13.12
表面张力和表面能
当画笔浸入水中时，刷毛在水中自由波动。 取出来后，刷毛粘在一起。 这种效应背后的原因是表面张力。
假设一个装满液体的烧杯。 液体中的大分子在所有侧面与周围分子受到相同的吸引力。 然而，由于本体分子，表面分子受到向下的净吸引力。 液体表面的行为就像一张拉伸的膜，它往往会最小化表面积。 液体的这种特性称…
液体表面表现得像一张被拉伸的膜，意味着它们倾向于使表面积最小化。这一性质称为表面张力。
液体内部的分子受到周围分子来自各个方向的相等吸引力。但表面分子则受到不对称的力。
表面分子所受的净向内力将其向下拉，从而产生表面张力。
为了将大分子物质移向表面，需要做功，从而增加表面分子的势能。这部分额外的能量即为表面能。
考虑一个带有可滑动金属丝的框架，将其浸入肥皂液中以形成具有上下两个表面的肥皂泡。表面张力垂直于可滑动金属丝并指向内部，因此需要施加外力以将金属丝向外拉动。单位长度上的这一作用力即为表面张力。
如果力拉动导线移动一段距离，从而增加表面积，则表面能等于所做的功。这表明表面张力也可定义为单位表面积的表面能。
View the full transcript and gain access to JoVE Core videos
Q1: Why do liquid surfaces behave like stretched membranes?
Surface molecules experience asymmetric attractive forces from surrounding bulk molecules, creating a net inward force. This pulls surface molecules downward, causing the liquid to minimize surface area. Bulk molecules, by contrast, experience equal forces on all sides. This asymmetry is the fundamental reason liquids behave like stretched membranes.
Q2: How is surface tension related to surface energy?
Surface tension is the force per unit length acting on a liquid surface, measured in N/m. Surface energy is the additional potential energy surface molecules possess due to their position. Surface tension can be defined as surface energy per unit surface area, expressed in J/m². Work done to increase surface area equals the product of surface tension and the area change.
Q3: Why do liquid drops form spherical shapes?
A sphere has the minimum surface area for any given volume. Since liquids minimize surface area due to surface tension, drops naturally adopt spherical shapes to reduce their potential energy. This geometric property explains why water droplets and bubbles appear round rather than irregular.
Q4: What causes a paint brush's bristles to stick together when removed from water?
Surface tension pulls the water film surrounding the bristles inward, drawing them together. When immersed, bristles move freely because water fills the space between them. Upon removal, the water surface contracts due to surface tension, causing the bristles to adhere to one another.
Q5: How can objects denser than water float on its surface?
Objects like needles can float on water because surface tension creates an inward force perpendicular to the liquid surface. This force acts like a stretched membrane, supporting objects that would otherwise sink. The surface behaves as an elastic barrier capable of supporting weight distributed over a small area.
Q6: What does the sliding wire experiment demonstrate about surface tension?
When a wire is dipped in soap solution, surface tension pulls it inward. External force is required to pull the wire outward against this inward pull. The force per unit length of wire gives the surface tension value. When the wire moves through a distance, the work done equals the surface energy gained from increased surface area.
Q7: Why do surface molecules have higher potential energy than bulk molecules?
Surface molecules experience net attractive forces only from below, creating an energy deficit compared to bulk molecules surrounded equally on all sides. Work must be done to move bulk molecules to the surface, increasing their potential energy. This additional energy is called surface energy and explains why liquids spontaneously minimize surface area.