| Artificial fog,as a aerosol with high cost efficiency,was widely used in military camouflage,aerosol fumigation,pesticide spraying,weather simulation and other fields.Recently,the preparation of it was concentrated in explosion and dispersion,which not only caused serious hazard to both environment and human health,but also exhibited short timeliness and poor stability.Moreover,the single efficacy of artificial fog could not meet the current demand,and little research focused on its systematic and fundamental mechanism of formation and stabilization.Therefore,based on the coupling between stability of supersaturated vapor and temperature,the process of fogging with vapor prepared by the thermal fogger,was explored,which high rate might be attributed to the huge difference in temperature before and after atomization.We merged theory with experiments to carry on a comprehensive study on the formation and stability of mechanism among different series of polyol fog,using pure water fog as a reference,from thermodynamics and kinetics.（1）Formation of different series of polyol fog.In this paper,a self-developed fog chamber was used for investigating the effects of different fog（pure water fog,polyol fog A,B and C）on the shielding of visible light,far-infrared way and the dynamic distribution of droplets.Among them,polyol fog agent（A）was the basic one,in which water was acted as the substrate and polyol was the auxiliary.Polyol agent（B）was a kind of micro-emulsion by the addition of small molecular ester and TX-100.Polyol agent（C）was further added with solid particles on the basis of polyol agent（B）.Results indicated that the formation of pure water fog not only need consume large amount of material,but also accompanied by poor fog,under the same low humidity environment（25℃,40%RH）.However,the consumption of series of polyol fog agents（A,B and C）only occupied 1/20 of that of pure water.And the corresponding fog showed strong stability,which exhibited an increasing trend based on the above order.（2）Formation mechanism among series of polyol fog.On the basis of classical thermodynamics,the entropy change in the whole fog formation process was calculated,which quantified the difference between series of polyol fog and water fog from the perspective of entropy.Results indicated that there were all an entropy reduction in the formation of water and polyol water fog.Thereinto,the range of entropy reduction（?S2）between polyol fog A and B was from 65.61 to 69.79 J·K-1,the value of which was less than that of pure water(72.35 J·K-1).The above-phenomenon reflected that it was much more difficult to break through the non-spontaneous stage for the formation of pure water fog in thermodynamics,and revealed the bottleneck of fogging macroscopitally.In addition,based on the intrinsic connection with nucleation theory,the critical Gibbs free energy and critical radius was further quantified during the formation of stable droplets from vapor,which further revealed the fogging mechanism from the perspective of energy.Meanwhile,two related quantitative calculation was elaborated,in which one focused on intrinsic physical properties and the other emphasized on fogging process.It turned out that the critical Gibbs free energies of water,glycerol and triethylene glycol were 3.107×10-20,2.659×10-20and 2.643×10-20J respectively,and the corresponding critical radius were0.319×10-3,0.317×10-3and 0.389×10-3μm respectively,according to physical properties.It was easy to see that the critical Gibbs free energy of water was significantly greater than the others,and the minimum value of the critical radius among three components belonged to that of glycerol.Based on this,we speculated that glycerol might form liquid core preferentially.Combined with the actual process in the present study,the supersaturation of water only was 0.673 under the mass concentration(4.63 g·m-3),which might be the principal reason for no stable droplets and thick fog,even if there was a large temperature difference.For polyol fog（A）and（B）,the supersaturated components were all glycerol and triethylene glycol,and the supersaturation of glycerol was higher than that of triethylene glycol.Besides.there was a wider variation of two substances in polyol fog（A）than that in polyol fog（B）.According to the critical Gibbs free energy(1.600×10-19-2.779×10-19,2.946×10-19-7.119×10-19J)and critical radius(0.779×10-3-1.027×10-3,1.300×10-3-2.021×10-3μm)of glycerol and triethylene glycol,it was consistent with the conclusion obtained by the analysis based on physical properties,as was that of polyol fog（B）.The preferential formation of liquid nuclei was equivalent to the reduced barrier for other steam in the condensation process,promoting the formation of fog.While,solid particles were added into polyol fog（C）,meaning heterogeneous nucleation.Results showed that supersaturated components,supersaturation,and critical radius of polyol fog（C）was the same as that of polyol fog（B）.However,its critical Gibbs free energy of glycerol(0.412×10-21J)and triethylene glycol(1.152×10-21J)was more than 100 times lower than that of values of polyol fog（B）,promoting the formation of fog vastly.（3）Stability mechanism among series of polyol fog.Based on stabilization time and dynamic changes of droplets of pure water fog and the optimal formula of series polyol fog,combining with coagulation coefficient,“Stokes”sedimentation,monolayer theory and physical properties of carbon materials,the differences between various fog and the mechanism of stability were revealed.Results indicated that the maximum stabilization time of pure water fog（200 g）was 20 min.And the average coagulation coefficient and sedimentation rate was 0.599×10-15m3·s-1and 34.1921 cm·s-1at 0 min,respectively.However,the stability time of polyol fog（A,B and C）under the condition of 50%shading,could reached to 120 min.At 0 min,the coagulation coefficient of polyol fog（A,B and C）was 1.755×10-15,1.665×10-15and 1.433×10-15m3·s-1respectively.And the corresponding sedimentation rate was 0.0418,0.0363 and 0.0262 cm·s-1.In addition,at 120 min,the value of coagulation coefficient and sedimentation rate were 0.797×10-15,0.617×10-15,0.646×10-15m3·s-1and 4.7314,0.9767,0.9440 cm·s-1.As could be seen from the above,the distribution of water droplets were relatively uniform,accompanied with susceptible settlement due to larger particle size.The distribution of droplets made by polyol fog agents was much wider and along with lower amplification,settlement rate and longer stabilization time.（4）Application of polyol fog in agricultural fog fumigation.Based on systematic analysis of the formation and stability mechanism of water fog and polyol fog,the application of polyol fog in agricultural atomization fumigation was proposed.In this study,with fog agents as a carrier,the inductive effect of melatonin on crops during the fumigation was optimized by the means of thermal atomization.Results showed that polyol fog agent has no effect on endogenous melatonin content,and the content and metabolism of anthocyanin of strawberry.In the fumigation group along with glycerol,endogenous melatonin content was higher than that of single fumigation group.Meanwhile,it further increased anthocyanin content,delayed the aging of fruits,and up-regulated sharply genes related to anthocyanin metabolic pathway,which was mainly due to the long suspension time of fumigation containing fog agents,and further improving the effect of fumigation.In conclusion,the formation and stability mechanism was clarified through progressive and systematic analysis of water fog and polyol fog from thermodynamics and dynamics.This study not only made up the lack of this research,and met the demand for the characteristics of with high stability,long-term effectiveness and low consumption,but also broke the bottleneck,clarifying the theoretical guidance and research direction for performance optimization and wide application. |
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