Manipulation of Contact Angle Hysteresis at Electrified Ionic Liquid-Solid Interfaces

Room-temperature ionic liquids (RTILs) are intriguing fluids that have drawn much attention in applications ranging from tribology and catalysis to energy storage. With strong electrostatic interaction between ions, their interfacial behaviors can be modulated by controlling energetics of the electrified interface. In this work, we report atomic-force-microscope measurements of contact angle hysteresis (CAH) of a circular contact line formed on a micron-sized fiber, which is coated with a thin layer of conductive film and intersects an RTIL-air interface. The measured CAH shows a distinct change by increasing the voltage $U$ applied on the fiber surface. Molecular dynamics simulations were performed to illustrate variations of the solidlike layer in the RTIL adsorbed at the electrified interface. The integrated experiments and computations demonstrate a new mechanism to manipulate the CAH by rearrangement of interfacial layers of RTILs induced by the surface energetics.

Figure 1

(a) Sketch of the AFM-based capillary force apparatus at a liquid-air interface. The Pt-coated hanging fiber and the metal cell work as two electrodes and the RTIL is filled in between. (b) Optical image of the RTIL meniscus in the vicinity of a vertical fiber with a diameter $d=3.5\mathrm{\mu }\mathrm{m}$. Red dots indicate the meniscus profile. The coordinate system for image analysis and simulations is shown.

Figure 2

(a) Variations of the measured capillary force $f$ and corresponding $\theta$ as a function of traveling distance $s$ during the first (black) and second (red) dips. Each hysteresis loop is measured when the fiber is first pushed downward (advancing $\to$, solid line) and then pulled upward (receding $\leftarrow$, dashed line) through an RTIL-air interface. Sketches on the right show experimental conditions. The shaded area on the fiber denotes prewetted surface with a SLL. $s^{\prime }$ denotes the position that divides bare Pt surface and SLL covered surface. (b) Measured hysteresis loops for different voltages $U=0.0\mathrm{V}$ (blue), 2.0 V (red), and 3.0 V (black) applied on the fiber surface. The measurements are made at a fixed fiber speed $v=5\mathrm{\mu }\mathrm{m}/\mathrm{s}$ and with a fiber diameter $d=1.5\mathrm{\mu }\mathrm{m}$.

Figure 3

(a) Measured capillary force per unit length $f/\pi d$ and the corresponding $\theta$ as a function of the applied voltage $U$ during advancing (solid symbols) and receding (open symbols) processes. Four fibers with different diameters $d$ are used in the experiment. Results from optical measurement [Fig. 3] and MD simulations [Figs. 4 and 4] are also shown. Error bars indicate standard deviations of the measurements at different positions on fiber. The dashed line indicates the threshold voltage. (b) Variations of the meniscus profile at different voltages $U$ applied on the fiber in advancing (solid symbols) and receding (open symbols) directions measured by optical images [Fig. 1]. The lines are fittings to Eq. (S1) in SM [25] with ${\theta }_i$ as the fitting parameter. (c) Measured CAH ($F_h$) as a function of $U$ with different probes. (d) $F_h$ measured with increasing or decreasing $U$.

Figure 4

(a) Snapshots from MD simulations showing ions in the two layers above Pt at $U=0.0\mathrm{V}$ (top) and 2.7 V (bottom). (b) Number density distribution of cation (black) and anion (red) as a function of distance $x$ away from the Pt surface at $U=0.0$ and 2.7 V. (c) Calculated interaction energy per unit area $E$ between different regions as a function of $U$. The dashed line indicates the threshold voltage. (d) Energy in different subdomains shows enhanced fluctuations in the two layers at 2.7 V. The dashed line indicates the mean value $E_{\mathrm{mean}}$ of $E_i$ in the two layers at 2.7 V. Inset shows the sketch of a pinning potential. (e) Snapshots from three-phase simulations showing ${\theta }_i$ and CAH at electrified surface $U=2\mathrm{V}$ (bottom). (f) Variations of $\theta$ in three-phase simulations show hysteresis loops for different $U$ similarly to Fig. 2. The red arrow indicates the frame shown in (e), and the corresponding $\theta$ is plotted in Fig. 3.