Marangoni-driven spreading of miscible liquids in the binary pendant drop geometry

Unveiling unique scaling behavior in miscible, volatile Marangoni spreading

We present a novel observation of the expansion of the outer tip radius of a fast-spreading ethanol-water film spreading over a deep substrate of water. The experimentally measured radius of the outer tip of the film (ro) and its velocity (Uo) display a complex scaling with time and drop properties. The variation showed by ro differed from the commonly observed scalings of t3/4 and t1/4. We propose novel scaling laws for ro and Uo by expressing ro as the sum of the radius of the stable part of the film rf and the length of the unstable part lp at the periphery of the stable part of the film, that had azimuthally uniformly spaced plumes. The radius of the stable part of the film scales as rf ∼ t1/4 since, while the film expands, the Marangoni stresses are balanced by viscous stresses within the film thickness. At the same time, lp ∼ t3/4 since the plumes grow at the periphery of the stable part of the film, with the driving surface tension stresses balanced by the viscous stresses in a shear layer below the plumes. Combining these two scaling laws yielded a novel, two-term scaling law for ro, which is close to a single power-law scaling ro ∼ t1/2. We obtain an expression for the dimensionless mean outer tip radius as , where t* = t/tξ, tξ = (rd4ρwμw/Δσ2)1/3 being the time scale. Similarly, we show that the dimensionless velocity scales as with the variables λ1 and λ2 being functions of t* and drop properties. These proposed scaling laws are shown to match our measurements, thereby validating the phenomenology of such miscible, volatile spreading.

Marangoni-driven spreading of a droplet on a miscible thin liquid layer

HYPOTHESIS

The coalescence of droplets with liquid-gas interfaces of different surface tensions is common in nature and industrial applications, where the Marangoni-driven film spreading is an essential process. Unlike immiscible fluids governed by triple contact line dynamics, the mixing between two miscible fluids strongly couples with the film spreading process, which are expected to manifest distinct power-law relations for the temporal increase in the film radius.

EXPERIMENTS

We experimentally investigate the Marangoni-driven film spreading phenomenon for a droplet with lower surface tension dropping onto a miscible, thin liquid layer. The temporal growth of the film radius was detected by using a novel deep convolutional neural network, the U2-net method. Scaling analysis was performed to interpret the spreading dynamics of the film.

FINDINGS

We find that the film radius exhibits a three-stage power-law relation over time, with the exponent varying from 1/2 to 1/8, and back to 1/2. The diffusion-affected Marangoni stresses in these three stages were derived, and two estimations of viscous stress were considered. Through estimating and balancing the viscous stress with the Marangoni stress, the three-stage power-law relation was derived and validated.

Mass production of lumenogenic human embryoid bodies and functional cardiospheres using in-air-generated microcapsules

Organoids are engineered 3D miniature tissues that are defined by their organ-like structures, which drive a fundamental understanding of human development. However, current organoid generation methods are associated with low production throughputs and poor control over size and function including due to organoid merging, which limits their clinical and industrial translation. Here, we present a microfluidic platform for the mass production of lumenogenic embryoid bodies and functional cardiospheres. Specifically, we apply triple-jet in-air microfluidics for the ultra-high-throughput generation of hollow, thin-shelled, hydrogel microcapsules that can act as spheroid-forming bioreactors in a cytocompatible, oil-free, surfactant-free, and size-controlled manner. Uniquely, we show that microcapsules generated by in-air microfluidics provide a lumenogenic microenvironment with near 100% efficient cavitation of spheroids. We demonstrate that upon chemical stimulation, human pluripotent stem cell-derived spheroids undergo cardiomyogenic differentiation, effectively resulting in the mass production of homogeneous and functional cardiospheres that are responsive to external electrical stimulation. These findings drive clinical and industrial adaption of stem cell technology in tissue engineering and drug testing.

Spontaneous water-on-water spreading of polyelectrolyte membranes inspired by skin formation

Stable interfaces between immiscible solvents are crucial for chemical synthesis and assembly, but interfaces between miscible solvents have been less explored. Here the authors report the spontaneous water-on-water spreading and self-assembly of polyelectrolyte membranes. An aqueous mixture solution containing poly(ethyleneimine) and poly(sodium 4-styrenesulfonate) spreads efficiently on acidic water, leading to the formation of hierarchically porous membranes. The reduced surface tension of the polyelectrolyte mixture solution drives the surface spreading, while the interfacial polyelectrolytes complexation triggered by the low pH of water mitigates water-in-water mixing. The synergy of surface tension and pH-dependent complexation represents a generic mechanism governing interfaces between miscible solvents for materials engineering, without the need for surfactants or sophisticated equipment. As a proof-of-concept, porous polyelectrolyte hybrid membranes are prepared by surface spreading, exhibiting exceptional solar thermal evaporation performance (2.8 kg/m²h) under 1-sun irradiation.

Stable interfaces between immiscible solvents are crucial for chemical synthesis and assembly, but interfaces between miscible solvents have been less explored. Here the authors report the spontaneous water-on-water spreading and self-assembly of polyelectrolytes.

Size-Dependent Dried Colloidal Deposit and Particle Sorting via Saturated Alcohol Vapor-Mediated Sessile Droplet Spreading

We experimentally and theoretically investigate a distinct problem of spreading, evaporation, and the associated dried deposits of a colloidal particle-laden aqueous sessile droplet on a surface in a saturated alcohol vapor environment. In particular, the effect of particle size on monodispersed suspensions and efficient self-sorting of bidispersed particles have been investigated. The alcohol vapor diffuses toward the droplet's curved liquid-vapor interface from the far field. The incoming vapor mass flux profile assumes a nonuniform pattern across the interface. The alcohol vapor molecules are adsorbed at the liquid-vapor interface, which eventually leads to absorption into the droplet's liquid phase due to the miscibility. This phenomenon triggers a liquid-vapor interfacial tension gradient and causes a reduction in the global surface tension of the droplet. This results in a solutal Marangoni flow recirculation and spontaneous droplet spreading. The interplay between these phenomena gives rise to a complex internal fluid flow within the droplet, resulting in a significantly modified and strongly particle-size-dependent dried colloidal deposit. While the smaller particles form a multiple ring pattern, larger particles form a single ring, and additional "patchwise" deposits emerge. High-speed visualization of the internal liquid-flow revealed that initially, a ring forms at the first location of the contact line. Concurrently, the Marangoni flow recirculation drives a collection of particles at the liquid-vapor interface to form clusters. Thereafter, as the droplet spreads, the smaller particles in the cluster exhibit a "jetlike" outward flow, forming multiple ring patterns. In contrast, the larger particles tend to coalesce together in the cluster, forming the "patchwise" deposits. The widely different response of the different-sized particles to the internal fluid flow enables an efficient sorting of the smaller particles at the contact line from bidispersed suspensions. We corroborate the measurements with theoretical and numerical models wherever possible.

Inkjet Bioprinting of Biomaterials

The inkjet technique has the capability of generating droplets in the picoliter volume range, firing thousands of times in a few seconds and printing in the noncontact manner. Since its emergence, inkjet technology has been widely utilized in the publishing industry for printing of text and pictures. As the technology developed, its applications have been expanded from two-dimensional (2D) to three-dimensional (3D) and even used to fabricate components of electronic devices. At the end of the twentieth century, researchers were aware of the potential value of this technology in life sciences and tissue engineering because its picoliter-level printing unit is suitable for depositing biological components. Currently inkjet technology has been becoming a practical tool in modern medicine serving for drug development, scaffold building, and cell depositing. In this article, we first review the history, principles and different methods of developing this technology. Next, we focus on the recent achievements of inkjet printing in the biological field. Inkjet bioprinting of generic biomaterials, biomacromolecules, DNAs, and cells and their major applications are introduced in order of increasing complexity. The current limitations/challenges and corresponding solutions of this technology are also discussed. A new concept, biopixels, is put forward with a combination of the key characteristics of inkjet printing and basic biological units to bring a comprehensive view on inkjet-based bioprinting. Finally, a roadmap of the entire 3D bioprinting is depicted at the end of this review article, clearly demonstrating the past, present, and future of 3D bioprinting and our current progress in this field.

Flower-like droplets obtained by self-emulsification of a phase-separating (SEPS) aqueous film

Self-emulsification, referring to the spontaneous formation of droplets of one phase in another immiscible phase, is attracting growing interest because of its simplicity in creating droplets. Existing self-emulsification methods usually rely on phase inversion, temperature cycling, and solvent evaporation. However, achieving spatiotemporal control over the morphology of self-emulsified droplets remains challenging. In this work, a conceptually new approach of creating both simple and complex droplets by self-emulsification of a phase-separating (SEPS) aqueous film, is reported. The aqueous film is formed by depositing a surfactant-laden aqueous droplet onto an aqueous surface, and the fragmentation of the film into droplets is triggered by a wetting transition. Smaller and more uniform droplets can be achieved by introducing liquid-liquid phase separation (LLPS). Moreover, properly modulating quadruple LLPS and film fragmentation enables the creation of highly multicellular droplets such as flower-like droplets stabilized by the interfacial self-assembly of nanoparticles. This work provides a novel strategy to design aqueous droplets by LLPS, and it will inspire a wide range of applications such as membraneless organelle synthesis, cell mimics and delivery.