Influence of Bond Number on Behaviors of Liquid Drops Deposited onto Solid Substrates

Abstract

The problem of spreading behaviors of pendant and sessile drops was studied experimentally and numerically under the action of gravity force and surface tension. Bond number was considered to be a main factor of the influence on shape behaviors of liquid drops. This study was performed in the framework of an experimental investigation of drop behaviors in microgravity onboard a Chinese satellite in future. The experiments were carried out in the Drop Tower of Beijing, which could supply about 3.6 s of microgravity (free-fall) time. The surface shape change of liquid drops was investigated and the contact angle variety in sessile and pendant drops were measured from normal gravity to microgravity. A sharp decrease and oscillatory variation of the contact angle for both sessile and pendant drops were found with the sudden decrease of Bond number. The succedent comparison between experimental and numerical results suggests that Bond number has a significant influence on the drop contact angle. Additionally, the drop shapes and the bulk flows inside sessile and pendant drops were analyzed numerically, and it was found that the bulk flows could affect the free-surface shape of liquid drops apparently. Comparison of the moving velocity of contact line between sessile and pendant drops indicated that the pendant drops had a faster response to Bond number.

The Thermodynamic Characterization of Wetting by Submitting a Sessile Drop to Microgravity

“…The experiments using the Drop Tower of Beijing [23] report a stabilization time from 0.8 to 1.2 seconds in the case of sessile drops, while by the experiments using the Drop Tower Facility of Queensland [22], the vibration time for sessile drops to reach equilibrium is less than 0.5 seconds. These authors carefully observed the transition from formal to reduced gravity using a 30 fps camera.…”

“…during the release of the capsule to the free fall. Recent experiments realized in 2012 by Diana et al [22] using the Drop Tower Facility of the Queensland University of Technology (Queensland, Australia) and Zhu et al [23] using the Drop Tower of Beijing (National Microgravity Laboratory, Beijing, China) showed that the simple wire cutting release mechanism that initiates the drop of the experimental rack induces vibrations on the drop. According to [22], the sudden variation of the gravity force induces oscillations in the sessile drop.…”

“…the surface tensions, and θY is the contact angle. evidence using microgravity by parabolic arc flights [21] and microgravity drop towers [22][23] have demonstrated that the effect of gravity on the contact angle is relevant even at very small drop volumes such as 5 µL. According to Allen [24], who studied the wetting of very small drops with small contact angles, a drop is small enough to neglect gravitational influences only if its volume is less than 1 µL.…”

The Thermodynamic Characterization of Wetting by Submitting a Sessile Drop to Microgravity

“…According to Leger, Joanny, Fujji and Nakae, only forces that become increasingly large at the contact line such as the viscous force on an advancing liquid can affect Young's law. However, recent experimental evidence using microgravity by parabolic arc flights [21] and microgravity drop towers [22][23] have demonstrated that the effect of gravity on the contact angle is relevant even at very small drop volumes such as 5 µL. According to Allen [24], who studied the wetting of very small drops with small contact angles, a drop is small enough to neglect gravitational influences only if its volume is less than 1 µL.…”

Surface modification of textured silicon and its wetting behaviour

“…These four distinct categories of wettability phenomenon occur as a result of balance between gravity and surface tension forces. [5] The wettability behaviours on the surface were originally defined by the Wenzel and Cassie-Baxter states. [6][7][8] The Wenzel assumes that the liquid wet the whole rough surface, whereas the Cassie-Baxter assumes that droplet partially wet the rough surface due to trapped air in the microstructures.…”