Insulating liquid droplets

The test procedure of lEC Publication 112-1979 refers to the relative resistance of solid electrical insulating materials to tracking up to 600 volts when the surface is exposed under electric stress to water with the addition of contaminants. A horizontal specimen at least 3 mm in thickness and 15x 15 mm plane area is loaded by two platinum electrodes with cross-section of 5 x 2 mm at 1 0.05 N (Fig. 3.116). During the test procedure, the surface between the electrodes is dampened by a liquid droplet 20 3 mm every 30 s. The following standard solutions are used ... 

Liquid dielectrophoresis (1-iDEP), which can displace insulating or semi-insulating liquids (defined as < ImS/m) above dielectric layers [18]. Oil droplet obstacle from an intersecting channel into the main channel to create an oil meniscus as depicted in Fig. 1 [9]... 

Figure 1 illustrates the electrowetting effect, where a polarizable, conductive liquid droplet is initially at rest on a hydrophobic surface [4]. When an electrical potential is applied between the droplet and an insulated... 

A copper wire of 50 pm in diameter is connected to a power supplier to produce Joule heating. The copper wire is coated with a 10 pm-thick electric insulation layer. It is inserted into a fused silica capillary with an inner diameter of 149 pm. The capillary is placed in contact to a large liquid droplet which acts as a thermal reservoir for maintaining a constant ambient temperature. The centerlines of both the copper wire and the capillary are kept in a horizontal plane to avoid possible free convection in the capillary. With a 4 K temperature difference, a large temperature gradient of about 5 x lO" K/m can be reached. 

At low temperatures a pure semiconductor is a perfect insulator with no free carriers. Upon laser irradiation at a frequency greater than the semiconducting band gap, a high density of electron-hole pairs can be excited which, at liquid-helium temperatures, condense into small droplets of electron-hole plasma. These electron-hole (e-h) droplets have been discussed thoroughly in a dedicated volume of Solid State Physics that contains reviews of theoretical aspects (Rice, 1977) and experiments (Hensel et al., 1977). 

Figure 1.33 The electrowetting effect. A droplet of conducting liquid has a contact angle 0with a solid hydrophobic insulator (solid contour).

The solid/liquid interfacial energy is reduced on applying a voltage V between the droplet and a counter-electrode below the insulator. This decreases 0and leads to improved wetting of the solid by the droplet (dashed contour) [98] (by courtesy of RSQ. 

Figure 15. The electrowetting effect. (According to Mugele et al. [260].) (a) If a voltage V is applied between a liquid and an electrode separated by an insulating layer, the contact angle of the liquid-solid interface is decreased and the droplet flattens , (b) Hydrophobic surfaces enhance the effect of electro wetting. For electrowetting on dielectrics (EWOD) several individual addressable control electrodes (here on the bottom) and a large counter-electrode are used. The droplet is pulled to the charged electrodes.

Design methods are discussed elsewhere (417) and are based on limiting the maximum diameter of droplets entrained in the vapor stream. Maximum droplet diameter can be reduced by supplying a knockout facility upstream of the compressor. This can be either an enlargement of the vapor space above the top tray, an in-line liquid separator, or a separate knockout drum. Mist eliminators are effective and can be installed above the top tray or in the drum. If the vapor temperature is higher than the worst ambient conditions, the lines from the knockout facility to the compressor should either be kept short and insulated or be provided with liquid-removal facilities. 

The Paul trap is housed in a central climate chamber which can be cooled down to the temperature of liquid nitrogen. The pressure and composition of the background gas in this chamber can be controlled by a gas mixing and inlet system and is usually adapted to stratospheric conditions. This chamber is suspended inside an insulation chamber which is kept at high vacuum. This chamber is equipped with a quadrupole mass spectrometer to analyze the chemical composition of the atmosphere, which leaks from the central chamber through a small pinhole. This system of vacuum chambers is connected to a standard time-of-flight mass spectrometer by means of a droplet translator. This is a cold finger which can be used to... 

It is important to note here that although fairly general, the above analysis is not strictly valid for apolar liquids (e.g., silicone oil at the top of a conducting solid substrate). In such cases, the droplet may be modeled as an insulator with a permittivity of ei. The electric energy stored in that case can be estimated as ll2eiR (VIR) ade(0), where flde(0)is a shape factor which cannot be computed analytically but can be numerically fitted as [3]... 

---