Vibration-Enhanced Droplet Motion Modes: Simulations of Rocking, Ratcheting, Ratcheting With Breakup, and Ejection

Abstract

Power plant water usage is a coupling of the energy–water nexus; this research investigates water droplet motion, with implications for water recovery in cooling towers. Simulations of a 2.6 mm-diameter droplet motion on a hydrophobic, vertical surface were conducted in xflow using the lattice Boltzmann method (LBM). Results were compared to two experimental cases; in the first case, experimental and simulated droplets experienced 30 Hz vibrations (i.e., ±0.1 mm x-direction amplitude, ±0.2 mm y-direction amplitude) and the droplet ratcheted down the surface. In the second case, 100 Hz vibrations (i.e., ±0.8 mm x-direction amplitude, ±0.2 mm y-direction amplitude) caused droplet ejection. Simulations were then conducted for a wide range of frequencies (i.e., 10–100 Hz) and amplitudes (i.e., ±0.018–50 mm), resulting in maximum accelerations of 0.197–1970 m/s2. Under low maximum accelerations (e.g., <7 m/s2), droplets rocked upward and downward in rocking mode, but did not overcome the contact angle hysteresis and, therefore, did not move. As acceleration increased, droplets overcame the contact angle hysteresis and entered ratcheting mode. For vibrations that prompted droplet motion, droplet velocities varied between 10–1000 mm/s. At capillary numbers above approximately 0.0044 and Weber numbers above 3.6, liquid breakup was observed in ratcheting droplets (e.g., the formation of smaller child droplets from the parent droplet). It was noted that both x- and y-direction vibrations were required for droplet ejection.

Modeling Internal Flow Patterns of Sessile Droplets on Horizontally Vibrating Substrates

“…In addition to the studies of oscillating modes, efforts have been devoted to investigating the motion of droplets on vibrating substrates [18][19][20][21][22]. It was found that the effect of contact angle hysteresis was partially or completely mitigated by vibrations, and the vibrating frequency of the substrates (either asymmetric roughness or wettability gradient) must match the resonant frequency of the droplet to achieve maximum droplet motion efficiency.…”

The effects of Surfactin on sprayed droplets in flat fan, full cone, and low energy precision application bubbler nozzles: droplet formation and spray breakup

“…One of the main considerations for the irrigation design of sprinklers over LEPA is the reduction of field runoff in relation to the soil type, irrigation (well) capacity, field topography, and field management (Rogers et al, 2008). Droplet dynamics are impacted by fluid properties (e.g., surface tension, density, and viscosity), environmental factors (e.g., humidity, temperature), and external forces (gravity, pressure, flow, electric fields) (Leach et al, 2006;Ristenpart et al, 2006;Boreyko and Chen, 2009;Chen and Li, 2010;Nath and Boreyko, 2016;Chen et al, 2017;Nath et al, 2017;Huber et al, 2019;Noori et al, 2020;2021;Shams Taleghani and Sheikholeslam Noori, 2022;Kingsley and Chiarot, 2023). For nozzle applications, the nozzle design and geometry will also impact breakup length, spray angle, and droplet size (Fraser et al, 1962;Shavit and Chigier, 1995;Butler Ellis et al, 2001;Silva, 2006;Qin et al, 2010;Davanlou et al, 2015;Payri et al, 2015;Asgarian et al, 2020;Sijs and Bonn, 2020;Sijs et al, 2021;Jalili et al, 2023).…”

Droplet motion on sonically excited hydrophobic meshes

“…However, as the excitation frequency increases the droplets can overcome the pinning forces and move into a ratcheting mode. Furthermore, as the excitation frequency increases further, the droplet can jump and break into newborn droplets while ejected from the surface 12 . As the surface wetting changes to a superhydrophobic state, then, the sonic excitation can cause jumping of the droplets at a ratcheting mode.…”