Browsing by Subject "capillary"

When two liquid drops coalesce on a colloidal particle, the particle and merged drop may jump away from the surface in which it is located. This jumping mechanism has been viewed in many fungal spores. The powering mechanism of the jumping motion is the conversion of surface energy to kinetic energy upon coalescence, which, in this case, does not require a superhydrophobic surface. Although the jumping process of fungal ballistospores have been studied, a detailed study of the mechanism of the jumping process has not been explored, which has implications in areas such as weather cycles, crop diseases, and self-cleaning surfaces. In this thesis, we analyze two different colloidal catapult systems with different geometries and develop a capillary-inertial scaling law that closely matches the jumping behavior of the systems. The first system is a spherical particle with two similar-sized droplets on top. The second system closely mimics a ballistospore.

To experimentally verify the scaling hypothesis, an inkjet printer was used to print liquid droplets on the two geometries. Once the two drops coalesced in each case, the initial velocity of the projectile was measured. When measuring the velocity of each system in comparison to the relative size of droplets, the ballistospore-like jumping catapult experiment is more efficient in removing the particle, and closely matches the results of ballistospore data. This is confirmed by an interfacial numerical simulation.

Fiber-based coalescers are widely used to accumulate droplets from aerosols and emulsions, where the accumulated droplets are typically removed by gravity or shear. This thesis investigate self-propelled removal of droplets from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion toward the spontaneous departure. The self-propelled removal occurs above a threshold drop-to-fiber radius ratio, disrupting the power-law accumulation on a fibrous coalescer. The departure velocity approaches the capillary-inertial one at large radius ratios.

In experiments, the condensation process including self-propelled removal phenomenon was captured on Teflon-coated fibers with radius of 13~$mu$m and 40~$mu$m. The power law of condensation is obtained by plotting time and averaged radius of droplets condensed on fibers in 2-dimensional (2D) image at that time. Then, to better understand the mechanism resulting such self-propelled removal, droplet-pairs with equal size were manipulated on different radius of fibers (on cones with slowly varied radius). To simplify analysis, two droplets were aligned so that their center connection line was perpendicular to the axis of fiber. By using two high-speed cameras, two views of this removal process were captured simultaneously. Based on information obtained in those video-pairs, the velocity immediate after removal and drop-to-fiber radius ratio were extracted for every case. In plotting those velocity against the radius ratio, data-sets of different size of fibers were collapsed on a single curved, implying critical radius-ratio (at which the removal starts) and asymptotic removal velocity (when radius ratio is very large). In understanding the fluid field in this dynamic process, a 2D phase-field simulation were qualitatively compared with experimental observation, which help to explain why such self-propelled removal can happen on highly curved hydrophobic surface (micro-fiber), but not on flat hydrophobic surface. Understanding to this phenomenon can be useful in chemical industry, ventilation system, and oil separation, in all of which fibrous beds are used to separate aerosol from immiscible flow.