Laura Schmidt, PHY335, Dec. 2, 2003 Selective withdrawal using non-Newtonian fluids The process: withdrawing fluid through a pipette positioned above a two-fluid interface. Much work has already been done to characterize the interface profiles for Newtonian, but not non- Newtonian fluids. Newtonian fluids vs. non-Newtonian fluids Newtonian fluids follow Newton’s law of viscosity: the shear stress is proportional to the rate of shear strain, with the constant of proportionality defined as the viscosity, η. Newtonian: water, oil, glycerol. non-Newtonian: gels, pastes, polymer solutions I. Cohen, “Scaling dependence on the fluid viscosity ratio in the selective withdrawal transition,” 2001.

Selective withdrawal is used to coat microparticles I. Cohen et al., Science 292 (2001), The selective withdrawal process is one of the most viable means of coating micro- particles, crucial in -immobilized catalytic chemical processes -cell and tissue encapsulation and -local drug delivery. 0.5 mm Selective withdrawal is preferred over other cell coating techniques because it can produce uniform coats on different sized cells and has fewer restrictions on chemical composition.

The experimental set-up Q camera 8 cm oil solution pump pipette waste Polyethylene glycol diacrylate

The transition’s dependence on the system parameters A series of experiments will be performed to determine how the transition (from hump to spout profile) depends on the withdrawal rate (Q) and the height of the pipette above the interface (S). The profiles are recorded by a video camera and then transferred to a computer, where an already existing edge-tracing program tracks and records the profile. Q For example: Vary S and hold Q fixed (for a range of values). Record at transition: - S u, the transition tube height - , the radius of curvature of the hump tip - h, the height of the hump Then, the dependence on Q of S u, , and h can be seen and compared to the Newtonian fluid cases.