AFM, Tensiometer and Dr. Menon

Today we were shown how the Atomic Force Microscope (AFM)works. This is one of the things I have enjoyed most being here, just seeing and sometimes using all the different devices I have ready about. We will not use this instrument, but Ji Xu explained the principles of its operation and how to use it. He was very helpful and did not mind all the questions I asked. :-)

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, very gently, 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. 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. Ji Xu has hexagons that self-assembled as his polymer annealed. A close up of one of the hexagons is seen below, from the AFM, the other picture shows the sample in an optical microscope, the hexagons are the tiny dots to the left of the large drop.


After lunch, 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 when a force, using a tip, is applied to the surface of the film.