Shape Of Liquid Droplet Research Articles

Droplet Shape Representation Using Fourier Series and Autoencoders

The shape of liquid droplets in air plays an important role in the aerodynamic behavior and combustion dynamics of miniaturized propulsion systems such as microsatellites and small drones. Their precise manipulation can yield optimal efficiency in such systems. It is desired to have a minimal representation of droplet shapes using as few parameters as possible to automate shape manipulation using self-learning algorithms, such as reinforcement learning. In this paper, we use a neural compression algorithm to represent, with only two parameters, elliptical and bullet-shaped droplets initially represented with 200 points (400 real numbers) at the droplet boundary. The mapping of many to two points is achieved in two stages. Initially, a Fourier series is formulated to approximate the contour of the droplet. Subsequently, the coefficients of this Fourier series are condensed to lower dimensions utilizing a neural network with a bottleneck architecture. Finally, 5000 synthetically generated droplet shapes were used to train the neural network. With a two-real-number representation, the recovered droplet shapes had excellent overlap with the original ones, with a mean square error of ∼10−3. Hence, this method compresses the droplet contour to merely two numerical parameters via a fully reversible process, a crucial feature for rendering learning algorithms computationally tractable.

Evolution of nanoliter size fluid droplet on micropatterned surface

Static shape of liquid droplets on textured surface draws significant attention from the standpoint of several engineering applications, ranging from heat transfer to bio-printing. This paper discusses the equilibrium behavior of a nanoliter size droplet dispensed on a micropatterned surface. For a given combination of intrinsic wettability of the surface, liquid surface tension and the geometric morphology of the textured surface, the liquid droplet prefers to be on Cassie-Baxter, Wenzel or an intermediate, hybrid state. Here we have carried out an energy-based simulation of nanoliter size droplets on micropatterned surface, using an open-source surface evolver fluid interface tool SE-FIT. A simplified periodic geometry of rectangular (straight or tapered) micro-pillars of specified dimensions is chosen for the micropatterned surface. For a given solid surface texture, we found that droplet prefers to transit from Wenzel to Cassie state beyond a threshold intrinsic sessile contact angle (which the liquid would have subtended on a microscopically smooth surface of the same solid material). This critical transition contact angle is plotted against the roughness parameter. Present study helps in designing the wettability-engineered surfaces for specific engineering applications.

Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on a Micropyramid Patterned Surface.

Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena, and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on a topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation the octagonal shape of the solid-liquid interface is changed to a rectangle with corners cut upon increasing the salt concentration. Such a variation in the solid-liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid-liquid and liquid-vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats "slow slip-rapid slip" across row by row of micropyramids during the depinning stage.

Graphene-coated meshes for electroactive flow control devices utilizing two antagonistic functions of repellency and permeability.

The wettability of graphene on various substrates has been intensively investigated for practical applications including surgical and medical tools, textiles, water harvesting, self-cleaning, oil spill removal and microfluidic devices. However, most previous studies have been limited to investigating the intrinsic and passive wettability of graphene and graphene hybrid composites. Here, we report the electrowetting of graphene-coated metal meshes for use as electroactive flow control devices, utilizing two antagonistic functions, hydrophobic repellency versus liquid permeability. Graphene coating was able to prevent the thermal oxidation and corrosion problems that plague unprotected metal meshes, while also maintaining its hydrophobicity. The shapes of liquid droplets and the degree of water penetration through the graphene-coated meshes were controlled by electrical stimuli based on the functional control of hydrophobic repellency and liquid permeability. Furthermore, using the graphene-coated metal meshes, we developed two active flow devices demonstrating the dynamic locomotion of water droplets and electroactive flow switching.

Nucleation of liquid droplets and voids in a stretched Lennard-Jones fcc crystal.

The method of molecular dynamics simulation has been used to investigate the phase decay of a metastable Lennard-Jones face-centered cubic crystal at positive and negative pressures. It is shown that at high degrees of metastability, crystal decay proceeds through the spontaneous formation and growth of new-phase nuclei. It has been found that there exists a certain boundary temperature. Below this temperature, the crystal phase disintegrates as the result of formation of voids, and above, as a result of formation of liquid droplets. The boundary temperature corresponds to the temperature of cessation of a crystal-liquid phase equilibrium when the melting line comes in contact with the spinodal of the stretched liquid. The results of the simulations are interpreted in the framework of classical nucleation theory. The thermodynamics of phase transitions in solids has been examined with allowance for the elastic energy of stresses arising owing to the difference in the densities of the initial and the forming phases. As a result of the action of elastic forces, at negative pressures, the boundary of the limiting superheating (stretching) of a crystal approaches the spinodal, on which the isothermal bulk modulus of dilatation becomes equal to zero. At the boundary of the limiting superheating (stretching), the shape of liquid droplets and voids is close to the spherical one.

Microwrinkles: Shape-tunability and applications

Recently, spontaneously formed microwrinkle patterns on hard coating-capped elastomer surfaces have attracted the attention of both the scientific and applied research communities, because of their simple fabrication process and practical potential in diverse applications. The periodicity and statistical orientation of the microwrinkle stripes can be controlled by applying uniaxial or isotropic compressive strain. Thus, microwrinkles have been applied to cell patterning, optical gratings, pattern templates for further patterning, surfaces with anisotropic wetting properties, surfaces with unique adhesion properties, and metrology of ultrathin film properties. Our group has focused on tuning the structure of microwrinkles by exerting additional strain. This is almost impossible for micropatterns fabricated on a hard Si wafer; therefore, our technique is based on a soft substrate and the non-linear response of the system to external strain. The dynamic shape-tunability of the micropatterns shows potential for new applications, in which switching states of a system could be induced by a change in the physical boundary conditions, namely, the shape of microwrinkles.This feature article summarizes our laboratory’s recent work on controlling the stripe pattern of microwrinkles and the application of shape-tunable microwrinkles to liquid manipulation and liquid crystal alignment. For liquid manipulation, the microgrooves of the microwrinkles are used as an open channel capillary, in which the tunable groove depth controls the capillary action of the liquid in the grooves. Further changes in the direction of the microgrooves, which are filled with liquid, can induce the division of the liquids into small droplets. Such methods for shaping liquids are made possible by only macroscopic control of the strain applied to the sample. The alignment of a nematic liquid crystal was also investigated. Nematic liquid crystals can be aligned by anisotropic microgrooves; therefore, we have demonstrated that microwrinkles can be used for this purpose. In addition, because microwrinkles are shape-tunable, the liquid crystal alignment could be repeatedly switched.Our research demonstrates that shape-tunable microwrinkles provide new physical boundary conditions that can control the states of the bounded material, for example: the shape of liquid droplets and the alignment of a nematic director. We expect that many other systems that interact with the tunable boundary will lead to the discovery of new phenomena and technologies.

Effects of gravity on the shape of liquid droplets

We report the gravity effects on the shape and focal length changes of liquid droplets. As the droplet size decreases, the gravity effect is gradually weakened. Moreover, if the outside space of the droplet is filled with another immiscible liquid rather than air, the filled liquid helps to offset the gravity effect even though their densities do not match well. Good agreement between experiment and simulation is obtained. The negligible gravity effect enables us to improve the optical performances of the liquid lens by choosing suitable liquids without worrying their density mismatch.

Wetting fibers with liposomes

Giant unilamellar vesicles (GUVs) are deposited on glass microfibers. The vesicles adopt the classical “onduloidal” shape of liquid droplets on fibers. They spread by two simultaneous mechanisms: envelopment and emission of a precursor film. This film spreads faster than on a uniform plane surface and eventually stops, signaling the presence of defects on the rod. This fast spreading tenses the vesicles; transient pores open on the GUVs and the internal liquid leaks out. This process leads to a new technique for fiber coating.

The shape of liquid droplets in the presence of a quantized vortex

The self-consistent shapes are calculated for small superfluid drops containing a vortex, and for normal fluid drops attached to a vortex. Surface energies which depend on the local curvature and the distance from the vortex axis are given in terms of the bulk superfluid density, and the coherence length.