Electrowetting behavior of liquid drops has been followed in-situ and in-vacuum using XPS in a chemically resolved fashion, under both D.C. and A.C. excitations. Various liquid drops, compatible with the UHV conditions, consisting of an Ionic Liquid, DEME-TFSI, [N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide]), or Poly-Ethylene-Glycol (PEG, M.W. ~600 amu) and their mixtures. For the dielectric substrate, a ~300 nm thick silicon oxide (SiO₂/Si) without and with a thin (~20 nm) hydrophobic amorphous fluoropolymer coating, CYTOP (Bellex International Co.), has been employed. XPS data have been recorded both in the conventional scan and also in the fast (<1s) snap-shot modes. Intensity and position of the peaks, representing the liquid drops (F1s in the case of the IL, or C1s/O1s of the PEG) as well as those of the substrates (Si2p for the oxide only and F1s for the hydrophobic coated one) have been recorded under various electrical excitations. Under AC excitation at a fixed frequency, intensity modulations in the XPS peaks reveal geometrical changes of the drops, while the peak position modulations reveal electrical potentials developed. Monitoring binding energy modulations as a function of the changes in the AC frequency (10^-2 – 10⁴ Hz) allows us tap into ionic, dipolar and electronic contributions of the dielectric susceptibility of both the liquid drops and the substrates. Experimental details and various applications will be presented and discussed.

This work is partially supported by TUBITAK through Grant No. 215Z534