Overview of Haugstad research program
Our research interests involve three intertwined subjects:
(1) Scanning probe methods for studying condensed matter and microdevices.
(2) Surfaces, interfaces and films composed of polymers and/or small organic molecules.
(3) Energy dissipation at material interfaces.
Scanning probe studies of surfaces, interfaces, films and microdevices require the development of methodologies for data acquisition and interpretation. The interpretation of interactions between a scanning probe and condensed matter is aided by exploring parameter space: molecular weight, free volume, crystallinity, plasticizing molecules, etc. Most property-sensitive nanoprobes trigger energy dissipation (i.e., during interfacial sliding, rupture of contact, or cycling of compressive, tensile or shear strain). Polymeric/organic thin films often are employed to modify dissipation at interfaces (e.g., as lubricants or coatings).
Dominant mechanisms of energy dissipation in sliding friction on polymers have been identified as primary or secondary relaxations (i.e., dissipative rotational isomeric motions along the main chain or in side groups). Energy dissipation in amorphous domains (with rotational isomeric freedom) is strongly enhanced compared to highly crystalline domains, and a strong function of rate and temperature (akin to viscoelastic loss tangent). Water content also plays a large role in the case of hydrophilic polymers. In ongoing research, energy dissipation is being compared during shear, compression/tension and contact rupture.
Films studied to date range in thickness from nanometer- to micron-scale, comprised of solution-cast synthetic homopolymers of primarily hydrophobic or hydrophilic character, or biologically derived polymers like gelatin, physisorbed or chemisorbed to inorganic model substrates. Some work has involved more complex systems like polymer blends, polymer-polymer interfaces, block copolymers, or polymer-inorganic composites. Other work has examined plasma modification, crosslinking (network formation), or surfactant inclusion (e.g., emulsion delivery). Technological applications of interest include tribology (friction/lubrication/wear), coating/defects, biomedical insertion/interaction, organic semiconductor devices, adhesive/release media, and protein or waste water filtration.
Analytical methods development includes several operating modes of scanning probe microscopy as follows.
(a) Sliding friction: variable velocity/temperature/humidity, calibration.
(b) Shear force modulation: low frequency, Fourier analysis.
(c) Rapid force curve mapping modes (pulsed force mode / peakforce tapping): energy dissipation during rupture of adhesive contact.
(d) Dynamic force: attractive/repulsive regimes, spatially resolved approach-retract curves, parameter space.
(e) Dynamic methods sensitive to surface potential and capacitance, such as Kelvin-probe force microscopy.
(f) Multifrequency methods: higher eigenmode imaging, contact resonance methods, single-pass EFM/KFM.
Physical topics have included