Droplet Microfluidic Method for Estimating the Dynamic Interfacial Tension of Ion-Crosslinked Sodium Alginate Microspheres.

The research focuses on optimizing the production of hydrogel microspheres using droplet microfluidics for pharmaceutical and bioengineering applications. A semiempirical method has been developed to predict the dynamic interfacial tension at the interface of ion-cross-linked sodium alginate microsphere-sunflower oil modified with glacial acetic acid and Tween 80 surfactant. These microspheres are produced in a small-scale coaxial device that is manufactured using affordable DLP/LCD 3D printing technology with a transparent photopolymer. The method was tested to design the minireactor in the device, which allows for the production of fully cross-linked microspheres that are ready for use at the output of the reactor without additional cross-linking steps in the microsphere collector. The mathematical expression for estimating the interfacial tension at the moment of formation of a hydrogel microsphere includes the Reynolds number for a two-phase liquid, the Ohnesorge number, and the surface tension at the liquid-air interface for continuous medium flow (modified oil). The reliability of the prediction is confirmed for continuous medium and dispersed phase flow rates of 0.8-3.2 and 0.01-0.08 mL/min, respectively. The evolution of the interfacial tension from the moment the microspheres formed and the estimated ultimate interfacial tension in a cross-linked hydrogel-modified oil system contributed to the reliable determination of the linear size of a minireactor. The ultimate interfacial tension of 76.5 ± 0.3 mN/m was determined using the Young-Laplace equation, which is based on measuring the surface free energy of the hydrogel as soft matter using the Owens-Wendt method. Additionally, the equilibrium static contact angle of the fully cross-linked hydrogel surface wetted with oil is measured using the sessile drop method. From a practical perspective, a method for optimizing and streamlining the high-tech manufacturing of cross-linked polymer microspheres and mini- and microchannel devices for use in bioengineering and pharmaceutical applications is suggested.

Not in black or white, encryption of grayscale images by donor-acceptor Stenhouse adducts.

Invisible inks have been applied for the secrecy of texts, symbols and binary images. Based on the photochromism of donor-acceptor Stenhouse adducts (DASAs) in the solid-state promoted by ester-containing molecules, we report the encryption of grayscale information by controlling the kinetics of photoisomerization.

A standing Leidenfrost drop with Sufi whirling.

When a water drop is placed on a hot solid surface, it either undergoes explosive contact boiling or exhibits a stable state. In the latter case, the drop floats over an insulating layer of vapor generated by rapid vaporization of water at the surface/drop interface; this is known as the Leidenfrost state. Here, we discuss a previously unrecognized steady state in which a water drop "stands" on a hot smooth surface. In this state, the drop stabilizes itself with partial adhesion on the hot surface, leading to unique deformation and rotation behavior reminiscent of Sufi whirling-a form of spinning dance. Our analysis of this standing Leidenfrost state reveals the underlying mechanisms that drive the drop's stable partial adhesion and subsequent deformation with rotation. The heat-transfer efficiency of this standing state is up to 390% greater than that of the traditional floating Leidenfrost state.

Impinging blood droplets on different wettable surfaces: Impact phenomena, contact line motion, post-impact oscillation and dried stains.

Understanding the underlying hydrodynamics of impinging blood droplets and finding out the physical parameters determining the bloodstain characteristics are of great importance in blood related forensic investigations. In this work, the impact of non-Newtonian blood droplets on solid surfaces ranging from lyophilic to superlyophobic was systematically investigated and compared to that of Newtonian droplets with a similar dynamic shear viscosity. We show that impinging blood droplets behave as low-viscosity Newtonian droplets in the short-time spreading, which is dominated by capillary and inertial forces, but their non-Newtonian viscoelasticity would notably affect the droplet retraction and post-impact oscillation occurring in large timescales. Whereas the strong liquid-solid adhesion and the non-Newtonian elongational viscosity hinder droplet recoiling and thus alter the impact phenomena on lyophobic and superlyophobic surfaces, the shear and elongational viscosities are coupled to result in higher damping coefficients of oscillating blood droplets after deposition, in comparison to that of impinging Newtonian droplets. The size of the dried bloodstain was found to be different from both the maximum spreading radius of the droplet that can reach during impact and the final radius of the deposited droplet after oscillation, and their correlations are highly dependent on the impact velocity and surface wettability. Moreover, the morphologic characteristics of the bloodstains would also be changed by varying either the impact velocity or the surface wettability. We envision that these findings can not only find applications in the bloodstain pattern analysis, but also provide useful information for medical diagnosis based on blood droplet test.

Aerodynamic Super-Repellent Surfaces.

Repelling liquid drops from engineering surfaces has attracted great attention in a variety of applications. To achieve efficient liquid shedding, delicate surface textures are often introduced to sustain air pockets at the liquid-solid interface. However, those surfaces are prone to suffer from mechanical failure, which may bring reliability issues and thus limits their applications. Here, inspired by the aerodynamic Leidenfrost effect, we present that impacting drops are directionally repelled from smooth surfaces supplied with an exogenous air layer. Our theoretical analysis reveals that the synchronized nonwetting and oblique bouncing behavior is attributed to the aerodynamic force arising from the air layer. The versatility and practicability of our approach allow for drop repellency without the aid of any surface wettability treatment and also avoid the consideration of mechanical stability issues, which thereby provides a promising candidate for the applications that necessitate liquid shedding, e.g., resolve the problem of tiny raindrop adhesion on the automobile side window during driving.

Controlling Isomerization of Photoswitches to Modulate 2D Logic-in-Memory Devices by Organic-Inorganic Interfacial Strategy.

Logic-in-memory devices are a promising and powerful approach to realize data processing and storage driven by electrical bias. Here, an innovative strategy is reported to achieve the multistage photomodulation of 2D logic-in-memory devices, which is realized by controlling the photoisomerization of donor-acceptor Stenhouse adducts (DASAs) on the surface of graphene. Alkyl chains with various carbon spacer lengths (n = 1, 5, 11, and 17) are introduced onto DASAs to optimize the organic-inorganic interfaces: 1) Prolonging the carbon spacers weakens the intermolecular aggregation and promotes isomerization in the solid state. 2) Too long alkyl chains induce crystallization on the surface and hinder the photoisomerization. Density functional theory calculation indicates that the photoisomerization of DASAs on the graphene surface is thermodynamically promoted by increasing the carbon spacer lengths. The 2D logic-in-memory devices are fabricated by assembling DASAs onto the surface. Green light irradiation increases the drain-source current (Ids ) of the devices, while heat triggers a reversed transfer. The multistage photomodulation is achieved by well-controlling the irradiation time and intensity. The strategy based on the dynamic control of 2D electronics by light integrates molecular programmability into the next generation of nanoelectronics.

Interfacial Nanoblisters Formed in Water Serving as Freestanding Platforms for Measuring Elastic Moduli of Polymeric Nanofilms.

Polymeric nanofilms have been widely utilized in diverse cutting-edge technologies, yet accurately determining their elastic moduli remains challenging. Here we demonstrate that interfacial nanoblisters, which are produced by simply immersing substrate-supported nanofilms in water, represent natural platforms for assessing the mechanical properties of polymeric nanofilms using the sophisticated nanoindentation method. Nevertheless, high-resolution, quantitative force spectroscopy studies reveal that the indentation test must be performed on an effective freestanding region around the nanoblister apex and meanwhile under an appropriate loading force, to obtain load-independent, linear elastic deformations. The nanoblister stiffness increases with either decreasing its size or increasing its covering film thickness, and such size effects can be adequately rationalized by an energy-based theoretical model. The proposed model also enables an exceptional determination of the film elastic modulus. Given that interfacial blistering is a frequently occurring phenomenon for polymeric nanofilms, we envision that the presented methodology would stimulate broad applications in relevant fields.

Penetration and ligament formation of viscoelastic droplets impacting on the superhydrophobic mesh.

Spraying occurs by the impact of water droplets on the superhydrophobic wire meshes by liquid penetration during the spreading and recoiling. We have shown that adding a small amount of high molecular weight polymer (PEO) alters the ligaments formation and stabilizes them due to its high elasticity. Consequently, it suppresses droplet spray during droplet spreading and recoiling (recoil penetration). In the wide range of the impact velocities, the penetrated ligaments retracted back to the mesh after reaching the maximum length and eventually merged with the droplet on the mesh. The empirical fitting shows that the ligament evolution follows the parallel spring-dashpot model of Kelvin-Voigt. The additive polymer also changes the recoil penetration mechanisms from cavity collapse to cavity detachment due to the higher retraction velocity of the cavity near the mesh that is induced by the upward flow formed by the retraction of the ligaments to the mother droplet. A model based on mass conservation is proposed to calculate the variation of the maximum ligament size.

Liquid-Pressure-Guided Superhydrophobic Surfaces with Adaptive Adhesion and Stability.

The dynamic application environments of superhydrophobic surfaces, such as in the manufacturing, chemical, and garment industries, require the fast adaptiveness of the surfaces to their surroundings. Despite the progress in materials and structural design of superhydrophobic surfaces, simultaneously achieving high superhydrophobic stability and low adhesion by traditional design is still challenging. Here, a liquid-pressure-guided superhydrophobic surface with self-adjustable solid-liquid stability, and adhesion is demonstrated when reacting to the dynamic environmental requirements. To understand the underlying adaptive processes, the liquid impalement dynamics is imaged in three dimensions by confocal microscopy and the stability and superhydrophobicity are measured in varied systems. It is envisioned that the design strategy of liquid-pressure-guided superhydrophobic surfaces with dynamic anti-infiltration capability can stimulate the development of stable superhydrophobicity under complex wetting conditions.

Spectacular Behavior of a Viscoelastic Droplet Impinging on a Superhydrophobic Mesh.

Spray formation using the droplet impact on superhydrophobic mesh surfaces is particularly important because of its application in different industries. The present study revealed that adding a trivial amount of the poly(ethylene oxide) (PEO) polymer to a water droplet can considerably change the impact phenomena on the superhydrophobic mesh surfaces and suppress the spray formation. Droplet rebound is observed only in a narrow range of impact velocities of PEO aqueous droplets when the tiny filaments still connect the surface and droplet. Rebound suppression and deposition of the PEO aqueous droplet is attributed to the higher interaction between the polymer chains and the superhydrophobic mesh surface. After a critical impact velocity and We number which is independent of the PEO concentration, the liquid penetrates the mesh pores. The penetrated liquid formed the ligaments that grow until they reach the maximum length and surprisingly retract back to the mesh surface and the mother droplet. The ligaments destabilized at low PEO concentrations (c = 0.5 and 1 g/L) and a mesh opening size of H = 357 µm to the crest swell droplets when the droplet size is reduced by increasing the impact velocity. The ligament fragmentation and droplet detachment are observed only at high impact velocities when c = 0.5 and 1 g/L and H = 357 µm. The result shows that the PEO additive does not significantly affect the maximum spreading diameter. An empirical model to calculate the maximum spreading factor is developed.

Successive Rebounds of Impinging Water Droplets on Superhydrophobic Surfaces.

When a water droplet strikes a superhydrophobic surface, there may be several to a few tens of rebounds before it comes to rest. Although this intriguing multiphase flow phenomenon has received a great deal of attention from interfacial scientists and engineers, the underlying dynamics have not yet been completely resolved. In this paper, we report on an experimental investigation into the bouncing behavior of water droplets impinging on macroscopically flat superhydrophobic surfaces. We show that the restitution coefficient, which quantifies the energy consumed during impact and rebound, exhibits a nonmonotonic dependence on the Weber number. It is the droplet-surface friction that restricts the rebound height of the impinging droplet, so its restitution coefficient increases with the Weber number when the impact velocity is below a critical value. Above this value, the viscous friction within a thin liquid layer close to the superhydrophobic surface becomes dominant, and thus, the restitution coefficient decreases sharply. On the basis of energy analyses, semiempirical formulas are proposed to describe the restitution coefficient, and these can be employed to predict the number of successive rebounds of impinging droplets on superhydrophobic surfaces.

Elasticity-to-Capillarity Transition in Soft Substrate Deformation.

Whereas capillarity controls fluid dynamics at submillimeter scale and elasticity determines the mechanics of rigid solids, their coupling governs elastocapillary deformations on soft solids. Here, we directly probed the deformations on soft substrates induced by sessile nanodroplets. The wetting ridge created around the contact line and the dimple formed underneath the nanodroplet were imaged with a high spatial resolution using atomic force microscopy. The ridge height nonmonotonically depends on the substrate stiffness, and the dimple depth nonlinearly depends on the droplet size. The capillarity of the substrate overcomes the elasticity of the substrate in dominating the deformations when the elastocapillary length is approximately larger than the droplet contact radius, showing an experimental observation of the elasticity-to-capillarity transition. This study provides an experimental approach to investigate nanoscale elastocapillarity, and the insights have the potential to kick-off future work on the fundamentals of solid mechanics.

Salvinia-like slippery surface with stable and mobile water/air contact line.

Superhydrophobic surfaces are widely used in many industrial settings, and mainly consist of rough solid protrusions that entrap air to minimize the liquid/solid area. The stability of the superhydrophobic state favors relatively small spacing between protrusions. However, this in turn increases the lateral adhesion force that retards the mobility of drops. Here we propose a novel approach that optimizes both properties simultaneously. Inspired by the hydrophobic leaves of Salvinia molesta and the slippery Nepenthes pitcher plants, we designed a Salvinia-like slippery surface (SSS) consisting of protrusions with slippery heads. We demonstrate that compared to a control surface, the SSS exhibits increased stability against pressure and impact, and enhanced lateral mobility of water drops as well as reduced hydrodynamic drag. We also systematically investigate the wetting dynamics on the SSS. With its easy fabrication and enhanced performance, we envision that SSS will be useful in a variety of fields in industry.

Macrodrop-Impact-Mediated Fluid Microdispensing.

High-resolution fluid dispensing techniques play a critical role in modern digital microfluidics, micro-biosensing, and advanced fabrication. Though most of existing dispensers can achieve precise and high-throughput fluid dispensing, they suffer from some inherent problems, such as specially fabricated dispensing micronozzles/microtips, large operating systems, low volume tunability, and poor performance for low surface tension liquids and liquids containing solid/liquid additives. Herein, the authors propose a facile, low-frequency micro dispensing technique based on the Rayleigh-Plateau instability of singular liquid jets, which are stimulated by the air cavity collapse arising in the impact of microliter drops on non-wetting surfaces. This novel dispensing strategy is capable to produce single microdrops of low-viscosity liquids with a tunable volume from picoliters to nanoliters, and the operational surface tension range covers most laboratory solvents. The dispensing function is implemented without using small-dimension nozzles/tips and enables handling diverse complex liquids. Moreover, the rather simple operating platform allows the integration of the whole dispensing function into a handy portable device with a low cost. Employing this microdispensing technique, the authors have controlled microchemical reactions, handled liquid samples in biological analysis, and fabricated smart materials and devices. The authors envision that this rational microdrop generator would find applications in various research areas.

Droplet impact on pillar-arrayed non-wetting surfaces.

Droplet impact on pillar-arrayed polydimethylsiloxane (PDMS) surfaces with different solid fractions was studied. The lower and upper limits of Weber number, We, for complete rebound of impacting droplets decreased with decreasing solid fractions. Gaps were visible during the spreading and retraction processes of bouncing droplets on the surface with a solid fraction of 0.06 while no gaps were observed during the retraction process when We was greater than its upper limit, indicating that there existed a transition from the Cassie-Baxter wetting state to the Wenzel wetting state. Therefore, a novel model accounting for the penetration of a liquid into the cavities between the pillars was developed to predict the upper limit of the impact velocity of bouncing droplets. At high We, partial rebound was observed for surfaces with solid fractions of 0.50 and 0.20 while a sticky state was observed for the surface with a solid fraction of 0.06. Moreover, surface roughness has a great influence on the contact time of bouncing droplets. Besides, the maximum spreading parameter was found to follow a scaling law of We1/4.

Polymeric Microparticles Generated via Confinement-Free Fluid Instability.

In-fiber fluid instability can be harnessed to realize scalable microparticles fabrication with tunable sizes and multifunctional characteristics making it competitive in comparison to conventional microparticles fabrication methods. However, since in-fiber fluid instability has to be induced via thermal annealing and the resulting microparticles can only be collected after dissolving the fiber cladding, obtaining contamination-free particles for high-temperature incompatible materials remains great challenge. Herein, confinement-free fluid instability is demonstrated to fabricate polymeric microparticles in a facile manner induced by the ultralow surface energy of the superamphiphobic surface. The polymer solution columns break up into uniform droplets then form spherical particles spontaneously in seconds at ambient temperature. This method can be applied to a variety of polymers spanning an exceptionally wide range of sizes: from 1 mm down to 1 µm. With the aid of microfluidic spinning instrument, a large quantity of microparticles can be obtained, making this method promising for scaling up production. Notably, through simple modification of the feed solution configuration, composite/structured micromaterials can also be produced, including quantum-dots-labeled fluorescent particles, magnetic particles, core-shell particles, microcapsules, and necklace-like microfibers. This method, with general applicability and facile control, is envisioned to have great prospects in the field of polymer microprocessing.

Charge Density Gradient Propelled Ultrafast Sweeping Removal of Dropwise Condensates.

Continuous sweeping of dropwise condensates is an effective form of vapor to liquid transition in terms of thermal transport at a solid/liquid interface. However, using conventional approaches, it is difficult to simultaneously achieve small activating size and fast departure of condensed droplets with high efficiency, due to the insufficient driving force compared to adhesion. Here, we propose an unexplored method to stimulate a frequent sweeping removal of dropwise condensates at ultrahigh efficiency on a superhydrophobic substrate, aided by a charge density gradient (CDG). We show that the CDG can be injected inside a superhydrophobic substrate on which the condensate droplet with jump-induced charges starts to sweep at a small size down to the microscale followed by quick snowball-like growing and chase-like propelling. The incorporation of the CDG on a superhydrophobic substrate enables a continuous, fast, frequent, long-range, and gravity-independent droplet removal during condensation, making this strategy a promising solution for diverse applications in water harvesting, antifogging, and anti-icing.

What Can Probing Liquid-Air Menisci Inside Nanopores Teach Us About Macroscopic Wetting Phenomena?

Solid surfaces with excellent nonwetting ability have drawn significant interest from interfacial scientists and engineers. While much effort was devoted to investigating macroscopic wetting phenomena on nonwetting surfaces, the otherwise microscopic wetting has received less attention, and the surface/interface properties at the microscopic scale are not well resolved and correlated with the macroscopic wetting behavior. Herein, we first characterize the nanoscopic morphology and effective stiffness of liquid-air interfaces inside nanopores (nanomenisci) on diverse nonwetting nanoporous surfaces underneath water droplets using atomic force microscopy. Detailed three-dimensional imaging of the droplet-surface contact region reveals that water only slightly penetrates into the nanopores, allowing for quantitative prediction of the macroscopic contact angle using the Cassie-Baxter model. By gradually increasing the scanning force, we observe incrementally wetting of nanopores by water, and dewetting occurs when the force is lowered again, exhibiting reversible wetting-dewetting transitions. Further, nanoindentation measurements demonstrate that the nanomenisci show apparent elastic deformation and size-dependent effective stiffness at small indenting forces. Finally, we correlate the effective stiffness of the nanomenisci with the transition from complete rebound to partial rebound for impinging droplets on nanoporous surfaces. Our study suggests that probing the physical properties of the liquid-air menisci at the nanoscale is essential to rationalize macroscopic static and dynamic wetting phenomena on structured surfaces.

Surface-Charge-Assisted Microdroplet Generation on a Superhydrophobic Surface.

The ability to generate and manipulate droplets down to microscales has attracted great attention in a variety of applications, such as in printing, microreactors, and biological assays. However, the production of microdroplets is often limited by special equipment or the size of needles. Here, an unexplored and facile approach is demonstrated; microdroplets can be generated and trapped yet not pinned on a micro-nano-structured superhydrophobic surface by controllable surface charge during drop impact. Tiny droplets with a size at a scale of tens of microns to millimeters are generated by simply changing the impacting velocity, the size of the impact drop, or impact frequency. Theoretical analysis suggests the generation of the microdroplet as a result of the surface-charge-regulated adhesion, competing with liquid dynamic and interfacial energy. The distribution of surface charge which determines the size and the location of the microdroplet is at the top of the micro-nano-structured surface and dependent on the pressure field applied on the surface during the drop impact. The mobility of the resulting microdroplet that can be easily manipulated without liquid retention is also shown, by taking advantage of the shielding property of the surface charge. This facile yet effective method provides a promising candidate for the realization of tiny droplet-generating and -manipulating applications.

Droplets Self-Born in the Dynamic Polymer for Generating Functional Coatings.

Droplet-embedded structures are useful in functionalizing polymer composites but difficult to prepare. Herein, we report a facile self-born method for creating droplets in supramolecular gels to mediate the material's functions. This method is based on the amplification of the defects of polymer matrices generated in curing by swelling-driving reconfiguration of supramolecular polymer networks. The system of poly(urea-co-polydimethylsiloxane) that can cross-link via hydrogen-bond interaction is used to demonstrate our concept. The elastomer matrices are prepared via a casting method and exhibit a heterogeneous structure with both strong- and weak-cross-linking domains. When these materials are swelled in solvents, solvent molecules concentrate in the weak-cross-linking domains to nucleate. With the reconfiguration of the matrices, the nuclei grow into pure droplets, leading to the formation of droplet-embedded structures. This method is applicable to different material systems. We also show that obtained coatings with such droplet-embedded structures exhibit various interesting properties including self-replenishment of the surface liquid, mechanoresponsiveness, and self-healing ability. Moreover, after the droplets are consumed, this method can be used to regenerate the droplet-embedded structure for refunctionalizing the materials. Therefore, we envision its applications in preparation of many useful polymer composites.