We develop a phase-field multiphase lattice Boltzmann model to systematically investigate the dynamic behaviour of a droplet passing through a microfluidic T-junction, especially focusing on the non-breakup of the droplet. Detailed information on the breakup and non-breakup is presented, together with the quantitative evolutions of driving and resistance forces as well as the droplet deformation characteristics involved. Through comparisons between cases of non-breakup and breakup, we find that the appearance of tunnels (the lubricating film between droplet and channel walls) provides a precondition for the final non-breakup of droplets, which slows down the droplet deformation rate and even induces non-breakup. The vortex flow formed inside droplets plays an important role in determining whether they break up or not. In particular, when the strength of vortex flow exceeds a critical value, a droplet can no longer break up. Additionally, more effort has been devoted to investigating the effects of viscosity ratio between disperse and continuous phases and width ratio between branch and main channels on droplet dynamic behaviours. It is found that a large droplet viscosity results in a small velocity gradient in a droplet, which restricts vortex generation and thus produces lower deformation resistance. Consequently, it is easier to break up a droplet with larger viscosity. Our work also reveals that a droplet in small branch channels tends to obstruct the channels and have small vortex flows, which induces easier breakup too. Eventually, several phase diagrams for droplet flow patterns are provided, and the corresponding power-law correlations ( $l_{0}/w=\unicode[STIX]{x1D6FD}Ca^{b}$ , where $l_{0}/w$ is dimensionless initial droplet length and $Ca$ is capillary number) are fitted to describe the boundaries between different flow patterns.

Two-phase (liquid, vapor) flow in confined spaces is fundamentally interesting and practically important in many practical applications such as thermal management, offering the potential to impart high thermal transport performance owing to high surface-to-volume ratio and latent heat released during liquid/vapor phase transition. However, the associated physical size effect, in coupling with the striking contrast in specific volume between liquid and vapor phases, also leads to the onset of unwanted vapor backflow and chaotic two-phase flow patterns, which seriously deteriorates the practical thermal transport performances. Here, we develop a thermal regulator consisting of classical Tesla valves and engineered capillary structures, which can switch its working states and boost its heat transfer coefficient and critical heat flux in its “switched-on” state. We demonstrate that the Tesla valves and the capillary structures serve to eliminate vapor backflow and promote liquid flow along the sidewalls of both Tesla valves and main channels, respectively, which synergistically enable the thermal regulator to self-adapt to varying working conditions by rectifying the chaotic two-phase flow into an ordered and directional flow. We envision that revisiting century-old design can promote the development of next generation cooling devices towards switchable and very high heat transfer performances for power electronic devices.

Microfluidic systems with splitting structures are excellent for increasing emulsion production. However, traditional two-dimensional (2D) lithographic systems require complex modification of the microchannel surfaces and achieve only 2D splitting of the droplets. Herein, we present new glass capillary microfluidic devices that perform three-dimensional (3D) splitting of droplets. These devices are simply constructed using different structural glass capillaries as the collection microchannels of the droplet microfluidic systems. We demonstrate that the devices are able to produce a 3D split of both single emulsions and double emulsions into two and three portions, respectively. These emulsions, after the splitting process, still have high monodispersity. We believe that this new technique for 3D splitting could be widely used, not only in the field of microfluidics but also in chemical/biological applications (e.g., drug delivery, micro-dispersion, etc.).