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Contactless droplet transport has gained intensive attention in recent years. In this study, the motion of a sessile binary droplet or a pure water droplet under the contactless Marangoni effect has been investigated experimentally and theoretically. The concrete form of the driving force originates from the evaporation of a pure liquid "source" droplet was achieved and verified by our experimental data. For a "target" droplet consisting of pure water, we discovered a non-negligible increase in its spreading radius R during motion. Based on the experimental results, we proposed a linear approximation between R and the droplet spacing and successfully introduced its influence on the driving force calculation through the perturbation theory. [ABSTRACT FROM AUTHOR]

We numerically study the head-on collisions of two immiscible droplets of different components and focus on the effects of droplet inertia and interfaces, which are expected to play a crucial role in the interaction between the two droplets. A ternary-fluid diffuse-interface method is used here after being validated by comparing against experiments of the collision between an aqueous droplet and a silicone oil droplet. In order to figure out how the droplet inertia and interfaces affect the dynamic behavior after the collision, axisymmetric simulations are performed with various Weber number We and surface tension ratio λ, i.e., the ratio of the surface tension coefficient of the liquid–liquid to the liquid–gas interfaces. Their effects on the film thickness, maximal deformation of the colliding droplets, and the corresponding contact time are investigated. To describe the collision dynamics, we propose an equivalent surface tension σ* based on the analysis of the energy conservation and morphology of the colliding droplets. Using the equivalent surface tension σ*, we theoretically predict the film thickness, maximal spreading time, and deformation of the colliding droplets. The theoretical predictions are in good agreement with the numerical results. [ABSTRACT FROM AUTHOR]

The rupture of coaxial liquid jets in co-flow focusing is studied experimentally and numerically, aiming to deepen the understanding of interface coupling and its effect on the morphology of the generated compound droplets. The weak, transitional, and strong coupling regimes of interface instabilities are identified in experiments by varying the flow rate ratio of the inner jet to the coaxial jets (denoted by rQ). The dynamics of coaxial liquid jets are further resolved by direct numerical simulations after being validated against experiments. The flow structures and pressure contours for the coaxial jets can be given under different coupling conditions. Moreover, scaling laws are proposed to correlate the breakup length of coaxial jets and the sizes of compound droplets and encapsulated cores with rQ, and a good agreement is obtained. It can be found that the transition from weak, transitional and strong coupling occurs approximately at rQ = 0.15 and rQ = 0.4, respectively. This work provides underlying insight into the interface coupling of coaxial liquid jets, which would provide theoretical guidance for microcapsule generation in various potential applications. [ABSTRACT FROM AUTHOR]

The incompressible two-phase flows are simulated using combination of an etching multiblock method and a diffuse interface (DI) model, particularly in the complex domain that can be decomposed into multiple rectangular subdomains. The etching multiblock method allows natural communications between the connected subdomains and the efficient parallel computation. The DI model can consider two-phase flows with a large density ratio, and simulate the flows with the moving contact line (MCL) when a geometric formulation of the MCL model is included. Therefore, combination of the etching method and the DI model has potential to deal with a variety of two-phase flows in industrial applications. The performance is examined through a series of numerical experiments. The convergence of the etching method is firstly tested by simulating single-phase flows past a square cylinder, and the method for the multiphase flow simulation is validated by investing drops dripping from a pore. The numerical results are compared with either those from other researchers or experimental data. Good agreement is achieved. The method is also used to investigate the impact of a droplet on a grooved substrate and droplet generation in flow focusing devices. [ABSTRACT FROM AUTHOR]

Axisymmetric droplet spreading is investigated numerically at relatively large rates of spreading, such that inertial effects become important. Results from two numerical methods that use different means to alleviate the stress singularity at moving contact lines (a diffuse interface, and a slip-length-based level-set method) are shown to agree well. An initial inertial regime is observed to yield to a regime associated with Tanner's law at later times. The spreading rate oscillates during the changeover between these regimes. This becomes more significant for a fixed (effective) slip length when decreasing the value of an Ohnesorge number. The initial, inertia-dominated regime is characterized by a rapidly extending region affected by the spreading, giving the appearance of a capillary wave travelling from the contact line. The oscillatory behaviour is associated with the rapid collapse that follows the point at which this region extends to the entire droplet. Results are presented for the apparent contact angle as a function of dimensionless spreading rate for various values of Ohnesorge number, slip length and initial conditions. The results indicate that there is no such universal relation when inertial effects are important. [ABSTRACT FROM AUTHOR]

In the pure axisymmetric flow focusing (AFF), it is difficult to generate uniform droplets due to the random breakup of bulk flows. Therefore, applying external perturbations is a desirable approach to control the monodispersity of the droplets and makes it possible to produce uniform droplets at relatively high flow rates. In the present study, the effects of different external perturbations (waveform shape, frequency f and amplitude A) on the droplet generation are numerically investigated. When the focused phase is singly actuated, the size of the uniform droplets can be obtained and manipulated by adjusting f and A. In particular, the jet breakup has the same frequency as the external perturbation in the generation of uniform droplets. There exists a cutoff frequency beyond which the perturbation cannot control the jet breakup, even with large A. This is found to be associated with the critical condition for the onset of the Rayleigh-Plateau instability. In addition, the reservoir effect of the cone in the AFF effectively reduces the influence of the perturbation on the liquid supply to the liquid jet, accounting for the presence of jetting at low f and large A. Moreover, we apply the perturbations either singly to the focusing phase or simultaneously to the focused and focusing phases and assess their effects on the jet breakup. Finally, the square waveform perturbations acting on the droplet generation are discussed. The present work provides a guidance of the actuation-aided AFF for practical applications of on-demand droplet generation. [ABSTRACT FROM AUTHOR]

We consider the growth rate of the height of the connecting bridge in rapid surface-tension-driven coalescence of two identical droplets attached on a partially wetting substrate. For a wide range of contact angle values, the height of the bridge grows with time following a power law with a universal exponent of 2/3, up to a threshold time, beyond which a 1/2 exponent results, that is known for coalescence of freely-suspended droplets. In a narrow range of contact angle values close to 90°, this threshold time rapidly vanishes and a 1/2 exponent results for a 90° contact angle. The argument is confirmed by three-dimensional numerical simulations based on a diffuse interface method with adaptive mesh refinement and a volume-of-fluid method. [ABSTRACT FROM AUTHOR]