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3. 12. J. K. Moon

3. 12. J. K. Moon . Diagram of the experimental apparatus. 2-ml water drop (0.2mM KCl ) in silicone oil. coalescence. Non-coalescence. Coalescence would reduce the system’s energy by minimizing the total surface area and corresponding surface energy. coalescence. Metal wires . +.

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3. 12. J. K. Moon

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  1. 3. 12. J. K. Moon

  2. Diagram of the experimental apparatus 2-ml water drop (0.2mM KCl) in silicone oil coalescence Non-coalescence Coalescence would reduce the system’s energy by minimizing the total surface area and corresponding surface energy.

  3. coalescence Metal wires + Dielectrophoretic forces cause the drop to move towards and contact the top electrode, thereby providing the drop a net charge. The drop then moves down towards the oppositely charged oil/water meniscus. The bottom edge of the small droplet becomes positively charged, whereas the oil/water meniscus beneath it becomes negatively charged. The opposite charges attract, forcing the drop into contact and hastening the overall rate of coalescence. • - + oil ----------- water

  4. Non-coalescence + Above a critical field strength Ec, drops fail to coalesce The leading edge of the drop noticeably elongates. This elongation results from the electrical stress acting on the charged liquid/liquid interface. After apparent contact with the meniscus, however, the cone recoils and the drop reverses direction, moving upwards against gravity • - + ------- After bouncing, the drop typically moves back to the top electrode. On contact with the metal, the drop again reverses direction and migrates back to the oil/water meniscus, whereupon it bounces again, repeating the cycle.

  5. Why do the droplets bounce? Bouncing of a ball ~ combination of inertia and elasticity For perfectly elastic materials all of the kinetic energy before and after the bounce is conserved. The relative magnitudes of inertia and surface tension Inertia is negligible A comparison of inertia to viscous drag NO!! Inertia is negligible

  6. Why do the droplets bounce? Thermal Marangoni flows Joule heating, which scales linearly with ionic conductivity At sufficiently high salt concentrations (> 10–2 M) the critical force is independent of the concentration Maxwell stresses acting on the oil/water interface in the meniscus bridge. These stresses scale as E2and are responsible for the cone formation prior to contact, but after contact they no longer play a dominant role. NO!!

  7. Why do the droplets bounce? The Young–Laplace equation approximates the capillary pressure inside the bridge

  8. the capillary pressure in the bulk of the drops (each assumed to have radius R) the pressure difference between the bulk of the drop and the meniscus bridge For sufficiently steep cones Fluid moves from the meniscus back into the drop, driving pinch-off

  9. Critical cone angle. whether coalescence occurs or not depends only on the cone angle following contact

  10. Non-coalescence behaviour in different liquid systems. The universal nature of the non-coalescence behaviour can explain observations in very different fields.

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