This review focuses on experimental work on nonlinear phenomena in microfluidics, which for the most part are phenomena for which the velocity of a fluid flowing through a microfluidic channel does not scale proportionately with the pressure drop. Examples include oscillations, flow-switching behaviors, and bifurcations. These phenomena are qualitatively distinct from laminar, diffusion-limited flows that are often associated with microfluidics. We explore the nonlinear behaviors of bubbles or droplets when they travel alone or in trains through a microfluidic network or when they assemble into either one- or two-dimensional crystals. We consider the nonlinearities that can be induced by the geometry of channels, such as their curvature or the bas-relief patterning of their base. By casting posts, barriers, or membranes─situated inside channels─from stimuli-responsive or flexible materials, the shape, size, or configuration of these elements can be altered by flowing fluids, which may enable autonomous flow control. We also highlight some of the nonlinearities that arise from operating devices at intermediate Reynolds numbers or from using non-Newtonian fluids or liquid metals. We include a brief discussion of relevant practical applications, including flow gating, mixing, and particle separations.

中文翻译：

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微流体中的非线性现象

本综述侧重于微流体中非线性现象的实验工作，在大多数情况下，流经微流体通道的流体速度与压降不成比例。示例包括振荡、流量切换行为和分叉。这些现象在质量上不同于通常与微流体相关的层流、扩散受限的流动。我们探索气泡或液滴在单独旅行或通过微流体网络在火车中旅行或组装成一维或二维晶体时的非线性行为。我们考虑了由通道的几何形状引起的非线性，例如它们的曲率或它们底部的浅浮雕图案。通过铸造柱子、障碍物、或位于通道内的膜——来自刺激响应或柔性材料，这些元件的形状、大小或配置可以通过流动的流体来改变，这可以实现自主流动控制。我们还强调了在中间雷诺数下操作设备或使用非牛顿流体或液态金属产生的一些非线性。我们简要讨论了相关的实际应用，包括流控、混合和颗粒分离。