Building water bridges in air: Electrohydrodynamics of the floating water bridge

Abstract

The interaction of electrical fields and liquids can lead to phenomena that defies intuition. Some famous examples can be found in Electrohydrodynamics as Taylor cones, whipping jets or noncoalescing drops. A less famous example is the Floating Water Bridge: a slender thread of water held between two glass beakers in which a high voltage difference is applied. Surprisingly, the water bridge defies gravity even when the beakers are separated at distances up to 2 cm. In the presentation, experimental measurements and simple models are proposed and discussed for the stability of the bridge and the source of the flow, revealing an important role of polarization forces on the stability of the water bridge. On the other hand, the observed flow can only be explained due to the non negligible free charge present in the surface. In this sense, the Floating Water Bridge can be considered as an extreme case of a leaky dielectric liquid (

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

“…Once the bridge is established current will flow followed by swelling of the bridge (Figures 8b, 9b) to 3-5 mm diameter depending on the conditions. In many of the liquids studied thus far the time from bridge ignition to swelling is between 10-500 msec and is largely a function of the applied voltage, separation distance, and liquid viscosity 8,22,37 .…”

“…The possibility of charge injection at sharp edges also exists and is consistent with the formation of heterocharge layers which generate electroconvective currents in the liquid bulk 22 thus linking the liquid bridge system with the Sumoto effect 12 . The governing EHD relationships for bridges are extensively covered elsewhere for water and other polar liquids 22,[36][37][38] . These theoretical approaches suffer certain limitations which should be considered when approaching experimental data.…”

“…Non-polar dielectric liquids (e.g., hexane) do not exhibit bridge formation. The dielectric liquids capable of supporting bridges thus far studied 8,22,37 lie within a well-defined group of physical parameters that establish a good starting point for further experimentation: low conductivity (σ < 5 μS/cm), moderate static relative permittivity (ε = 20-80), moderate to high surface tension (γ = 21-72 mN/m). Interestingly a wide range of viscosities (η = 0.3-987 mPa·sec) work in such bridges.…”

“…The Maxwell stress tensor treatment 36 is insensitive to field heterogeneities as well as non-uniformities in the liquid bridge. A pure EHD approach 37 provides steady state definitions of the electrogravitational number and its relationship to the bridge aspect ratio; however, the flow dynamics and important transient phenomena (e.g., bridge creation) are not predicted. Three dimensionless numbers are useful when analyzing the bridge's stability and are derived here as previously published by Marín & Lohse 37 .…”

“…They permit the exploration of the interaction of bulk and surface forces with externally applied electric fields. They open the opportunity to examine new means of mixing different liquids 37 ; changing chemical reaction kinetics 52 ; proton transport 44,45 ; and examining the response of biological systems to such conditions 53 . In addition these bridges allow direct access to the liquid surface without any physically subtending structures which has already yielded new spectroscopic information on the dynamics in liquid water 28 and hints not only at the existence of an electrically controlled state switch whereby new bulk properties emerge 31 but at the potential to examine liquid-liquid phase transitions 54 through an entirely new method.…”