5. Mechanism of Dynamic Force Microscopy in Fluid

5.1. Introduction

This section is devoted to a discussion of the mechanism of DFM in liquid, specifically water. The mechanism of the DFM in air has received considerable attention (Anczykowski, Kruger et al. 1996; Tamayo and Garcia 1996; Tamayo and Garcia 1997) as has the operation of DFM on clean surfaces in vacuum (Giessibl 1997). The DFM in fluid is less well understood. The interaction between an oscillating cantilever and a solid surface in fluid has been studied with the cantilever driven solely by thermal fluctuations (Roters, Gelbert et al. 1997) or at a very low amplitude of oscillation (O'Shea, Welland et al. 1994; Han, Lindsay et al. 1996; O'Shea and Welland 1998). O’Shea et al. (O'Shea, Lantz et al. 1998) demonstrated that low-amplitude DFM senses changes in the elastic and viscous properties of the surface at nm distances, and can therefore be operated in a non-contact mode. In this section we discuss operation of the DFM in water at the larger amplitudes typical of many imaging applications (Lantz, O'Shea et al. 1994; Han, Lindsay et al. 1996).

The discussion is organized as follows: (1) The criteria for choosing operating amplitude are presented and a procedure for amplitude calibration is outlined. (2) There are two current methods for exciting cantilever motion in DFM; one uses acoustic excitation and the other utilizes direct magnetic drive of a magnetic cantilever. The difference between these approaches is analyzed and illustrated with experimental data. (3) A formula for interpreting low frequency, high amplitude approach curves is derived and applied to surfaces under water. (4) Measurements of the cantilever response as a function of frequency and distance from a mica surface are presented and interpreted with the aid of numerical simulations. These confirm the interpretation of the low frequency approach curves which indicate that on initial ‘contact’ the tip is sensing the surface via compression of an interfacial fluid layer. Thus, it appears to be possible to operate the DFM in a non-contact mode in fluid even at high amplitude.