Snap-to-contact

I Pulsed Force Mode AFM In pulsed force mode AFM, a conventional contact mode AFM scan with feedback on the cantilever deflection is carried out. Simultaneously, the sample is modulated in the z direction sinusoidally (with variable frequency, e.g., 1 kHz) such that the tip breaks free from the surface periodically. The resulting plot of cantilever deflection versus time is shown in Figure 6.7. In the figure, it can be observed that the tip snaps to contact and then indents the sample. Upon retraction, the tip breaks free from the surface (pull-off) and the free cantilever rings down in air. Hence, instead of a slow acquisition of force-displacement data in... 

In the spectroscopy mode of AFM force-distance curves F(z) are recorded at one or more scan points after the z piezo has been adjusted to the force setpoint (contact mode). The classical shape, as illustrated in Section 5.3.4 and Figure 63, is obtained mostly with hard materials or simpler molecules. In air a meniscus of water is formed at the jump-to-contact. Due to the meniscus force the jump-to- and jump-oflf-contact separations differ largely and the area of the hysteresis loop becomes quite large. Force-distance curves can have various appearances. An overview is given in Reference [233]. Spectroscopy is rarely employed in the dynamic mode because with an oscillating probe the tip-sample separation is never well-defined. On the other hand the snap-on is avoided and the complete interaction potential can be inferred from the measured frequency versus distance curve employing simulations [234,235]. 

Far from the sample, the force is by definition zero. Close to the sample, the tip may experienee van der Waals attraetion, snap into contact, and subsequently bend due to increasing repulsive forces up to the peak force threshold. Upon retraction, the tip may stick to the sample and the cantilever bends down due to adhesive forces (the force drops below the zero force line), until the nanoprobe tip eventually snaps off the surfaee and the deflection returns to the base line deflection position. This procedure is repeated pixel by pixel. 

The decisive element in exocytosis is the interaction between proteins known as SNAREs that are located on the vesicular membrane (v-SNAREs) and on the plasma membrane (t-SNAREs). In the resting state (1), the v-SNARE synaptobrevin is blocked by the vesicular protein synaptotagmin. When an action potential reaches the presynaptic membrane, voltage-gated Ca "" channels open (see p. 348). Ca "" flows in and triggers the machinery by conformational changes in proteins. Contact takes place between synaptobrevin and the t-SNARE synaptotaxin (2). Additional proteins known as SNAPs bind to the SNARE complex and allow fusion between the vesicle and the plasma membrane (3). The process is supported by the hydrolysis of GTP by the auxiliary protein Rab. 

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In neurons, the SNARE complex consists of three main proteins the v-SNARE synaptobrevin or VAMP (vesicle-associated membrane protein), and two t-SNAREs, syntaxin and SNAP-25 (synaptosomal associated protein of 25 kD). Synaptobrevins traverse the synaptic vesicle membrane in an asymmetric manner a few amino acids are found inside the vesicle, but most of the molecule lies outside the vesicle, within the cytoplasm. Synaptobrevin makes contact with another protein anchored to the plasma membrane of the presynaptic neuron, syntaxin, which is associated with SNAP-25. Via these interactions, the SNARE proteins play a role in the docking and fusion of synaptic vesicles to the active zone. 

Like the snap back process, billow forming is a sheet pre-stretch process used with a male mold. The sheet is clamped across a pressure chamber and is billow pre-stretched by applying a low magnitude positive air pressure to the chamber. The male mold is then moved into the stretched sheet until the clamp seals on the mold periphery. Contact between the mold and controlling the rate at which the pressure chamber is vented to compensate for the advancing mold can regulate the sheet. 

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