Study On Bubble Breakup And Interfacial Dynamic In Microchannels

Microchemical process is an advanced technology emerged in the last two decades. The bubble behavior and dynamics in microchannel is one of the most important issues for this technology. The bubble breakup in the microchannels was investigated experimentally in present thesis, by using a high speed digital camera and a high resolution microscopy. The main contents in this study are listed as follows:Four various flow patterns including breakup with permanent obstruction(B1), breakup with part obstruction(B2), breakup with permanent tunnel(B3) and non-breakup(NB) were observed in asymmetric microfluidic T-junctions, and the transition mechanisms between them were studied. Two critical dimensionless bubble lengths were introduced to describe the transitions between B1 and B2 and that between B2 and B3. The bubble broke more easily in the asymmetric microfluidic T-junction used in present work than in the symmetric one.Bubble breakup with permanent obstruction in an asymmetric microfluidic T-junction was investigated experimentally. The experiment was conducted in a square microchannel with the width of 400 μm. The breakup process of bubbles could be divided into three stages: squeezing, transition and pinch-off stages. In the squeezing stage, the thinning of the bubble neck was mainly controlled by the velocity of the fluid flowing into the T-junction, and the increase of the liquid viscosity could promote this process. In the transition stage, the minimum width of bubble neck decreased linearly with time. In the pinch-off stage, the effect of the velocity of the fluid flowing into the T-junction on the thinning of the bubble neck became weaker, and the increase of the liquid viscosity would delay this process. The bubble length had little influence on the whole breakup process of bubbles. A relationship for predicting the breakup time was also proposed: 0.93/ 2( /)c c c T T u T w-(28).The bubble breakup with partly obstruction in a microfluidic T-junction was investigated experimentally. A tunnel characterizing the bubble breakup with partly obstruction appeared between the bubble tip and the microchannel wall in the squeezing stage of the breakup, which had less effect on the evolution of the bubble neck. However, the evolutions of the bubble tip were obviously different before and after tunnel’s appearance. By means of the analysis on the dynamics of the bubble tip, some important parameters such as the final bubble length and the leakage volume were studied and discussed profoundlyThe critical condition for bubble breakup in the microfluidic flow-focusing junction was studied. The analysis on the experimental observations showed that the critical condition was closely related to two time-scales: the breakup time and deformation time. By analyzing the dynamical gas-liquid interface shape of the bubble flowing through the microfluidic flow-focusing junction, the beginning and ending moments of the breakup and deformation processes were defined. Based on analyzing the effects of several factors on the breakup and deformation process, the mathematical models were constructed for the breakup time and the deformation time, respectively. The critical condition for bubble breakup was finally obtained from these two models.