Snap-off

On a microscopic scale (the inset represents about 1 - 2mm ), even in parts of the reservoir which have been swept by water, some oil remains as residual oil. The surface tension at the oil-water interface is so high that as the water attempts to displace the oil out of the pore space through the small capillaries, the continuous phase of oil breaks up, leaving small droplets of oil (snapped off, or capillary trapped oil) in the pore space. Typical residual oil saturation (S ) is in the range 10-40 % of the pore space, and is higher in tighter sands, where the capillaries are smaller. 

The eurrent waveform, this time, is a negative linear ramp whose slope is -VoJL. When the power switeh onee again turns on, the diode snaps off and the eurrent now flows through the input power souree and the power switeh. The induetor s eurrent (imin) just prior to the switeh being turned on, beeomes the initial eurrent the power switeh must then initially pass. 

Here are two more examples of penny-pinching. The piston of a reciprocating engine was secured to the piston rod by a nut, which w as locked in position by a tab washer. When the compressor was overhauled, the tightness of this nut was checked. To do this, the tab on the washer had to be knocked down and then knocked up again. This weakened the washer so that the tab snapped off in service, the nut worked loose, and the piston hit the end of the cylinder, fracturing the piston rod. 

Abklingzeit,/. decay period, abklopfen, v.t. beat beat off, knock off scale off (Med.) percuss, abklbren, v.t. boil out (for redyeing). abkoalleUt v.t. go off, explode. — v.t. fire off. abkneifen, v.t. pinch off, nip off. abknicken, v.t. bend (sharply) crack or snap off. 

Barring direct measurement of foam texture, we adopt the following reasoning. Because of the generation of foam bubbles by the snap-off and division mechanisms (4), bubble sizes are expected to be approximately that of pore bodies. Thus, the linear bubble density should scale roughly as n 6/Dwhere... 

FHE NOTCH HERE COMPLETELY -AROUND STEM GRASP BOWL OF GLASS AND STRIKE STEM AGAINST HARD OBJECT STEM WILL SNAP OFF EVENLY... 

Vibrator on-off switch. Turns the vibrator on or off. It s a spring-return switch so you must hold the switch in the on position. Let go, and it snaps off. 

BioRad produces Bio-Spin disposable polypropylene chromatography columns (Catalog No. 732-6008) that can be packed with up to 1.2 ml gel filtration media. Their micro Bio-Spin columns hold 0.8 ml media. Both are autoclavable columns that have snap-off column tips, polyethylene media bed supports, and fit standard centrifuge tubes. 

A typical container closure system is a plastic (usually HDPE) bottle with a screw-on or snap-off closure and a flexible packaging system, such as a pouch or a blister package. A typical closure consists of a cap — often with a liner — frequently with an inner seal. If used, fillers, desiccants, and other absorbent materials are considered primary packaging components. 

Snap off the bottom closure and place the column in 1.5-mL microcentrifuge tubes. Then, prespin the column for 1 min at 735g. 

Just about everyone has heard stories of compressed gas tanks being knocked over, thus snapping off the main valve, turning the compressed gas tank into a rocket. The Compressed Gas Association claims that this orifice is too small for the tank to become a projectile. However, if the tank contains a flammable gas, the question of whether the tank will turn into a rocket is of secondary importance. 

The wiggly line is a graphical way of indicating an incomplete structure it shows where we have mentally snapped off the C02H group from the rest of the molecule. 

By reducing the pore size of the membrane Rp, droplets become smaller, just as when we increase the cross-flow velocity of the continuous phase (and therefore increase the shear stress on the wall t ,). Addition of surfactants leads to a lower interfacial tension a, and will therefore lead to smaller droplets. (The relation between the interfacial tension at the time of droplet snap-off and the concentration of surfactants is, however, a complex one). 

Systems with Droplet Snap-Off Induced by Interfacial Tension... 

Figure 15.16. Schematic representation of the processes inside an SPG membrane. As soon as an oil droplet emerges, it will have a different Laplace pressure difference with the continuous phase. At a certain droplet size, it will be lower than the Laplace pressure inside the oil phase in the pore. When the pores are highly interconnected, the oil in the pore can be replaced by the continuous phase, leading to spontaneous snap-off.

With other membranes, such as ceramic membranes, the interconnectivity can be less. In that case, the continuous phase cannot enter the pore, and the oil flowing out has to be replaced by oil, coming from other pores. The cylinder of oil in the pore remains intact, and no spontaneous snap-off takes place. This may be the reason that only some membranes show this effect. 

Further, when we put a high pressure over the membrane, the flow rate of the oil through the membrane pores wUl be fast, and at the time when the continuous phase should replace the oil in the pore, it does not have the time. Therefore, what one sees is that the spontaneous snap-off process only takes place at low flow rates. At higher flow rates droplets will stiU be formed, but they will be larger, and they will be detached by the cross-flow of the continuous phase over the membrane. 

When will this spontaneous snap-off take place If the droplet has radius Rj (thus giving rise to Laplace pressure difference with the continuous phase of la/Rj), the point at which the oil will start to have a pressure gradient from pore to droplet is ... 

So, as soon as the droplet (radius) is twice as large as the pore (radius), there is the possibility of spontaneous snap-off It depends on the shape of the pore structure and on the flow rate of the oil— how large the final droplets will be—but it is usually close to this value. 

One sees in Figure 15.18 also that this spontaneous snap-off only occurs at lower flow rates. At higher flow rates, the continuous phase does not have the time to replace the oil. The droplets now become larger, and will be dragged away by the cross-flow. Also here, one sees the transition from interfacial tension-induced spontaneous snap-off towards shear-induced snap-off by the cross-flow over the membrane. 

A second, new class of processes is that of membrane and micro-channel emulsification. A to-be-dispersed phase is here pushed through pores of a membrane or through micro-engineered micron-scale channels. At the pore or channel mouth, droplets are formed. These droplets can spontaneously detach from the pore or channel mouth (interfacial tension driven snap-off), due to the distortion of the droplet shape when it is still attached to the mouth. At higher fluxes or with channel mouths not giving a strong shape distortion, droplets are sheared off by a cross-flowing continuous phase. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

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