Static creating drops

A typical static sessile drop is created by a microsyringe with an automated plunger to place a tiny drop of water on the polymer surface. Ideally, the polymer sample should be in a humidity chamber to minimize the water evaporation that would change the shape, and thus the C A of the droplet. The shape of the droplet is captured by a camera and CA is measured by an image analysis software [30]. 

Where surface-active agents are present, the notion of surface tension and the description of the phenomena become more complex. As fluid flows past a circulating drop (bubble), fresh surface is created continuously at the nose of the drop. This fresh surface can have a different concentration of agent, hence a different surface tension, from the surface further downstream that was created earlier. Neither of these values need equal the surface tension developed in a static, equiUbrium situation. A proper description of the flow under these circumstances involves additional dimensionless groups related to the concentrations and diffusivities of the surface-active agents. 

Pressure drop Flow of air through the fluid-bed processor is created by the blower or a fan located downstream from the process chamber. This fan imparts motion and pressure to air using a paddle-wheel action. The moving air acquires a force or pressure component in its direction of motion because of its weight and inertia. This force is called velocity pressure and is measured in inches or millimeters of water column. In operating duct systems, a second pressure that is independent of air velocity or movement is always present. Known as static pressure, it acts equally in all directions. In exhaust systems, such as fluid-bed processors, a negative static pressure exists on the inlet side of the fan. Total pressure is thus a combination of static and velocity pressures. Blower size is determined by calculating... 

The static mixer creates a hydrodynamic environment where critical viscous shear stresses deform drops, causing them to break. The dispersion process decreases the drop sizes and increases the... 

The interfacial tension may be determined to within about 1% accuracy with the spinning-drop method (127, 128). It is an absolute and static method that requires only small samples and, in contrast to most other methods, does not depend on the wettability of a probe, such as a ring or Wilhelmy plate. The stabilizing surfactant is commonly used at concentrations in the bulk continuous phase that are far above the critical micelle concentration (erne). This ensures that the concentration remains above the erne after adsorption on to the vastly extended interface has taken place, which is clearly needed to maintain emulsion stability. It is tempting, therefore, to assume that the interfacial tension in the finished emulsion equals that between the unemulsified bulk phases and that it remains constant when a mother emulsion is diluted with continuous phase in order to create a series of emulsions in which only O is varied (67). This may be a reasonable assumption when a pure surfactant is used, but there is evidence that this may not be so when impure commercial surfactants or surfactant mix-... 

Another control is lift planning. That helps ensure a safe load and lift. In some cases, there is a need to complete precise load calculations that account for many factors. Hoisting apparatus should have load limits clearly marked. The ratings assume a static load. Jerked loads or loads dropped some distance create dynamic conditions. They place inertial loads on the rigging and structure and may overload them. Cranes, for example, should have a load chart affixed to the operator s cab and an operator s manual in the cab. The operator must know the weight of the load. That and other information allow the operator to use the load chart and ensure the capacity of the equipment is not exceeded. 

Effect of Surfactants. For dilute dispersions, the presence of surfactants influences drop size only by reducing interfacial tension. To a first approximation, the drop size may be estimated within the framework developed above using the static interfacial tension in the presence of surfactant. However, drop stretching and breakup occur rapidly. As new interface is created, the rate at which surfactant diffuses to the surface may not be sufficient to maintain a constant interfacial tension. The dynamic a will vary from the static value in the presence of a surfactant to the valne for a clean interface. Phongikaroon (2001) found that for this reason, drop sizes prodnced in a rotor-stator mixer with a surfactant-laden system of known static a were larger than those produced for a clean system of the same o. 

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