A lattice Boltzmann method is used to explore the effect of surfactants on the unequal volume breakup of a droplet in a T-junction microchannel, and the asymmetry due to fabrication defects in real-life microchannels is modelled as the pressure difference between the two branch outlets ( $\Delta {P^\ast }$ ). We first study the effect of the surfactants on the droplet dynamics at different dimensionless initial droplet lengths ( $l_0^\ast $ ) and capillary numbers (Ca) under symmetric boundary conditions ( $\Delta {P^\ast } = 0$ ). The results indicate that the presence of surfactants promotes droplet deformation and breakup at small and moderate $l_0^\ast $ values, while the surfactant effect is weakened at large $l_0^\ast $ values. When the branch channels are completely blocked by the droplet, a linear relationship is observed between the dimensionless droplet length ( $l_d^\ast $ ) and dimensionless time ( ${t^\ast }$ ), and two formulas are proposed for predicting the evolution of $l_d^\ast $ with ${t^\ast }$ for the two systems. We then investigate the effect of the surfactants on the droplet breakup at different values of $\Delta {P^\ast }$ and bulk surfactant concentrations ( ${\psi _b}$ ) under asymmetric boundary conditions ( $\Delta {P^\ast } \ne 0$ ). It is observed that, as $\Delta {P^\ast }$ increases, the volume ratio of the generated droplets ( ${V_1}/{V_2}$ ) decreases to 0 in both systems, while the rate of decrease is higher in the clean system, i.e. the presence of surfactants could cause a decreased pressure difference between the droplet tips. As ${\psi _b}$ increases, ${V_1}/{V_2}$ first increases rapidly, then remains almost constant and finally decreases slightly. We thus establish a phase diagram that describes the ${V_1}/{V_2}$ variation with $\Delta {P^\ast }$ and ${\psi _b}$ .

In this paper, the dynamic behavior of a self-propelled droplet along a conical fiber is simulated by using an improved lattice Boltzmann color-gradient method. This method is developed on the basis of our recently developed density ratio model [Zhang et al., Int. Commun. Heat Mass Transfer, 137, 106284 (2022).], but a wetting boundary condition is added to account for the moving contact line on an arbitrary solid surface. First, this method is validated against the analytical droplet shapes and contact angles for droplets surrounded by matrix fluids of different densities on flat and spherical surfaces, and the spontaneous transport of a droplet on a conical fiber. This method is then adopted to systematically study the effects of the Bond number (Bo), surface wettability (θ), cone hemi-angle (α), and droplet volume on the droplet dynamic behavior. In each case, the results show that the droplet climbing velocity first increases and then decreases, and a velocity fluctuation is observed, which is due to that the apparent receding and advancing contact angles do not simultaneously reach the equilibrium contact angle. As droplet volume increases, the equilibrium droplet height monotonically increases. As Bo or θ increases, the droplet climbing height and the wetting area both decrease. We also found that the equilibrium climbing height first increases and then decreases with α, and its maximum is reached around α=2.5°.

As a novel type of catalytic Janus micromotor (JM), a double-bubble-powered Janus micromotor has a distinct propulsion mechanism that is closely associated with the bubble coalescence in viscous liquids and corresponding flow physics. Based on high-speed camera and microscopic observation, we provide the first experimental results of the coalescence of two microbubbles near a JM. By performing experiments with a wide range of Ohnesorge numbers, we identify a universal scaling law of bubble coalescence, which shows a cross-over at dimensionless time $\tilde{t}$ = 1 from an inertially limited viscous regime with linear scaling to an inertial regime with 1/2 scaling. Due to the confinement from the nearby solid JM, we observe asymmetric neck growth and find the combined effect of the surface tension and viscosity. The bubble coalescence and detachment can result in a high propulsion speed of ∼0.25 m s⁻¹ for the JM. We further characterise two contributions to the JM’s displacement propelled by the coalescing bubble: the counteraction from the liquid due to bubble deformation and the momentum transfer during bubble detachment. Our findings provide a better understanding of the flow dynamics and transport mechanism in micro- and nano-scale devices like the swimming microrobot and bubble-powered microrocket.

The purpose of this study is to use a weak light source with spatial distribution to realize light-driven fluid by adding high-absorbing nanoparticles to the droplets, thereby replacing a highly focused strong linear light source acting on pure droplets. First, Fe₃O₄ nanoparticles with high light response characteristics were added to the droplets to prepare nanofluid droplets, and through the Gaussian light-driven flow experiment, the Marangoni effect inside a nanofluid droplet was studied, which can produce the surface tension gradient on the air/liquid interface and induce the vortex motion inside a droplet. Then, the numerical simulation method of multiphysics field coupling was used to study the effects of droplet height and Gaussian light distribution on the flow characteristics inside a droplet. Nanoparticles can significantly enhance the light absorption, so that the Gaussian light is enough to drive the flow, and the formation of vortex can be regulated by light distribution. The multiphysics field coupling model can accurately describe this problem. This study is helpful to understand the flow behavior and heat transfer phenomenon in optical microfluidic systems, and provides a feasible way to construct the rapid flow inside a tiny droplet by light.

The emerging application of a microbubble-driven micromotor encounters such a dilemma: persistence of long-distance transport is strongly desired, whereas the bubble micromotor usually manifests circular or spiral motions, which are supposed to be random owing to Brownian dynamics at the microscale. However, this assumption based on the model of active Brownian motion is debatable for bubble micromotors, and what mechanism could lead to deterministic motion of the bubble micromotor is still unclear. In this work, we show a counterintuitive phenomenon that a smaller bubble-driven Janus micromotor is more likely to manifest a straight trajectory than a larger one. Our model explains that the microbubble position determines the shape of the trajectory as the bubble introduces a propulsion force and a rotational torque to drive the motion of the micromotor. We perform numerical simulation to reveal how interfacial hydrodynamics coupled with surface chemical reaction determines the bubble nucleation position. Surprisingly, the result of predictable bubble position indicates a deterministic mechanism for the circular motion of a bubble-driven Janus micromotor, which shows good agreement with our experimental observation. Our findings provide valuable insight for developing manipulation strategies for bubble-driven micromotors/microrobots based on controllable deterministic mechanisms.

With the development of society, crowds in public buildings are dense, and the subsequent spread of diseases in places with high population density has attracted widespread attention. In the real situation, the spread of the disease has its complexity, such as the influence of factors such as the movement of people and the intervention of external measures, so it is necessary to further refine the description of the process of infection transmission. Based on the social force model, this article focuses on the impact of microscopic crowd movement in public places on the dynamics of infection. Taking a typical pedestrian exit process in a subway station as an example, we numerically simulated the infection problem among small-scale people in public places, observed the impact of pedestrian movement characteristics on infection transmission, put forward corresponding prevention and control measures, and verified the effectiveness of the measures.