NONCONTACT 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 si nusoidally activating resonances of a vibrating membrane to which the sample is mounted. A piezoelectric transducer glued to the back-side o f the steel membrane allows a fully electronic feedback to be set-up. Both bare capillary tubes and micropipettes previously coated with a s ilicone lacquer were found to reproducibly approach to the sample surf ace when measuring the interaction force as a function of tip-sample d istance. Nevertheless, significant differences are found depending whe ther the capillary was filled or not. In these spectra, the onset of i nteraction manifests in both a significant reduction of vibration ampl itude as well as a pronounced phase shift with respect to the free mem brane oscillation. Time stable feedback operation regulating on a cons tant interaction gradient is presented. (C) 1998 Elsevier Science Ltd. All rights reserved.