Shang Luoran, Xu Ke, Lu Peter J, Abbaspourrad Alireza, Zhao Yuanjin, Weitz David A
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.
The Shanghai Key Laboratory of Medical Epigenetics, the International Co-Laboratory of Medical Epigenetics and Metabolism (Ministry of Science and Technology), Institutes of Biomedical Sciences, Fudan University, Shanghai 200032, China.
Proc Natl Acad Sci U S A. 2024 Oct 15;121(42):e2403953121. doi: 10.1073/pnas.2403953121. Epub 2024 Oct 10.
Droplets of one fluid in a second, immiscible fluid are typically spherical in shape due to the interfacial tension between the two fluids. Shear forces can lead to droplet deformation when they are subjected to flow, and these effects can be further modified when the droplet is stabilized by a surfactant due to a flow-induced gradients in the surfactant concentration. An alternative method of stabilizing droplets is through the use of colloidal particles, whose stabilization behavior is intrinsically different from molecular surfactants. Under the same flow condition, a gradient of particle concentration can result in the jamming of particles in regions with a high packing density, making the interface solid-like, albeit only under compression and not tension. However, how this asymmetry in the surfactant properties alters the droplet shape under shear is unknown. Here, we show that shear of particle-stabilized droplets can lead to a remarkable array of shape deformations as the droplets flow through a constrained microchannel. The shear-induced migration of particles on the surface results in the formation of an elastic shell at the back of the droplet, which can wrinkle and invaginate, ultimately leading to a unique core-shell structure. The shapes depend on the Peclet number of the flow, reflecting the balance of shear forces that drive the particles and diffusion that randomizes them. These findings highlight the consequences of the asymmetry in the forces between the particles and provide a unique method to controllably create droplets with a vast array of different shapes.
一种流体在第二种不混溶流体中的液滴通常呈球形，这是由于两种流体之间的界面张力所致。当液滴受到流动作用时，剪切力会导致液滴变形，并且当液滴通过表面活性剂稳定时，由于表面活性剂浓度的流动诱导梯度，这些影响会进一步改变。另一种稳定液滴的方法是使用胶体颗粒，其稳定行为与分子表面活性剂本质上不同。在相同的流动条件下，颗粒浓度梯度会导致颗粒在堆积密度高的区域发生堵塞，使界面呈固体状，尽管仅在压缩而非拉伸时如此。然而，这种表面活性剂性质的不对称如何在剪切作用下改变液滴形状尚不清楚。在这里，我们表明，当颗粒稳定的液滴流经受限微通道时，剪切作用会导致一系列显著的形状变形。颗粒在表面的剪切诱导迁移导致在液滴背面形成一个弹性壳，该壳会起皱和内陷，最终形成独特的核壳结构。形状取决于流动的佩克莱数，反映了驱动颗粒的剪切力与使颗粒随机化的扩散之间的平衡。这些发现突出了颗粒间力不对称的后果，并提供了一种独特的方法来可控地创建具有大量不同形状的液滴。
