NUCLEATION LABORATORY

OVERVIEW OF RESEARCH INTERESTS

    Current research in the Nucleation Laboratory involves, primarily, nucleation and nucleation-related phenomena.  Essentially, nucleation is the name given to the physical process whereby one phase (the "mother" phase) first begins to make the transition to another, more stable, phase (the "daughter" phase).  Nucleation occurs often in nature although usually under conditions requiring a surface or a "seed" to be present in order to start the nucleation process.  This is termed heterogeneous nucleation and common examples are:  rain, fog, and dew formation, as well as the boiling of liquids and the freezing of water.  In the laboratory, however, nucleation is studied under a variety of carefully controlled conditions.  When those conditions are specially chosen, nucleation can be made to occur in the complete absence of foreign surfaces or "seeds."  This is called homogeneous nucleation and is the subject of much scientific investigation involving the study and application of phase transitions.

    In spite of the immense practical significance of nucleation and in spite of the fact that scientists and engineers have been studying it for decades, there is much we do not yet understand about the process by which it actually occurs.  For example, we know that, if we shine light of certain wavelengths on certain supersaturated vapors, we make the vapor nucleate!  In fact, this is thought to occur to a significant extent in our own atmosphere (e.g. smog and gas-to-particle conversion).  Even though this process has been investigated for a number of years, there is still disagreement as to the mechanism by which the process occurs.  One of my research group's on-going efforts is to learn more about this process of photo-induced nucleation (PIN) and to try and utilize this interesting effect for other scientific and engineering applications.  For example, we currently are investigating the production of ultrafine metallic and non-metallic particles by photo-induced nucleation.  The small particles that we produce under these unusual conditions may have novel chemical, physical, and electronic properties that could make them practically useful.

    We have observed during the course of our nucleation investigations that the presence of non-nucleating gases (i.e. the background gas in the diffusion cloud chamber) can affect the nucleation process in extraordinary ways.  Nucleation researchers have tended to ignore the presence of non-nucleating background gases in describing results of nucleation experiments and in describing the operational aspects of their experimental methods.  We have found that both the amount of background gas and the kind of background gas play an important role in the operation of the diffusion cloud chamber and, quite possibly, in the nucleation process.  We have developed a special high pressure version of the diffusion cloud chamber that allows us, for the first time, to carry out nucleation experiments at elevated temperatures and total pressures.  It has been through our use of this high pressure chamber (HPCC) for our nucleation investigations, that we have been able to study this unusual behavior.  Utilizing this HPCC, we are not only able to carry out nucleation experiments at elevated pressures and temperatures; but we are also able to make, for the first time, vapor to liquid nucleation measurements in close proximity to the critical point of a vapor.  These types of measurements are normally quite difficult to make, but the results have the potential to be of considerable scientific, engineering, and practical interest.

    While most of our research efforts deal with vapor-to-liquid nucleation, I am also interested in the inverse process, liquid-to-vapor nucleation.  This process, often called cavitation, involves superheating liquids to temperatures far above their normal boiling points (we can also accomplish the same effect by "stretching" liquids at lower temperatures).  One interesting result of this process is that we can create and maintain liquid droplets at quite high temperatures (well above the normal boiling point) but at low external pressures (e.g. one bar) which behave as novel, high temperature chemical reactors.  We have, in fact, carried out neat chemical reactions that, under normal conditions, can only be done in the presence of added catalysts.

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Chemical Engineering