Non-contact feedback for scanning capillary microscopy

We demonstrate the development of a dynamic probing mode for scanning capillary microscopy. Tip-sample distance variations are induced by sinusoidally activating resonances of a vibrating membrane to which the sample is mounted. A piezoelectric transducer glued to the back-side of the steel membrane allows a fully electronic feedback to be set-up. Both bare capillary tubes and micropipettes previously coated with a silicone lacquer were found to reproducibly approach to the sample surface when measuring the interaction force as a function of tip-sample distance. Nevertheless, significant differences are found depending whether the capillary was filled or not. In these spectra, the onset of interaction manifests in both a significant reduction of vibration amplitude as well as a pronounced phase shift with respect to the free membrane oscillation. Time stable feedback operation regulating on a constant interaction gradient is presented. (C) 1998 Elsevier Science Ltd. All rights reserved.