(11.4.2)--6.4-2GasificationandCondensation.pdf

Gasification and Condensation 2Gasification and Condensation 2 Influence of Surface Curvature on Saturation Vapor Pressure4 The Critical Radius for the Condensation of a Drop、Condensation Nuclear5 Supersaturated Vapor,Metastable State,Cloud Chamber64.Influence of Surface Curvature on Saturated Vapor Pressure The additional pressure generated by the surface tension of the curved liquid surface,which can be expressed by the increased or decreased height of the liquid lever in a fully wetting(or non-wetting)capillary,is responsible for the difference of saturation vapor pressures between the curved liquid surface and the flat liquid surface at the same temperature,which has been proved by experiment.Imagine that a closed container close to a heat source with temperature T is partially filled with a liquid with density of and molar mass of Mm,in which a capillary equipped with completely wettable(or not wet at all)liquid,whose inner radius is r,is vertically inserted.Obviously,the rise or fall of the height of liquid level in the capillary can be expresses as follows:2/hgr （Positive means completely wetting,while negative means non-wetting at all）Suppose that when the gas-liquid equilibrium is reached(p0 the saturation vapor pressure of the point right close to the flat surface,prand the saturation vapor pressure of the point near the concave surface in the capillary,which represents the saturation vapor pressure coexisting with the concave surface with radius of r),the isothermal atmospheric pressure formula can be used to find:0expmrM ghppRTrRTMppmr2ln0（Negative means concave,while positive means convex）Saturated vapor pressure at concave surface is low(bubble)Saturated vapor pressure at convex surface is high(droplet)The saturated vapor pressure in the bubble of liquid at the same temperature is lower than that near the flat liquid surface,and the saturated vapor pressure near the drop is higher than that near the flat liquid surface.For example:at 291K,the surface tension coefficient of water reaches 0.073 N/m,whose density stands at 1103 kg/m3，m108r0115.1pprm107r0011.1pprrRTMppmr2ln0 910 mr02.966rpp The smaller the droplet grows,the higher the saturated vapor pressure essential for the droplet to reach equilibrium will become,and the tougher and more tricky it will be to form and cultivate a drop.5.The Critical Radius for the Condensation of a Drop,Condensation Nuclear Condensation frequently takes place at the liquid surface when saturated vapor has been exposed to the liquid,while only by with the condensation nucleus at the center to gradually increase its size will the condensation of drops within the vapor can take place.rRTMppmr2ln0 The atmospheric temperature being T,the corresponding saturated vapor pressure of the flat liquid surface stands at p0.However,the actual vapor pressure in the atmosphere reaches p,which means that the radius rc of the drop,a physical quality that can achieve equilibrium with p,can be obtained from the above equation,as long as the pr is substituted by p.)/ln(20ppRTMrmccrr ppr1.To discussBeing supersaturated,such droplets can increase continuously;2.crr pprBeing undersaturated,the droplets will evaporate and disappear.3.Condensation conditions:(1)Saturated vapor pressure;(2)Condensation nucleus4.Supersaturated vapor pressure.02lnmcMppRTr 6.Supersaturated Vapor,Metastable State,Cloud Chamber what is required for condensation of vapor into droplets is a certain amount of condensation nucleus.Despite the higher vapor pressure than the saturated vapor pressure at this temperature,either insufficient condensation nucleus or rather small condensation nucleus will inhibit the formation and growth of the droplets,which means that supersaturation will appear,and such vapor is regarded as supersaturated vapor or supercooled vapor.supercooled vapormetastable The Cloud Chamber(an instrument for displaying high-speed charged particles and their tracks)in nuclear physics is also designed on the basis of this phenomenon.Artificial rainfallThanks！