Measurement of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate

A novel instrument allows for the first time measurements of the lateral adhesion forces at a solid-liquid interface, $f_{\parallel }$, in a way that is decoupled from the normal forces, $f_{\perp }$. We use it to measure how $f_{\parallel }$ between a drop and a surface is influenced by different $f_{\perp }$ and different histories of drop resting periods on the surface prior to sliding, $t_{\mathrm{rest}}$. The variation of $f_{\parallel }$ with $t_{\mathrm{rest}}$ is similar for different $f_{\perp }$ and always plateaus as $t_{\mathrm{rest}}\to \infty$. We show that the $f_{\parallel }$ plateau value is higher when $f_{\perp }$ is lower. This seemingly counterintuitive result is in agreement with recent theories.

Figure 1

The experimental setup of the CAB. A rotating arm has a closed chamber (1) at one end and a counterbalance (3) at the other. The chamber, drawn with its door open, holds a light source and a camera between which the drop is placed as shown in the right inset. The signal from the camera is transferred to a control box (2) which runs on battery and which further transfers the signal wirelessly to a computer placed nearby outside the rotating assembly (not shown). The angular velocity is monitored using an encoder (5) that touches a round enlargement in the shaft which in turn is connected to a dc motor (4). Thus force measurements are coupled with the in situ video signal of the sliding object (drop in this study). By independent manipulation of the angular velocity (measured in 5) and the tilt angle (1), the CAB allows for any combination of normal and lateral forces.

Figure 2

The procedure of a single force datum measurement exemplified using the system of hexadecane drop on a Teflon surface [17]. (a) The variation of the angular velocity, $\omega$, during the measurement time: the drop is allowed to rest in the stationary CAB for a prescribed period, $t_{\mathrm{still}}$, after which $\omega$ is gradually increased until, at a certain critical value, ${\omega }_c$, the drop starts sliding along the surface. The drop is pinned to the surface from right after placement until just before ${\omega }_c$ is reached; this whole time is termed $t_{\mathrm{rest}}$. We consider pendant and sessile drops. (b) Drop pictures as taken at different stages of the measurement. From (i) to (ii) no lateral force is applied and the drop is symmetric and pinned to the surface; during the active stage, it is deformed as shown in (iii) but it is still pinned to the surface. Once ${\omega }_c$ is reached the drop slides and hence in (iv) we see only part of it in the frame. We see that the lateral force required to slide the drop is higher when the normal force is lower.

Figure 3

The drop retention force, $f_{\parallel c}$, required for the onset of lateral motion of a $3.3\mu \mathrm{l}$ of sessile (▴) and pendant (▿) hexadecane drops on an OTA treated mica surface [23] as a function of the time, $t_{\mathrm{rest}}$, that the drop rested on the surface prior to sliding. We see that the lateral force required to slide the pendant drops is higher than that for sessile drops.

Figure 4

The drop retention force after reaching a time plateau, $f_{\parallel \infty }$ (cf. Fig. 3), required for the onset of lateral motion of a $0.5\mu \mathrm{l}$ hexadecane drop on an OTA treated mica surface [18] versus the normal force, $f_{\perp }$, that the drop experiences. The lateral force is normalized by unit length ($V$ is the drop’s volume).