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Constant diameter NeedleProbes™ for Atomic Force Microscopy (AFM) have been developed and are opening up a new area of research for studies at the liquid/air interface. The NeedleProbes' cylindrical geometry enables researchers to measure surface tension, contact angle, meniscus height, evaporation rate and viscosity - all in a single force distance curve.
New, constant diameter NeedleProbes are enabling unique measurements at the liquid/air interface. These NeedleProbes' cylindrical geometry enables a new area of research for liquid studies. "AFM can now be used to measure surface tension, contact angle, meniscus height, evaporation rate and viscosity - all in a single force distance curve" says Mehdi Yazdanpanah, inventor of the NeedleProbes. The new metallic probes have recently been introduced for a variety of applications, many of which had been impractical for AFM.
Atomic force microscopy routinely takes advantage of force-distance curves to measure interactions between an AFM tip and a sample surface. An example would be extending the tip to a liquid surface so the wetting force can be measured. Although standard probes can detect wetting force, surface tension and contact angle are very complicated to determine. Imprecise knowledge of the length of contact and angle of the meniscus with the sides of the tip make such measurements impractical using standard AFM probes. Furthermore, with increasing insertion depth, the attractive force of standard tips continues to grow as the contact line increases. The large force can eventually overcome the cantilever and result in the unintended immersion of the cantilever into the liquid.
The NeedleProbes' differentiating feature is the cylindrical shape, in contrast to the pyramidal or conical shape of most standard AFM probes. With the NeedleProbes, the liquid's attractive force is constant as the tip is immersed. When a NeedleProbe has a second region with a larger diameter, there is a step change in force when this larger region enters the liquid. The figure shows a diagram that differentiates between a conventional probe and a NeedleProbe entering a liquid medium.
The surface tensions for several low molecular weight liquids were measured with the NeedleProbes. Results were between -4.2 % and +8.3 % of standard reported values. Also, the F-D curves showed well-defined stair-step events on insertion and retraction from partial wetting liquids, compared to conventional AFM probes. A NeedleProbe 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.
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