1,The properties of a liquid ,especially its surface ,are of paramount importance to the biologist and medical scientist . The surface properties of a liquid arise from the interactions among liquid molecules ,and the interactions among the molecules of liquid and those of other substances .In this chapter ,we will discuss some important surface properties of liquid ,which including the surface energy and surface tension ;the additional pressure across a curved surface of a liquid ;the capillarity and some surface tension phenomena on medicine and biology .,Chapter 5 phenomena on liquid surface,2,surface tension and surface energy The additional pressure under a curved surface Capillarity,The physical meaning of surface tension coefficient Distributions of air molecules and pressure on the earth,3.To know Air embolism,Chapter 5 phenomena on liquid surfaces,3,Section 1 surface energy and surface tension,Section 2 the additional pressure of a curved Surface of liquid,Section 3 capillary action,Section 4 air embolism,Chapter 5 phenomena on liquid surfaces,,4,surface properties,Surface tension,Surface energy,,Section 1 surface energy and surface tension,,,,5,,Liquid has the tendencies to contract its surface to a minimum area the sphere has the smallest surface area for a given Volume In all shapes,,表面张力和表面能,surface properties,Eg: Mercury drops on a glass plate water drops on a paraffin ,takes the spherical shape,,6,7,The surface behaves ,in fact ,as if it were a stretched rubber sheet that constantly tried to decrease its area . There is ,therefore ,a tension acting on a cross any imaginary line drawn in the surface . It proves that surface of liquid has tension (张力). It is called surface tension (表面张力).,Surface tension,,8,If we divide a liquid surface into two sides by an imaginary line drawn in the surface ,The material on either side of the line exerts a pull ,or the surface tension ,lies in (or tangent to )the surface ,and is perpendicular to the line . Its magnitude is proportional to the length of the line l,f = l,,,表面张力和表面能,Magnitude,Direction,lies in (or tangent to )the surface and is perpendicular to the line,9,Surface tension,,10,coefficient of surface tension, Depends only on its properties and the temperature . It decreases as the temperature increases,11,Question: Surface tension(产生,液面为何收缩) It is known that the equivalent (平衡的) distance between two molecules is denoted by r0 which is about 10-10 m in length. Generally speaking, when the distance between two molecules, denoted by d, is less than r0, the force between the two molecules is repulsion (斥力) and when the distance between two molecules,12,is larger than r0, the force between them should be gravitation (引力). However, when d 10-9m, the gravitation between two molecules can be taken as zero. Only if a molecule is in the sphere of radius of 10-9 m, it can be gravitated by the center of molecule. This sphere is called molecular action sphere, we draw a thin layer on the surface of a liquid. The thickness of the layer is equal to the radius of the molecular action sphere and the layer is called surface layer.,13,表面张力起源于液体分子力—引力 10-10m—10-9m,分子力作用半径：10-9m 分子力能够发生的作用的最大分子间距.,分子力是使固体、液体分子聚集的主要因素,分子力作用球： molecular action sphere 以分子中心为球心、以分子力作用半径为半径所作的球。
表面张力和表面能,Surface energy,,14,15,The molecules in the surface layer and those in the liquid experience different applied forces. In liquid, the molecules are gravitated by their neighbor molecules in all directions and the total force for each molecule is zero, while the molecules in the surface layer have different gravitations in different directions.,The direction of total force acting on the molecules in the surface layer is perpendicular to the surface layer, pointing to the inside of the liquid.,a molecule of a liquid,16,,,The forces which molecule A is attracted with by neighboring molecules are balanced,For molecule B near the surface the sum of all these force produces a resultant directed into the liquid,molecule A ：,molecule B、C：,,1. Interface,,,,表面层,,表面张力和表面能,Why the liquid surface has the tendency to contract , we compare a molecule of a liquid near the interface with a molecule interior of the liquid .,Surface energy,,17,2. Properties of the interface 表面层的特点,表面层内的分子受到垂直液面指向液体内部的引力,要把液体内部的分子移到表面层中来，就要克服上述引力做功，从而使分子势能增加，即表面层内的分子具有较大的势能——表面能。
forces,energy,,表面张力和表面能,Surface energy,explain,,表面能,液体表面层分子比内部分子所多出的势能总和18,3. Surface energy (表面能),Surface tension F can be described by the surface tension coefficient (表面张力系数) . The relation between them is,where L is the length of a line on the liquid surface.,19,You can imagine (想象) that there are two forces (surface tensions) pulled (拉) from both sides of the line. (= Ftension/L) expresses the surface tension per unit length. It is found that different liquid has different , higher temperature corresponds to (对应于) lower value of . Let’s see the relationship between the magnitude of surface tension and surface energy.,20,3.surface e。