This review mainly studies the development status, limitations, and future directions of modular microfluidic systems. Microfluidic technology is an important tool platform for scientific research and plays an important role in various fields. With the continuous development of microfluidic applications, conventional monolithic microfluidic chips show more and more limitations. A modular microfluidic system is a system composed of interconnected, independent modular microfluidic chips, which are easy to use, highly customizable, and on-site deployable. In this paper, the current forms of modular microfluidic systems are classified and studied. The popular fabrication techniques for modular blocks, the major application scenarios of modular microfluidics, and the limitations of modular techniques are also discussed. Lastly, this review provides prospects for the future direction of modular microfluidic technologies.

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The motion of bubbles in an ultrasonic field is a fundamental physical mechanism in most applications of acoustic cavitation. In these applications, surface-active solutes, which could lower the surface tension of the liquid, are always utilized to improve efficiency by reducing the cavitation threshold. This paper examines the influence of liquids’ surface tension on single micro-bubbles motion in an ultrasonic field. A novel experimental system based on high-speed photography has been designed to investigate the temporary evolution of a single bubble in the free-field exposed to a 20.43 kHz ultrasound in liquids with different surface tensions. In addition, the R-P equations in the liquid with different surface tension are solved. It is found that the influences of the surface tension on the bubble dynamics are obvious, which reflect on the changes in the maximum size and speed of the bubble margin during bubble oscillating, as well as the weaker stability of the bubble in the liquid with low surface tension, especially for the oscillating bubble with higher speed. These effects of the surface tension on the bubble dynamics can explain the mechanism of surfactants for promoting acoustic cavitation in numerous application fields.

For better applications of low-frequency sonophoresis in transdermal interstitial fluid extraction and transdermal drug delivery, the microscopic mechanism of skin permeabilization by low-frequency sonophoresis have drawn much attention. This paper examines the influence of the skin’s texture density on single micro-bubble’s motion near a skin-like wall in an ultrasonic field. A novel experimental system based on high-speed photography has been designed to investigate the temporary evolution of single bubble near skin-like walls with different texture densities exposed to a 20.51kHz ultrasound in liquids. In addition, the dynamic characteristics of single bubbles near skin-like walls with different texture densities are analyzed. It is found that the influences of the texture density on the bubble dynamics are obvious, which reflect on the changes in the maximum size of the bubble margin during bubble oscillating and the moving speed of the bubble to the wall, as well as the collapse time and the jet speed during collapses, especially for the bubble with an ultra-high texture density. These effects of the texture density on the bubble dynamics can explain the mechanism of skin texture structure for promoting acoustic cavitation in related application fields.

Microfluidics technology plays an important role in modern analytical instruments, while the modular design of microfluidics facilitates the reconfiguration of analytical instrument functions, making it possible to deploy on-demand systems in the field. However, modular design also faces the challenges such as connection reliability and reconfiguration convenience. Inspired by the self-locking structure of the Rubik’s cube, a modular, reconfigurable microfluidic instrument architecture is proposed in this paper. The system has a self-locking structure of Rubik’s cube components and an O-ring-based alignment and sealing mechanism, which enables reliable interconnection and rapid rearrangement of microfluidic modules by simply rotating the faces of the microfluidic cube. In addition, the system is capable of integrating a variety of customized modules to perform analysis tasks. A proof-of-concept application of detecting multiple pollutants in water is demonstrated to show the reconfigurable characteristics of the system. The findings of this paper provide a new idea for the design of microfluidic analytical instrument architectures.