(Invited) Octadecylphosphonic Acid Self-Assembled Monolayers in Low Voltage Electrowetting-on-Dielectric Systems
We studied self-assembled monolayers on anodic aluminum oxide. The monolayer films are formed using various solvents, immersion times, and formation temperatures. Characterization used contact angles and AFM to assess the quality of the films. Contact angles were studied in both a static and a dynamic configuration to assess the SAMs ability to serve as a hydrophobic layer for EWOD systems.
Static contact angles are measured optically through a Ramé-Hart goniometer system, allowing full electrowetting characterization, such as in Figure 1. The three regions shown are low voltage EWOD, which is modeled according to the Lippman – Young (LY) curve at low voltage: cos(θ) = cos(θ0) + cV2/2γ; where c is capacitance, V is applied voltage, γ is the gas-liquid interface tension, and θ0is the equilibrium angle. This is followed by a saturation region, giving a limiting angle below which an increase in electric field has no effect. Finally, at high field the monolayer desorbs causing a large drop in wetting angle.
Dynamic contact angles describe three phase boundary motion giving “advancing” and “receding” contact angles, with their difference defining the contact angle hysteresis (CAH). The measurements are made using the Wihelmy plate method and a Krüss tensiometer; the technique consists in dipping the plate in the liquid electrolyte, measuring the force as a function of the immersion depth. CAH gives an insight into the homogeneity of the SAM films, since any discontinuity in the SAM may obstruct the drop movement; this is observed in the saturation region of Figure 1. Additionally, the raw force-depth data from the tensiometer system show how the contact angle changes across the sample and through repeated cycles, as shown in Figure 2. The deviation from a linear behavior during immersion and emersion (Figure 2) suggests the presence of inhomogeneities in the SAM; the lines in fact smooth out for SAMs formed at higher immersion time.
 F. Mugele and J.-C. Baret, “Electrowetting: from basics to applications,” J. Phys. Condens. Matter, vol. 17, no. 28, pp. R705–R774, Jul. 2005.