اسئلة الامتحانات - Surface tension is a...

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Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids. Surface tension (denoted with the Greek variable gamma) is defined as the ratio of the surface force F to the length d along which the force acts: gamma = F / d Causes The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area. A molecule at the interface has a force acting upon it from the oil lying immediately above the interface and water molecules lying below the interface. The resulting forces are unbalanced and give rise to interfacial tension. The interfacial tension is the force per unit length required to create a new surface.
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In dealing with hydrocarbon systems , it is necessary to consider not only the interface between a gas and a liquid but also the forces that are active at the interface between two immiscible liquid phases and between the liquids and solids. The combination of all the active surface forces detrmines the wettability and capillary pressure of a porous rock. WETTABILITY Wettability is the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids. Wettability refers to the interaction between fluid and solid phases. In a reservoir rock the liquid phase can be water or oil or gas, and the solid phase is the rock mineral assemblage. Wettability is defined by the contact angle of the fluid with the solid phase. Interfacial tension (SIGMA) is the energy per unit area (force per unit distance) at the surface between phases. It is commonly expressed in Newton/meter (also, dynes/cm). Adhesion tension (AT) is expressed as the difference between two solid-fluid interfacial tensions. A negative adhesion tension indicates that the denser phase (water) preferentially wets the solid surface
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•An adhesion tension of 0.0 indicates that both phases have equal affinity for the solid surface. When 2 or more fluids are present, there are at least 3 sets of forces acting on the fluids and affecting hydrocarbon recovery. DEFINITIONS A T = σ wo COS ө Where : A T is the adhesion tension, dyne /cm σ so is the interfacial tension between the solid and oil .
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