Strong rotating flow in stationary droplets in low power budget using wire electrode configuration

We explore a simple strategy of generating strong rotating flow in a stationary surface‐droplet, using an intricate interplay of local electrical and thermal fields. Wire electrodes are employed to generate on‐spot heating without necessitating any elaborate micro‐fabrication, which causes strong local gradients in electrical properties to induce mobile charges into the droplet. Applying a low voltage (～10 V), strong rotational velocity of the order of mm/s can be achieved in the system, within the standard operating ranges of operating and geometrical parameters. Further, altering the diameter of the electrode, vortices can be tuned locally or globally in low power budget, without incurring any droplet oscillations. These results may turn out to be of immense consequence in enhancing micromixing in a plethora of droplet based applications ranging from thermal management to medical diagnostics to be potentially employed in resource‐limited settings.     

Interfacial tension in polyelectrolyte systems exhibiting associative liquid–liquid phase separation

A continued interest in polyelectrolyte phase diagrams guides the study of interfacial phenomena driven by polyelectrolyte complexation. The liquid–liquid interfaces formed by associative phase separation of oppositely charged synthetic and natural polyelectrolytes provide measurement challenges addressed by force-sensitive methods and deformed droplet retraction. The ultralow interfacial tension, typical of these systems, is sensitive to salt concentration and temperature and displays universal features described by mean-field theory. Several areas of fundamental development and novel applications of charge complexation for interfacial study and examples from membraneless organelles and biomolecular condensates are described.     

Acoustically responsive polydopamine nanodroplets: A novel theranostic agent

Ultrasound-induced cavitation has been used as a tool of enhancing extravasation and tissue penetration of anticancer agents in tumours. Initiating cavitation in tissue however, requires high acoustic intensities that are neither safe nor easy to achieve with current clinical systems. The use of cavitation nuclei can however lower the acoustic intensities required to initiate cavitation and the resulting bio-effects in situ. Microbubbles, solid gas-trapping nanoparticles, and phase shift nanodroplets are some examples in a growing list of proposed cavitation nuclei. Besides the ability to lower the cavitation threshold, stability, long circulation times, biocompatibility and biodegradability, are some of the desirable characteristics that a clinically applicable cavitation agent should possess. In this study, we present a novel formulation of ultrasound-triggered phase transition sub-micrometer sized nanodroplets (~400 nm) stabilised with a biocompatible polymer, polydopamine (PDA). PDA offers some important benefits: (1) facile fabrication, as dopamine monomers are directly polymerised on the nanodroplets, (2) high polymer biocompatibility, and (3) ease of functionalisation with other molecules such as drugs or targeting species. We demonstrate that the acoustic intensities required to initiate inertial cavitation can all be achieved with existing clinical ultrasound systems. Cell viability and haemolysis studies show that nanodroplets are biocompatible. Our results demonstrate the great potential of PDA nanodroplets as an acoustically active nanodevice, which is highly valuable for biomedical applications including drug delivery and treatment monitoring.     

Continuous Flow Pickering Emulsion Catalysis in Droplet Microfluidics Studied with In Situ Raman Microscopy

Pickering emulsions (PEs), emulsions stabilized by solid particles, have shown to be a versatile tool for biphasic catalysis. Here, we report a droplet microfluidic approach for flow PE (FPE) catalysis, further expanding the possibilities for PE catalysis beyond standard batch PE reactions. This microreactor allowed for the inline analysis of the catalytic process with in situ Raman spectroscopy, as demonstrated for the acid-catalyzed deacetalization of benzaldehyde dimethyl acetal to form benzaldehyde. Furthermore, the use of the FPE system showed a nine fold improvement in yield compared to the simple biphasic flow system (FBS), highlighting the advantage of emulsification. Finally, FPE allowed an antagonistic set of reactions, the deacetalization–Knoevenagel condensation, which proved less efficient in FBS due to rapid acid-base quenching. The droplet microfluidic system thus offers a versatile new extension of PE catalysis.     

Discontinuous Galerkin finite element method applied to the coupled unsteady Stokes/Cahn-Hilliard equations

Two-phase flows driven by the interfacial dynamics are studied by tracking implicitly interfaces in the framework of the Cahn-Hilliard theory. The fluid dynamics is described by the Stokes equations with an additional source term in the momentum equation taking into account the capillary forces. A discontinuous Galerkin finite element method is used to solve the coupled Stokes/Cahn-Hilliard equations. The Cahn-Hilliard equation is treated as a system of two coupled equations corresponding to the advection-diffusion equation for the phase field and a nonlinear elliptic equation for the chemical potential. First, the variational formulation of the Cahn-Hilliard equation is presented. A numerical test is achieved showing the optimal order in error bounds. Second, the variational formulation in discontinuous Galerkin finite element approach of the Stokes equations is recalled, in which the same space of approximation is used for the velocity and the pressure with an adequate stabilization technique. The rates of convergence in space and time are evaluated leading to an optimal order in error bounds in space and a second order in time with a backward differentiation formula at the second order. Numerical tests devoted to two-phase flows are provided on ellipsoidal droplet retraction, on the capillary rising of a liquid in a tube, and on the wetting drop over a horizontal solid wall.     

STATIC SPREADING OF DROPLET IMPACT ON SOLID SURFACE：INFLUENCE FACTOR

通过建立液滴撞击固体平壁的静态铺展力学平衡的数学模型,从理论上得到了静态铺展半径与液滴物性参数、以及液滴与固体壁面接触角之间关系的数学表达式,将理论结果与数值模拟的结果进行了比较,两者吻合较好.比较了不同条件下液滴的静态铺展半径的变化规律,分别得到了液滴密度、体积、表面张力和接触角等因素对液滴静态铺展半径的影响规律.     

Droplet impact on wetted structured surfaces

In this study, we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities. The time evolution of crown geometry is strongly affected by the surface structure. When the thickness of the liquid film is larger than the structure height, the expanding speed of the crown base radius is independent of the structure width. However, if the liquid film thickness is equal to the structure height, the crown base expands slower as...     

Crystallization of interphase droplets in emulsion explosive matrices

The instability of emulsion explosive matrices is mainly due to the crystallization of interphases as oversaturated aqueous solutions of nitrate salts. The principal features of crystallization for this type of emulsion have been previously studied; however, there is no consensus regarding the mechanism of crystallization for an emulsion explosive matrix. This study is devoted to the investigation of the crystallization behavior of interphase droplets. By monitoring the mass change of emulsions during their aging process, it was found that the mass of the emulsions remains almost constant and that water still completely existed in the emulsion system after crystallization of the interphase droplets. The ammonium nitrate (NH₄NO₃) crystals in the emulsion explosive matrices were then separated successfully using a simple method. The thermal behavior of pure NH₄NO₃ and crystals in the emulsion explosive matrices was studied by differential scanning calorimetry-thermogravimetric analysis (DSC-TG) at a heating rate of 10?K/min. The experimental results show that the thermal behavior of the crystals in the emulsions was exactly the same as for pure NH₄NO₃, meaning that only NH₄NO₃ crystallized from the emulsion explosive matrices with no water crystals. Thus, it could be concluded that after crystallization of the dispersed drops in the emulsion explosive matrices, pure NH₄NO₃ crystals and new smaller droplets were produced.     

基于各向异性表面的液滴驱动

驱动液滴实现各种动态行为在生物医学、微流控、痕量检测等领域具有重要应用.液滴的驱动主要依赖于对液滴不同位置受力的调节.具有浸润性差异或结构差异的各向异性表面,在对液滴进行驱动时具有操作简便、节约能源等优势,逐渐成为液滴操控领域的研究热点之一.本文结合本课题组的研究工作,对近年来利用各向异性表面驱动液滴的相关研究进行了综...