Dispensing fluids properties

The above-mentioned properties are required at the orifice, if the fluid is heated or cooled. Higher viscosities are tolerated in fluid delivery system if this does not create a pressure drop that limits the desired maximum frequency. For high density fluids such as molten metals, the fluid properties should be converted to kinematic values to determine if The fluid properties are acceptable for dispensing. 

In addition to its traditional role in graphics application, inkjet printing is considered as a versatile manufacturing tool for materials fabrication (11). The unique feature in these applications is the possibility of dispensing and the precise positioning of very small volumes of the desired fluid. The mechanisms of drop formation have been reviewed along with the relationship of the fluid properties to become printable. 

In the electronics industry, the list of commonly dispensed fluids includes adhesives (anaerobic, cyanoacrylates, epoxy, and UV) conformal coatings flux (liquid and paste) RTV silicone (room-temperature vulcanization — curing without heat) solder mask, and solder paste. Given the vast differences in the properties of these fluids, in particular their viscosities and typical dispensed volume, dispensing... 

The correct treatment of boundaries and boundary effects is crucial to simulation methods because it enables macroscopic properties to be calculated from simulations using relatively small numbers of particles. The importance of boundary effects can be illustrated by considering the following simple example. Suppose we have a cube of volume 1 litre which is filled with water at room temperature. The cube contains approximately 3.3 X 10 molecules. Interactions with the walls can extend up to 10 molecular diameters into the fluid. The diameter of the water molecule is approximately 2.8 A and so the number of water molecules that are interacting with the boundary is about 2 x 10. So only about one in 1.5 million water molecules is influenced by interactions with the walls of the container. The number of particles in a Monte Carlo or molecular dynamics simulation is far fewer than 10 -10 and is frequently less than 1000. In a system of 1000 water molecules most, if not all of them, would be within the influence of the walls of the boundary. Clecirly, a simulation of 1000 water molecules in a vessel would not be an appropriate way to derive bulk properties. The alternative is to dispense with the container altogether. Now, approximately three-quarters of the molecules would be at the surface of the sample rather than being in the bulk. Such a situation would be relevcUit to studies of liquid drops, but not to studies of bulk phenomena. 

Flack and co-workers developed a complex model that included the effects of evaporation on the rheological properties of the viscous fluid. Their work established the idea that only fluid viscosity, angular speed, and evaporative effects are important in determining the final film thickness. Dispense volume, dispense rate, and other factors seem not to be particularly critical in determining the final film thickness as long as the wafer is spun for a sufficiently long time. Yet, in spite of evaporative effects, the final thickness /if of the fluid can be fairly well predicted with an inverse power law relationship [Eq. (11.13)], where C is a constant depending on the viscosity and contains the effects of viscous forces. 

Specialty fluids are often engineered for their end use, but careful consideration must be given to their properties as dispensed. These include, but are not limited to ... 

Based on certain thermodynamic guidelines, a set of materials are selected. Based on the criteria defined in Fluid Requirements ink-jettable inks are formulated. An ink-jet printer or jetlab-II type printing system (see Figure 11-30) is employed to dispense an array of spots onto a substrate or into wells in different combinations of metal inks. The printed samples are individually or collectively processed so that all processing variables are explored. The sample properties are measured and archived along with composition data in a database. Finally, the database is mined to find optima and make connections between the composition of the catalysts, their structure and properties. 

This dielectrophoresis backgroxmd only serves as a brief overview these technologies need to be explored in more detail before incorporating them into a Lab-on-a-Chip system. Lab-on-a-Qiip systems integrate techniques of small fluid and sample handling with detection or process capabilities. Dielectrophoresis can be incorporated into these systems to manipulate, separate, or trap cells as well as control small amoxmts of fluid. This technology can be used to trap cells for additional analysis, separate cell types based on dielectric properties, dispense picoliter droplets, or used for similar manipulative applications. 

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