Dustin E. Stansbury and Jon M. Jones Research Mentor, Dr. Tonya Coffey Department of Physics

1. ‘THE PEPSI CHALLENGE’: THE EFFECT OF MACROBUBBLES ON THEVISCOSITY OF FLUIDS AT A SOLID-LIQUID INTERFACE Dustin E. Stansbury and Jon M. Jones Research Mentor, Dr. Tonya Coffey Department of Physics Appalachian State University

2. Motivation • Nanotribology: the study of frictional forces at the atomic scale (10-9m) • Importance to the development of nanomachines • Micro-Electro-Mechanical Systems (MEMS) Sandia National Laboratories

3. Motivation • Changes in drag force at the solid/liquid interface • Nanobubbles—bubbles formed on the surface of hydrophobic materials having a height of 10-100nm above the material surface • Formation of nanobubbles first proposed by Steve Granick who found, using a SFA, that a fluid introduced to a gaseous environment could vary from non-slip (high viscosity) to partial-slip (lower viscosity) activity at interface when sheared between two surfaces. • Implications in lubricants

4. Research Analogy • Macrobubbles formed on the surface of a solid material placed in a gaseous environment should show similar characteristics found by Granick of bubbles formed at the nanoscale • Create a gaseous environment placing a solid shearing surface in a carbonated solution, in this case soda.

5. Apparatus • Research Quartz Crystal Microbalance (RQCM) • The RQCM uses an oscillating quartz crystal and measures crystal frequency and resistance very accurately over wide ranges. • These measurements correspond to the solid-liquid interactions that we are interested in (i.e. the viscosity and elasticity of the interaction)

6. RQCM Operation In this experiment piezoelectric crystals were utilized. These crystals oscillate at a certain frequency, and, since they are part of an electrical circuit, have a measurable resistance. As the crystal oscillates from side to side, a particular film or substance (in this case a liquid) will ‘slip’ on the surface interface. This slippage relates to the viscosity of the solid (our crystal)-liquid (soft drink) interaction. As time goes by, the soft drink being tested will go flat. As the drink goes flat less carbon dioxide bubbles will be present on the surface of the crystal, and should theoretically correspond to higher viscosity. FIG 1: MAXTEK 1” Crystal (Front View) FIG 3: Single-plane vibration (xy) FIG 2: Diagram of crystal vibration

7. Theory • The loading effect of liquid at the QCM interface causes a change in • resonance frequency equal to * (1) • Loading of the crystal also causes a change in the series resistance that • is related to the amount of slippage of the liquid on the crystal surface. • The more slippage, the less the change in resistance, the less the • slippage, the more of a change in the series resistance. *Taken from the Maxtek RQCM Operation and Service Manual

8. Procedure • The RQCM was ‘tuned’ to the liquid being used • The computer was set to buffer the first few data points • Resistance and Delta Frequency Data were converted into ascii format (.dat files) and further analysis was done using Mirocal Origin.

9. Data

10. Diet Coke…

11. Diet Pepsi

12. Results & Implications • Found an increase in viscosity of soda over time. • Bubbles formed at a solid-liquid interface decrease the effective viscosity of the liquid at the interface, causing a decrease in drag at the surface. • Macroscopic results resemble those found earlier by Granick and support the theory of nanobubbles’ effects on fluid viscosity at a solid interface.

13. Future Research… • Perform similar study on bubbles at the nanoscale • Study of nanoscale “shock absorbers” • Study of slippage of different hydrocarbons in metal surface in ultra-high vacuum • Vacuum chamber