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Commercially available AFM probes are typically pyramidal or conical. While such probes detect wetting force, it can be complicated to extract surface tension and contact angle from the force distance (F-D) curves on liquid surfaces. These problems include imprecise knowledge of the length of the contact line and the angle of the meniscus with the facets of the tip. Figure 1a shows the F-D curve of a tapered AFM probe on water. The F-D extension curve (red) of the probe into the liquid and retraction curve (blue) from the liquid both show a roughly parabolic shape, with near vertical curves at entry into and just prior to removal from the liquid. With increasing insertion depth the attractive force continues to grow, corresponding to the increasing length of the contact line. This large and growing wetting force may even overwhelm the cantilever stiffness and result in the unintended immersion of the cantilever into the liquid.
In contrast, constant diameter High Aspect Ratio Needle-Probes (Figure 2) with atomically flat surfaces can be used for force-distance (F-D) AFM measurements of wetting and drag forces made with the probes. For a probe that has a constant diameter tip, there is a step change in force when the tip first touches the liquid (Fig. 1b). Then the force remains constant as the tip is extended until the meniscus contacts a second section of the probe that is larger in diameter. At this point in the can the wetting force increases again in a stepwise fashion. Therefore, a constant diameter tip has the desirable feature of maintaining a stable and constant wetting force over a range of insertion depths. A single F-D measurement similar to Figure 1b, with this constant diameter Needle-Probes can be used to estimate surface tension, contact angle, meniscus height, evaporation rate and viscosity altogether from a single F-D experiment.
Figure 1. Comparison of F-D curves of liquids for AFM tips that are (a) standard tapered AFM tips with pyramidal cross section and (b) constant and/or dual constant diameter. The liquid is dibasic ester measured at room temperature.
The surface tensions for several low molecular weight liquids that were measured with these probes were between -4.2 % and +8.3 % of standard reported values. Also, the F-D curves show well-defined stair-step events on insertion and retraction from partial wetting liquids, compared to the continuously growing attractive force of standard tapered AFM probe tips. A nanoneedle with a step change in diameter at a known distance from its end produced two steps in the F-D curve from which the meniscus height was determined. The step features enable meniscus height to be determined from distance between the steps, as an alternative to calculating the height corresponding to the AFM measured values of surface tension and contact angle.
Figure 2. SEM images of (a) a constant diameter (150 nm) needle and (b) a dual-diameter needle with two fused needles each 228 nm in diameter
