Study On Properties And Applications Of Bulk Nanobubbles

Since the concept of bulk nanobubbles was first proposed 40 years ago,the researches on nanobubbles has continued to advance in debates.As the existing gas dissolution and diffusion theory(E-P theory)and the theory of Laplace Pressure Bubble Catastrophe theory predict that nano bubbles cannot exist stably,researchers have tried to find various indirect evidences to prove that nanobubbles can exist stably.Firstly,based on the specific fluorescent probe molecule(C₆N₄)synthesized by our cooperative team,we explored the indirect and direct evidence of the stable existence of nanobubbles.The probe molecules produce fluorescence effect only when gathering at gas-liquid interface.In this thesis,the degassing process,the concentration of the particles and the intensity of fluorescence formed by the aggregation of the probe molecules at the gas-liquid interface of ultrapure water and water containing nanobubbles prepared were compared.The results show that the degassing process of water containing nanobubbles is more intense,and lasts longer;the number of particles in water containing nanobubbles per milliliter is 1.13 × 108,which is much larger than that in ultrapure water(7.74 × 106);the peak fluorescence intensity of water containing nanobubbles is about 2.5 times of that of ultrapure water,indicating that there are a lot of gas-liquid interfaces in water containing nanobubbles.The three experiments describes the difference between water containing nanobubbles and ultrapure water from three aspects:the content of gas,the number of particles per milliliter and the area of gas-liquid interface.The results provided indirect evidence for the existence of stable nanobubbles.In addition,the probe molecule C₆N₄ was added into the water containing the nanobubbles,and then the nanospheres with fluorescence formed by the aggregation of the probe molecules on the gas-liquid interface were obtained by confocal laser scanning microscope.The result provides direct evidence of existence of stable nanobubbles.The above experiments provide indirect and direct evidence for the stable existence of nanobubbles,which is helpful to end the academic debate on the existence of nanobubbles.Based on the above experimental results,the properties of nanobubbles were studied.This thesis focuses on whether the nanobubbles can produce hydroxyl radicals or hydrogen peroxide.Firstly,the water containing nanobubbles was detected by electron spin resonance,and the hydroxyl radicals was not detected because the concentration is too low.Secondly,based on fluorescence spectrophotometry,the concentration of hydroxyl radical in water containing nanobubbles was detected to be 6.58 ?M/L by using the probe molecule terephthalic acid,and the maximum fluorescence intensity increases with the increase of bubble concentration.These results indicate that the formation of hydroxyl radicals in water containing nanobubbles is related to nanobubbles.It is possible that hydroxyl radicals are formed during the collapse of nanobubbles.Thirdly,the effect of ultrasonic cavitation on the content of hydroxyl radicals was studied.The results show that the ultrasonic cavitation process itself will produce hydroxyl radicals,but the existence of nanobubbles further increased the concentration of hydroxyl radicals in a short time.Finally,based on fluorescence spectrophotometry,the presence of hydrogen peroxide in water containing nanobubbles was detected by using the probe molecule amplex-red,and it was found that the generation of hydrogen peroxide was persistent.Therefore,this may be related to the rupture of nanobubbles.Although the existing studies suggest that hydroxyl radicals are produced in the process of nanobubble rupture,the formation mechanism is still not fully clear.In this paper,we mainly studied the effect of water containing nano bubbles on the germination and growth of bean seeds(mung bean,soybean and red bean).The results showed that compared with the water containing only air nanobubbles,the water containing both air nanobubbles and hydrogen nanobubbles could accelerate the germination process of seeds and improve the germination rate of seeds.In addition,water containing nanobubbles can reduce the relative conductivity of leaves and promote the formation of chlorophyll,which can protect and promote the growth of plants.Water containing nanobubbles reduced chlorophyll content in soybean leaves.This may be due to the existence of reactive oxygen species in the water containing nanobubbles exceeding the toxic threshold of plant cells,and then causing damage to the cells.Therefore,it is very important to control the concentration and the gas composition of nanobubbles for improving agricultural production.