Electrowetting of Hydrofluoroether Liquid Droplet at a Gold Electrode/Water Interface: Significance of Lower Adhesion Energy and Static Friction Energy

Abstract

We explored the electrowetting behavior of a hydrofluoroester solvent, Novec 7100 (Novec), as a liquid droplet on a Au(111) electrode in water (0.05 M KClO 4 ). Comparison with the electrowetting of hexadecane (HD) highlighted the significance of the lower adhesion energy and static friction energy of Novec than those of HD. The electrode potential-dependent contact angle θ of a Novec droplet showed little hysteresis. When potentials were set by means of potential steps, a Novec droplet increased its θ at more positive potentials than the potential of zero charge (pzc) of the Au(111) electrode. We found that the key factor of the electrowetting behavior for Novec is its low adhesion energy and static friction energy. The static friction energy of the oils to the Au(111) electrode surface was evaluated by a comparative analysis of the potential dependence of the interfacial tension at the solid/water interface, Δγ S/W −E curve, calculated from electrochemical surface charge data and the experimental cos θ−E curve: 2.6 mN/m for HD and 0.95 mN/m for Novec. When Br − was added in the aqueous solution to allow its adsorption on the Au surface surrounding a Novec droplet, the potential of maximum cos θ was shifted to negative. Overall, although the Novec droplet showed a narrower range of θ change than a HD droplet, the Novec droplet seldom got stuck to the surface as far as potential step was used, reflecting the narrower plateau region of θ near the pzc. Also, the specific adsorption of a coexistent anion was a significant factor of θ. This work has featured the significance of a slippy droplet on an electrode surface, giving an impact on the technology of microfluid transportation control by electric potentials.

Manipulation of a Nonconductive Droplet in an Aqueous Fluid with AC Electric Fields: Droplet Dewetting, Oscillation, and Detachment

“…The oil droplet is adhesive on the parylene surface, increasing the barrier of contact line motion. 51,73 This can be further proved by the large contact line friction factor given in Table 1.…”

“…When an AC electric field is applied, the CA hysteresis is also reduced due to the periodical depinning of the contact line from the surface. ,, However, for the present oil/water system, the threshold voltage still exists because we used a naked parylene surface. The oil droplet is adhesive on the parylene surface, increasing the barrier of contact line motion. , This can be further proved by the large contact line friction factor given in Table .…”

“…The disturbance of the contact line is slow when f is low, and the transient input electrical force splits the droplet at the base when the voltage is applied. Differently at the high frequencies, the contact line is disturbed violently and the whole droplet is actuated effectively, , and as a result, the whole droplet is removed from the substrate.…”

“…The difference is that the actuated droplet is nonconductive, and its motion is caused by putting it into a conductive surrounding fluid. Thus, strictly speaking, the droplet manipulation is still within the scope of EW, and the setup can be categorized as an oil-in-water system (“oil/water system” for simplicity) in EW. ,, …”

“…Thus, strictly speaking, the droplet manipulation is still within the scope of EW, and the setup can be categorized as an oil-in-water system ("oil/water system" for simplicity) in EW. 15,50,51 For the oil/water system, it is the competitive effect of EW in the aqueous fluid that causes dewetting of the oil droplet and ultimately leads to its detachment. However, studies on the droplet dewetting/detachment in oil/water systems are rather limited.…”