Well it has been a while since I have posted but I have been working on the problem of obtaining images of micelles using the AFM. Sprinkled in there as well were presentations and qualification exams which also took time but this received the bulk of experimental attention. I finally got somewhere after emailing Dr. S. Manne who has quite a few publications which include micelle imaging and he suggested that I was walking before I learned to crawl (paraphrasing). He suggested I try graphite first (which was the actual surface used in his paper) to get the basics down and to use contact mode to get a protocol which I could reproduce. So I acquired the materials and started learning to use contact mode, as I had not used it before. Dr. Paul Ashby gave a seminar here at the University of Oklahoma last year and gave the advice that I advance the stage and "wag" the tip to see if I had reached the surface yet. I didn't seem to get anywhere with this (I understand why now) at first but continued with this mind set in contact mode. The breakthrough came from understanding (not just using but understanding) the force curve capability of the scope. The first two figures can help explain what I mean by this. The tip is extended into the solution and if the surface is not close enough there is only interaction with the fluid surrounding the tip (air or water), which is just noise (not pictured) but when there is a surface which gives a repulsive or attractive force the curve looks something like the curve seen in the Figure 1 (air with clean silicon surface). As the tip is advanced there is an increasing repulsive force and then an attraction, or snap-to point. Then as the tip is pulled away there is a slight adhesion force which causes the force to not line up on top of the approaching line, and then there is a snap off point, the straight vertical line, as the tip suddenly leaves the surface. Now if we now add water and micelles, we have added a layer to the surface which interacts with the tip. Figure 2 shows a small blip for the approaching line, which is where the tip has broken through the micelle surface and is now at the actual surface. As the tip is pulled away there is still an adhesion force and we see a snap-off. The trick is set the imaging setpoint (the force applied to the tip which keeps its height constant) to be just before the snap-to point. Any setpoint greater than this will push right through the micelle layer and you will only see the surface, which is smooth so you will essentially see nothing (many, many, many weeks/months/years of nothing). The hardest part of most of this was translating the protocols of others using different scopes to the scope we have here at OU. Anyway, the final products are seen in Figures 3 and 4, which are deflection images (working on translating to tapping mode currently). The images show lines of micelles, which follow the grains of the surface. For CTAB on graphite these are shown to be cylindrical micelles. In order to verify this with our set-up a sharper tip will be used to check the height of the micelles. A duller tip was used because I got tired of wasting all of my good, really sharp tips on seeing nothing so I used the slightly larger tip (but not so large that I would miss the micelles if they were there). There is still a lot of work to do but now that I have finally been able to get this to work and have a protocol which I can reproduce we can move forward again instead of being stuck in once place!! Next will work on imaging micelles using tapping mode (perhaps with a tip using magnetic backing to decrease the oscillation of the water) and then start working into the PMMA trenches.
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| Figure 1. Force Curve in air |
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| Figure 2. Force curve in water with micelles on surface |
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| Figure 3. CTAB micelles on surface following grains (deflection) |
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| Figure 4. CTAB micelles on surface (deflection) |
