NANO AT HOME: An Experiment That You Can Try
PLEASE NOTE: The Center for Nano- and Molecular Science and Technology (CNM) at The University of Texas at Austin (UT-Austin) cannot guarantee the accuracy or the safety of these activities. Some of these activities might pose safety hazards for young children, and all activities should be performed under the supervision of a responsible parent, teacher or adult. The CNM and UT-Austin do not assume any responsibility for these activities or their results. If you have questions, corrections, or comments please do not hesitate to contact the CNM.
The “Lightning Rod Effect”
When an object has an electrical charge, its electric fields are strongest at the sharpest features of that object. For example, electric fields are strongest at the sharp tip of a lightning rod, hence the name of the effect. This phenomenon also occurs at the nanoscale, where particles of metals such as gold or silver can produce oscillating electric fields when irradiated by particular wavelengths of light (plasmon resonance). These electric fields can be used to enhance other phenomena such as Raman light scattering. Researchers are currently studying how to make particles or combinations of particles that produce the strongest electric fields. Since the fields tend to be more intense at the sharp edges and corners of the particles, researchers are studying particles with many sharp edges.
Silver “star” being studied for its ability to enhance Raman signals with electric fields (note all the sharp edges). Picture from: Homan, K. A.; Chen, J.; Schiano, A.; Mohamed, M.; Wilets, K. A.; Murugesan, S.; Stevenson, K. J.; Emelianov, S. Adv. Funct. Mater., 2011, 21, 1673-1680.
The lightning rod effect can be demonstrated with magnetic fields as well as electric fields. This activity uses ferrofluid: a liquid containing magnetic nanoparticles that tends to flow to areas of strongest magnetic fields. (See: http://www.cnm.utexas.edu/magnetorheological-fluid/ for further description of magnetorheological fluids and ferrofluids.) Ferrofluid is commercially produced and is available through online vendors.
Iron-containing object with edges (such as hexagonal nuts)
Thin, strong plastic
To perform the activity, place the plastic over the strong magnet to protect it from staining by the ferrofluid. Carefully place the iron-containing object on the plastic so that the plastic does not tear. The magnetic field produced by the strong magnet passes through the iron object into the surroundings. The field is strongest at the edges and corners of the object. Use the eyedropper to place a few drops of ferrofluid onto the metal object. The ferrofluid will tend to concentrate at those edges and corners of the object where the magnetic field is strongest.
On the top: ferrofluid flows to the sharp edges of a hexagonal nut resting on a plastic sheet resting on the pole of a strong magnet. On the bottom: ferrofluid flows to the sharp edges of a metal sheet resting on a plastic sheet resting on the pole of a strong magnet. Note that the ferrofluid is thickest where the edges of the metal sheet come to a point.