized PPy(DBS) microstructure exhibited a super sticky surface while the reduced PPy(DBS) microstructure was super slippery to a DCM droplet. For the vertical capture-release of a DCM microdroplet, the PPy(DBS) microstructure was first oxidized at 0.6 V before being placed in contact with a DCM microdroplet. Due to a strong adhesion force, the microdroplet was immediately captured by the oxidized PPy(DBS) microstructure after contact. The microdroplet was then lifted away from the glass substrate with the PPy(DBS) microstructure for transportation. After the transportation, the DCM microdroplet was released from the PPy(DBS) microstructure at a potential of -0.9 V since the reduced PPy(DBS) microstructure behaved like a super slippery surface. Throughout this whole process, the DCM microdroplet remained fully intact and no residual liquid remained on the PPy(DBS) microstructure.

In summary, this study successfully demonstrated an ultra-low-voltage manipulation of an organic droplet in both lateral and vertical directions by using the electrochemical tunable wetting of PPy(DBS) surfaces. This technique has promising applications in microfluidics, antifouling, and biotechnologies.