Piezo-imaging

The third chapter, by Wasan and Nikolov, discusses fundamental processes in emulsions, i.e., ereaming/sed-imentation, flocculation, coalescence, and final phase separation. A number of novel experimental facilities for characterization of emulsions and the above-mentioned processes are presented. This chapter highlights recent techniques such as film rheometry for dynamic film properties, capillary force balance in eonjunetion with differential microinterferometry for drainage of curved emulsion films, Kossel diffraction, imaging of interdroplet interactions, and piezo imaging spectroscopy for drop-homophase coalescence rate processes. 

Most NC-AFMs use a frequency modulation (FM) teclmique where the cantilever is mounted on a piezo and serves as the resonant element in an oscillator circuit [101. 102]. The frequency of the oscillator output is instantaneously modulated by variations in the force gradient acting between the cantilever tip and the sample. This teclmique typically employs oscillation amplitudes in excess of 20 mn peak to peak. Associated with this teclmique, two different imaging methods are currently in use namely, fixed excitation and fixed amplitude. 

Images can be made in variable or constant force mode. In the latter case the difference signal from the photo detectors is used to adjust the distance between tip and surface, such that the force between the two, and thus the deflection of the cantilever, remains constant. An important advantage of working in constant force mode is that the overall orientation of the surface with respect to the z direction is not so critical, because the z piezo compensates for any inclination of the sample. 

Lithography With the STM Electrochemical Techniques. The nonuniform current density distribution generated by an STM tip has also been exploited for electrochemical surface modification schemes. These applications are treated in this paper as distinct from true in situ STM imaging because the electrochemical modification of a substrate does not a priori necessitate subsequent imaging with the STM. To date, all electrochemical modification experiments in which the tip has served as the counter electrode, the STM has been operated in a two-electrode mode, with the substrate surface acting as the working electrode. The tip-sample bias is typically adjusted to drive electrochemical reactions at both the sample surface and the STM tip. Because it has as yet been impossible to maintain feedback control of the z-piezo (tip-substrate distance) in the presence of significant faradaic current (vide infra), all electrochemical STM modification experiments to date have been performed in the absence of such feedback control. 

The systems that scan the piezos and record the image are similar to those used in atomic force microscopy. 

Figure 3.7. TMAFM images of a (001) surface of an as-received EDT-TTF-(CONHMe)2 single crystal measured under ambient conditions (2.5 im x 2.5 j.m) (a) topography and (b) phase. The phase angle is defined as the phase shift observed between the cantilever oscillation and the signal sent to the piezo-scanner driving the cantilever.

Fig. 11.5. A schematic of the feedback loop in an STM. The tunneling current, after the current amplifier and the logarithmic amplifier, is compared with a predetermined voltage, which represents the current setpoint. The error signal is processed by the feedback electronics, which typically contains an amplifier and an integration circuit. The output of the feedback electronics is applied to the z piezo, to keep the error between the actual tunneling current and the reference current very small. The voltage applied to the z piezo is recorded as the topographic image.

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