载体液滴经微细管成形过程动力学特性的实验与模拟

Experimental and Simulation Studies on Dynamic Formation of Sodium Alginate Drops in Miniature Tubes

利用微细管成形及后续交联和致孔过程,可以制备尺寸可控的载体微珠或晶珠。该过程中,微滴的成形是控制粒径的关键步骤,其成形过程动力学特性的研究具有重要意义。以海藻酸钠载体液滴为对象,通过高速摄影方法,用不同内径(1.1,1.6,1.9,2.2 mm)的微细管,对微滴成形过程中的形貌特征和界面演变动力学进行了实验研究,考察了微细管内径对液滴成形、滴落下落速度、液滴直径分布、颈缩线长度等的影响规律;进而,用VOF(volume of fluid)法中PLIC(piecewise linear interface calculation)的几何重构方法追踪非牛顿流体液滴形成过程的气液交界面,对相应液滴成形过程进行了模拟。结果表明,海藻酸钠溶液滴落下落速度、平均直径及颈缩线长度随着微细管内径的增加而增大;对于浓度2%的海藻酸钠溶液,以内径2.2 mm的微细管为例,当管内流速为30 mm·s^(-1),实验所得液滴脱落时最大颈缩线长度约为9.24 mm,滴落脱落时的速度约为6.09 mm·s^(-1),液滴直径约为5.8 mm;较内径1.1 mm的微细管所得液滴的参数值分别高119.04%、129.81%、39.13%;相应模拟所得液滴颈缩线长度、脱落时的速度及直径与实验结果的最大相对误差分别为8.7%、2.1%、8.6%,与实验结果基本一致,说明该方法适于微细管内非牛顿流体如海藻酸钠溶液成滴过程的模拟。

Injection methods using micro or miniature tubes followed by crosslinking or pore-forming cryogenic steps are effective in the fabrication of microbeads or cryogel-beads with controllable bead size distribution. The droplet formation process is crucial to the final size of microbeads and it is important to investigate dynamic characteristics of the droplet formation process. In this work, high-speed imaging was employed to study dynamic behaviors of sodium alginate droplet formation using different miniature tubes with inner diameters of 1.1, 1.6, 1.9 and 2.2 mm, respectively. The effects of tube diameter on droplet formation, dripping speed, diameter distribution and micro-thread length were investigated. Geometric reconstruction based on piecewise linear interface construction （PLIC） of the volume of fluid （VOF） method was used to capture gas-liquid interface, and the droplet generation process was numerically simulated. The experimental results show that drip velocity, mean diameter and micro-thread length of the sodium alginate droplets increase with the increase of tube diameter. For 2% （w/w） sodium alginate solution under tube inner diameter of 2.2 mm at flow velocity of 30 mm·s^-1, the maximum micro-thread length is 9.24 mm, the dripping speed is 6.09 mm·s^-1 and the mean diameter of droplets is 5.8 ram. These values are 119.04%, 129.81% and 39.13% higher than those obtained with tube inner diameter of 1.1 mm, respectively. The simulation results show that compared with experimental results, the maximum relative errors of the micro-thread length, dripping velocity and mean diameter of the droplets are 8.7%, 2.1% and 8.6%, respectively, which indicates that the simulation method is effective for describing and simulating droplet formation processes of non-Newtonian fluids like sodium alginate solution with micro or miniature tubes.