Wednesday July 25, 2007

As soon as we arrived, we headed for the microscopy lab to receive instruction on the operation of the Atomic Force Microscope (AFM). We will not use this instrument, but Ji Xu explained the principles of its operation and how to use it.


The AFM is not optical; it has a tip (visible only using an optical microscope) that vibrates vertically in response to an oscillating voltage, so that in effect it gently taps the surface of the sample at a constant amplitude as it moves along its surface. The constant amplitude of the tapping allows the tip to move up and down with the variations of the topography of the sample. The tip is attached to a cantilever which is just barely visible to the naked eye; this is attached to a black matrix that is large enough to be manipulated into position on the instrument (below, left). Prior to analyzing the sample, the AFM must be calibrated for that specific sample. First, the tip size is selected; the smaller the tip, the higher the possible magnification. Resolution is determined by the number of oscillations, or vibrations, of the tip on the sample surface, and magnification is again affected by the amplitude of the oscillation of the tip.



A laser beam shines down onto the cantilever and, as the tip moves across the topography of the sample, the beam is reflected at different angles. The reflected laser is collected by receptors that are analyzed and interpreted by the computer to create images. The image on the left of the screens below is the topography of the sample; higher structures appear lighter in color. The images on the right indicate the hardness (how densely the molecules in the sample are packed) of the sample: dark areas are hard and light areas are soft. Ji Xu has hexagons that self-assembled as his polymer annealed. A close up of one of the hexagons is seen below, left, while several are visible in the less magnified image seen below right.



Results are also graphed for interpretation (below, left). The sample can be manipulated under the AFM to a position that correlates to a position previously viewed under a high resolution optical microscope (below, right) so that images obtained from the optical microscope can be compared to the images obtained using the AFM.


After lunch, we prepared surfactant solutions of various concentrations that we will use in next week's wrinkling experiments. From the 0.5% solution prepared on Wednesday, a 1/4 dilution was made, resulting in a 0.1% solution. Twelve ml of the 0.1% solution was mixed with 8 ml of water to prepare a 0.06% solution; 18 ml of the 0.1% solution was diluted with 2 ml of water to create a 0.09% solution. A 0.12% solution was made by mixing 4.8 ml of 0.5% surfactant solution with 15.2 ml of water. We then set up the Tensiometer to measure the surface tension of the 0.03% solution again.

At 2 pm we went with Jaingshui to meet with Dr. Menon of the physics department in Hasbrouck. The first discussion focused on our progress with wrinkling, use of the reflectometer, and data analysis with ImageJ and Origin software. We then discussed problems that we could encounter using the surfactant. The surfactant is amphipathic, and the polar tails actually stick up from the surface of the water, making the environment at the surface of the drop different from the drop's internal environment. The same is true in a bowl of water/surfactant solution: the hydrophobic tails of the surfactant stick up while the hydrophilic heads are oriented toward the water. In the rest of the water, the hydrophobic tails of the surfactant are attracted to one another and form mycellae. The mycellae eventually form spheres (head to head/tail to tail attraction). If the spheres are broken apart (agitation, heat) they may reassemble as cylinders. If the concentration of the surfactant continues to increase, and the cylinders are broken apart, then lamellae may form. The formation of these various structures is dictated by the general rule that material tries to form the geometrical shape with the smallest surface area relative to its concentration.

Jiangshui then discussed the next focus of his research with Menon. Jaingshui will be working to create experimental evidence to support the mathematical explanation for wrinkling patterns.