Droplet-based microfluidics is a powerful tool for producing monodispersed micrometer-sized droplets with controlled sizes and shapes; thus, it has been widely applied in diverse fields from fundamental science to industries. Toward a simpler method for fabricating microparticles with front–back asymmetry in their shapes, we studied anisotropic gelation of alginate droplets, which occurs inside a flow-focusing microfluidic device. In the proposed method, sodium alginate (NaAlg) aqueous phase fused with a calcium chloride (${\mathrm{CaCl}}_2$) emulsion dispersed in the organic phase just before the aqueous phase breaks up into the droplets. The fused droplet with a front–back asymmetric shape was generated, and the asymmetric shape was kept after geometrical confinement by a narrow microchannel was removed. The shape of the fused droplet depended on the size of prefused NaAlg aqueous phase and a ${\mathrm{CaCl}}_2$ emulsion, and the front–back asymmetry appeared in the case of the smaller emulsion size. The analysis of the velocity field inside and around the droplet revealed that the stagnation point at the tip of the aqueous phase also played an important role. The proposed mechanism will be potentially applicable as a novel fabrication technique of microparticles with asymmetric shapes.

中文翻译：

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使用各向异性凝胶制备具有前后不对称形状的微粒

基于液滴的微流体是一种强大的工具，可用于生产尺寸和形状可控的单分散微米级液滴；因此，它已被广泛应用于从基础科学到工业的各个领域。为了制造具有前后不对称形状的微粒的更简单方法，我们研究了藻酸盐液滴的各向异性胶凝，它发生在流动聚焦微流体装置内。在所提出的方法中，海藻酸钠 (NaAlg) 水相与氯化钙 (${\mathrm{氯化钙}}_2$) 就在水相分裂成液滴之前分散在有机相中的乳液。产生具有前后不对称形状的熔融液滴，并且在去除窄微通道的几何限制后保持不对称形状。熔滴的形状取决于预熔的 NaAlg 水相的大小和${\mathrm{氯化钙}}_2$乳剂，并且在较小的乳剂尺寸的情况下出现前后不对称。对液滴内部和周围速度场的分析表明，水相尖端的停滞点也起着重要作用。所提出的机制将潜在地适用于具有不对称形状的微粒的新型制造技术。