Fundemental Studies of Surfaces by Scanning Probe Techniques
We are involved in studies of fundemental interactions between cantilevers and surfaces. Participants in this research include Professors George and Weimer from the Department of Chemistry, Dr. Al Fennelly of Teledyne Brown Engineering and gradute sudent Lelon Sanderson.
Two specific areas of focus have evolved through these efforts:
1. Quantitave Surface Force Gradient Measurements Using Atomic Force Microscopy
The accepted theory that reconciles attractive and repulsive forces has become known as the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory and may be simply stated as Vs = Vc + Va + Vr , where Vs is the total potential, Vc is the core repulsive potential due to Pauli exclusion, Va is the Van der Waals attractive potential and Vr is the double layer repulsive potential. The DLVO theory is widely regarded as a cornerstone for understanding colloidal systems and forces on the molecular scale. The objective of this study is to characterize the forces between two surfaces at the molecular scale using an atomic force microscope (AFM) and to relate the results quantitatively to parameters in DLVO theory. Investigations have been made using an AFM of surface forces present between a standard Si3N4 AFM tip and mica substrates for water, ethanol, and carbon tetrachloride. Results agree with those previously reported in the literature. Colloidal probes over mica and silica substrates are being used in ongoing research to provide easier geometries for comparison to the DLVO theory. Variations in surface forces as a function of pH and salt concentrations are being examined. The goal is to obtain a means of characterizing molecular scale forces over thin films such as aminopropyltriethoxysilane and polyethyleneglycol anchored to substrates. From a fundamental side, understanding of these forces is also important in analyzing the behavior of such molecules in solution, and the results can be used to select solutions for improving image resolution with the AFM.
2. Observation of Chaotic Motions in the Operation of a Scanning Probe Microscope
Chaotic motions have been observed now for many classes of phenomena, natural and man-made [1,2]. Recently, attention has focused on the nature of the oscillatory behavior of the cantilever used for "tapping mode" in scanning probe microscopy [3-6]. Investigations presented in this report demonstrate conditions where the transduced response of driven, oscillatory, tapping mode cantilevers in a scanning probe microscope (SPM) manifest chaotic motion. The applicability of the SPM to a wide range of physical sciences and the breadth of the potential causes for the phenomena warrants this announcement.
Nonlinear motion of tapping mode cantilevers has been previously reported in the SPM community. Gleyzes et al [7] reported on the bistable behavior of a vibrating tip operated near a solid surface. This is precursor to nonlinear and subsequent chaotic behavior. Fujisawa et al [8] reported results that they interpreted as an indication of stick-slip motion between the tip and sample. Stick-slip systems are well known to be operable in chaotic regimes for selected ranges of their control parameters. Joyce and Houston [9] operated an SPM with a feedback control to eliminate what they characterized as an "instability" or "jumping in". They thereby eliminated potential bistable, hysteretic behavior, precluding any opportunity of obtaining chaotic motion. Anczykowski et al. [10] also reported a hysteretic behavior in cantilever motion and regarded it as a phenomenon to be eliminated or to be accounted for in calibration.
The manifestations of so-called chaotic behavior in natural and man made
systems are not fully comprehended at the current level of knowledge. The
results in our study indicate a chaotic behavior for an oscillating cantilever.
While the existence of such phenomena is not new and unique in nature, the
observation of chaotic behavior in Scanning probe microscopy will provide
a new experimental avenue to test these ideas. The phenomena offers strong
potential for further studies into the natural laws that govern nanometer-scale
surface forces. At the very least, the observed chaotic motion strongly
suggests that there is a need to determine the conditions of operation under
which "linear approximations" are adequate and the circumstances
under which such approximations no longer apply. Our full experimental and
theoretical results and analyses will be reported elsewhere, along with
more detailed statistical studies of the data, including computations of
mutual information, singular value decompositions, attractor dimensionalities
and Lyapunov exponents. We believe that all the work on such systems published
to date is only a microcosm of what is to come.