Droplet Sorting and Manipulation on Patterned Two-Phase Slippery Lubricant-Infused Surface

Scalable Preparation of Liquid Infused Coatings for Lubrication of 103 m2 Dry Ski Slopes

To facilitate effective training for freestyle skiers on artificial dry ski slopes, it is crucial to reduce the friction coefficient of the slopes and closely match it with that of snow. Traditional lubrication methods, such as water or soapy water, come with multiple disadvantages, including water waste, which leads to environmental pollution, short-lived effectiveness, and high costs. In this study, we have successfully developed a method for the scalable preparation of a liquid-infused coating (LIC) by tandem spraying inexpensive and environmentally friendly SiO2 particles and silicone oil lubricants. Experimental results showed that the resulting LIC is capable of imparting slippery properties to various surfaces, regardless of the surface chemistry. Moreover, the presence of LIC could reduce the friction coefficient significantly. By carefully regulating the surface composition, we achieved a friction coefficient of 0.059 between a snowboard and the LIC-functionalized ski slope, closely matching that between the snowboard and snow in a typical skiing competition venue (∼0.06). We successfully applied LIC onto 103 m2 dry ski slopes, providing a training ground for professional freestyle skiers.

Comparison of Anti-Icing, Antifouling, and Anticorrosion Performances of the Superhydrophobic and Lubricant-Infused Coatings Based on a Hollow-Structured Kapok Fiber

The superhydrophobic surface and slippery liquid-infused porous surface (SLIPS)/lubricant-infused surface (LIS) have attracted increasing attention owing to their multifunctionality. However, their practical applications face several problems such as complex and inefficient preparation technology, loss of lubricant, and fragile microstructures. Therefore, new strategies for preparing microstructures must be developed for constructing superhydrophobic and lubricant-infused coatings. Herein, a low-cost and high-efficiency method for developing superhydrophobic and lubricant-infused coatings based on in situ grown TiO2 on the surface of a hollow kapok fiber (KF) is reported. The anti-icing, antifouling, and anticorrosion performance of the superhydrophobic and lubricant-infused coatings are compared. The superhydrophobic coating reduces the formation and accumulation of ice. The lubricant-infused coating exhibits an extremely low ice adhesion strength and durable anti-icing properties. The superhydrophobic and lubricant-infused coatings show the outstanding antifouling property of diatom; the superhydrophobic surface exhibits superior stability over LIS without an external force field. The lubricant-infused coating shows excellent corrosion resistance and durability when immersed in a 3.5% NaCl solution. The superhydrophobic coating loses its protection as a result of the corrosion media permeating the metal substrate via the electrolytic cell and coating interface, and the lubricant-infused coating provides lasting corrosion resistance because of the lubricant filling into the interface. Although the superhydrophobic coating is fragile and the lubricant-infused coating will lose lubricant, this simple and convenient approach can be repeated to keep the coatings active. This study provides new inspiration for the fabrication of superhydrophobic surfaces and LIS based on natural products.

Reduced Sliding Friction of Lubricant-Impregnated Catheters

During urethral catheterization, sliding friction can cause discomfort and even hemorrhaging. In this report, we use a lubricant-impregnated polydimethylsiloxane coating to reduce the sliding friction of a catheter. Using a pig urethra attached to a microforce testing system, we found that a lubricant-impregnated catheter reduces the sliding friction during insertion by more than a factor of two. This suggests that slippery, lubricant-impregnated surfaces have the potential to enhance patient comfort and safety during catheterization.

Polysulfonated covalent organic framework as active electrode host for mobile cation guests in electrochemical soft actuator

Tailoring transfer dynamics of mobile cations across solid-state electrolyte-electrode interfaces is crucial for high-performance electrochemical soft actuators. In general, actuation performance is directly proportional to the affinity of cations and anions in the electrolyte for the opposite electrode surfaces under an applied field. Herein, to maximize electrochemical actuation, we report an electronically conjugated polysulfonated covalent organic framework (pS-COF) used as a common electrolyte-electrode host for 1-ethyl-3-methylimidazolium cation embedded into a Nafion membrane. The pS-COF-based electrochemical actuator exhibits remarkable bending deflection at near-zero voltage (~0.01 V) and previously unattainable blocking force, which is 34 times higher than its own weight. The ultrafast step response shows a very short rising time of 1.59 seconds without back-relaxation, and substantial ultralow-voltage actuation at higher frequencies up to 5.0 hertz demonstrates good application prospects of common electrolyte-electrode hosts. A soft fluidic switch is constructed using the proposed soft actuator as a potential engineering application.

Droplet Self-Propulsion on Slippery Liquid-Infused Surfaces with Dual-Lubricant Wedge-Shaped Wettability Patterns

Young's equation is fundamental to the concept of the wettability of a solid surface. It defines the contact angle for a droplet on a solid surface through a local equilibrium at the three-phase contact line. Recently, the concept of a liquid Young's law contact angle has been developed to describe the wettability of slippery liquid-infused porous surfaces (SLIPS) by droplets of an immiscible liquid. In this work, we present a new method to fabricate biphilic SLIP surfaces and show how the wettability of the composite SLIPS can be exploited with a macroscopic wedge-shaped pattern of two distinct lubricant liquids. In particular, we report the development of composite liquid surfaces on silicon substrates based on lithographically patterning a Teflon AF1600 coating and a superhydrophobic coating (Glaco Mirror Coat Zero), where the latter selectively dewets from the former. This creates a patterned base surface with preferential wetting to matched liquids: the fluoropolymer PTFE with a perfluorinated oil Krytox and the hydrophobic silica-based GLACO with olive oil (or other mineral oils or silicone oil). This allows us to successively imbibe our patterned solid substrates with two distinct oils and produce a composite liquid lubricant surface with the oils segregated as thin films into separate domains defined by the patterning. We illustrate that macroscopic wedge-shaped patterned SLIP surfaces enable low-friction droplet self-propulsion. Finally, we formulate an analytical model that captures the dependence of the droplet motion as a function of the wettability of the two liquid lubricant domains and the opening angle of the wedge. This allows us to derive scaling relationships between various physical and geometrical parameters. This work introduces a new approach to creating patterned liquid lubricant surfaces, demonstrates long-distance droplet self-propulsion on such surfaces, and sheds light on the interactions between liquid droplets and liquid surfaces.

Design, fabrication, and applications of bioinspired slippery surfaces

Bioinspired slippery surfaces (BSSs) have attracted considerable attention owing to their antifouling, drag reduction, and self-cleaning properties. Accordingly, various technical terms have been proposed for describing BSSs based on specific surface characteristics. However, the terminology can often be confusing, with similar-sounding terms having different meanings. Additionally, some terms fail to fully or accurately describe BSS characteristics, such as the surface wettability of lubricants (hydrophilic or hydrophobic), surface wettability anisotropy (anisotropic or isotropic), and substrate morphology (porous or smooth). Therefore, a timely and thorough review is required to clarify and distinguish the various terms used in BSS literature. This review initially categorizes BSSs into four types: slippery solid surfaces (SSSs), slippery liquid-infused surfaces (SLISs), slippery liquid-like surfaces (SLLSs), and slippery liquid-solid surfaces (SLSSs). Because SLISs have been the primary research focus in this field, we thoroughly review their design and fabrication principles, which can also be applied to the other three types of BSS. Furthermore, we discuss the existing BSS fabrication methods, smart BSS systems, antifouling applications, limitations of BSS, and future research directions. By providing comprehensive and accurate definitions of various BSS types, this review aims to assist researchers in conveying their results more clearly and gaining a better understanding of the literature.

On-Chip Liquid Manipulation via a Flexible Dual-Layered Channel Possessing Hydrophilic/Hydrophobic Dichotomy

The hydrophilic/hydrophobic cooperative interface provides a smart platform to control liquid distribution and delivery. Through the fusion of flexibility and complex structure, we present a manipulable, open, and dual-layered liquid channel (MODLC) for on-demand mechanical control of fluid delivery. Driven by anisotropic Laplace pressure, the mechano-controllable asymmetric channel of MODLC can propel the directional slipping of liquid located between the paired tracks. Upon a single press, the longest transport distance can reach 10 cm with an average speed of ∼3 cm/s. The liquid on the MODLC can be immediately manipulated by pressing or dragging processes, and versatile liquid-manipulating processes on hierarchical MODLC chips have been achieved, including remote droplet magneto-control, continuous liquid distributor, and gas-producing chip. The flexible hydrophilic/hydrophobic interface and its assembly can extend the function and applications of the wettability-patterned interface, which should update our understanding of complex systems for sophisticated liquid transport.

Multifunctional Droplets Formed by Interfacially Self-Assembled Fluorinated Magnetic Nanoparticles for Biocompatible Single Cell Culture and Magnet-Driven Manipulation

The ability to encapsulate and manipulate droplets with a picoliter volume of samples and reagents shows great potential for practical applications in chemistry, biology, and materials science. Magnetic control is a promising approach for droplet manipulation due to its ability for wireless control and its ease of implementation. However, it is challenged by the poor biocompatibility of magnetic materials in aqueous droplets. Moreover, current droplet technology is problematic because of the molecule leakage between droplets. In the paper, we propose multifunctional droplets with the surface coated by a layer of fluorinated magnetic nanoparticles for magnetically actuated droplet manipulation. Multifunctional droplets show excellent biocompatibility for cell culture, nonleakage of molecules, and high response to a magnetic field. We developed a strategy of coating the F-MNP@SiO2 on the outer surface of droplets instead of adding magnetic material into droplets to enable droplets with a highly magnetic response. The encapsulated bacteria and cells in droplets did not need to directly contact with the magnetic materials at the outer surface, showing high biocompatibility with living cells. These droplets can be precisely manipulated based on magnet distance, the time duration of the magnetic field, the droplet size, and the MNP composition, which well match with theoretical analysis. The precise magnetically actuated droplet manipulation shows great potential for accurate and sensitive droplet-based bioassays like single cell analysis.

Application of droplet migration scaling behavior to microchannel flow measurements

In confined channels in low Reynolds number flow, droplets drift perpendicular to the flow, moving across streamlines. The phenomenon has proven useful for understanding microfluidic droplet separation, drug delivery vehicle optimization, and single-cell genomic amplification. Particles or droplets undergo several migration mechanisms including wall migration, hydrodynamic diffusion, and migration down gradients of shear. In simple shear flow only wall migration and hydrodynamic diffusion are present. In parabolic flow, droplets also move down gradients of shear. The resulting separation depends on parameters including particle size and stiffness, concentration, and flow rate. Computational methods can incorporate these effects in an exact manner to predict margination phenomena for specific systems, but do not generate a descriptive parametric dependence. In this paper, we present a scaling model that elucidates the parametric dependence of margination on emulsion droplet size, volume fraction, shear rate and suspending fluid viscosity. We experimentally measure the droplet depletion layer of silicone oil droplets and compare the results to theoretical scaling behavior that includes hydrodynamic diffusion and wall migration with and without an added shear-gradient migration. Results demonstrate the viability and limitations of applying a simple scaling behavior to experimental systems to describe parametric dependence. Our conclusions open the possibility for parametric descriptions of migration with broad applicability to particle and droplet systems.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

Manipulation and control of droplets on surfaces in a homogeneous electric field

A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. It is shown that the repulsive dipole force efficiently suppresses the coalescence of droplets moving on a liquid-infused surface (LIS). Using a combination of experiments, numerical computations and semi-analytical models, the dependence of the repulsion force on the droplet volumes, the distance between the droplets and the electric field strength is revealed. The method allows to suppress coalescence in complex multi-droplet flows and is real-time adaptive. When the electric field strength exceeds a critical value, tip streaming from the droplets sets in. Based on that, it becomes possible to withdraw minute samples from an array of droplets in a parallel process.

Control of droplet coalescence is a major challenge of droplet microfluidics. Here, the authors show that homogenous external electric field can induce dipoles inside droplets, which can be used to withdraw samples from an array of droplets.

Non-wetting Liquid-Infused Slippery Paper

Liquid-infused slippery surfaces have replaced structural superhydrophobic surfaces in a plethora of emerging applications, hallmarked by their favorable self-healing and liquid-repelling characteristics. Their ease of fabrication on different types of materials and increasing demand in various industrial applications have triggered research interests targeted toward developing an environmental-friendly, flexible, and frugal substrate as the underlying structural and functional backbone. Although many expensive polymers such as polytetrafluoroethylene have so far been used for their fabrication, these are constrained by their compromised flexibility and non-ecofriendliness due to the use of fluorine. Here, we explore the development and deployment of a biodegradable, recyclable, flexible, and an economically viable material in the form of a paper matrix for fabricating liquid-infused slippery interfaces for prolonged usage. We show by controlled experiments that a simple silanization followed by an oil infusion protocol imparts an inherent slipperiness (low contact angle hysteresis and low tilting angle for sliding) to the droplet motion on the paper substrate and provides favorable anti-icing characteristics, albeit keeping the paper microstructures unaltered. This ensures concomitant hydrophobicity, water adhesion, and capillarity for low surface tension fluids, such as mustard oil, with an implicit role played by the paper pore size distribution toward retaining a stable layer of the infused oil. With demonstrated supreme anti-icing characteristics, these results open up new possibilities of realizing high-throughput paper-based substrates for a wide variety of applications ranging from biomedical unit operations to droplet-based digital microfluidics.

Architecture-Driven Fast Droplet Transport without Mass Loss

Spontaneous droplet transport without mass loss has great potential applications in the fields of energy and biotechnology, but it remains challenging due to the difficulty in obtaining a sufficient driving force for the transport while suppressing droplet mass loss. Learning from the slippery peristome of Nepenthes alata and wedge topology of a shorebird beak that can spontaneously feed water against gravity, a combined system consisting of two face-to-face hydrophilic slippery liquid-infused porous surfaces (SLIPS) with variable beak-like opening and spacing was proposed to constrain the droplet in-between and initiate fast droplet transport over a long distance of 75 mm with a maximum speed of 12.2 mm·s^-1 without mass loss by taking advantage of the Laplace pressure gradient induced by the asymmetric shape of the constrained droplet. The theoretical model based on the Navier-Stokes equation was developed to interpret the corresponding mechanism of the droplet transport process. In addition, in situ sophisticated droplet manipulations such as droplet mixing are readily feasible when applying flexible 304 stainless foil as the substrate of SLIPS. It is believed that extended research would contribute to new references for the precise and fast droplet motion control intended for energy harvest and water collection devices.

Droplet Mobility on Slippery Lubricant Impregnated and Superhydrophobic Surfaces under the Effect of Air Shear Flow

The focus of this study is to investigate and compare the behavior of a droplet on superhydrophobic (SHS) and slippery lubricant impregnated (SLIPS) surfaces under the effect of air shear flow. In this regard, both experimental and numerical analyses have been conducted to compare their performance on droplet mobility under different air speeds. Two different lubricants have been utilized to scrutinize their effect on droplet movement. The numerical simulations have been performed based on the volume of fluid method coupled with the large eddy simulation turbulent model in conjunction with the dynamic contact angle method in addition to a model that can represent the effect of lubricants on slippery surfaces. The numerical simulations are compared with the experimental study in order to shed light on the underlying mechanisms. The results showed that under the same conditions, the critical velocity for droplet movement on the superhydrophobic surfaces is lower than that on the slippery lubricant impregnated surfaces due to the smaller droplet base diameter and the larger contact angle. The hydrodynamics of droplet mobility on superhydrophobic surfaces exhibits a rolling behavior while for the slippery lubricant impregnated surfaces a combination of rolling and sliding is observed. Beyond the critical airflow speed, a complete droplet shedding on all surfaces occurs. The wetting length and position of the droplet on superhydrophobic and slippery surfaces have been measured. On slippery surfaces, the speed of droplets is greatly affected by the lubricant properties while similar behavior in the wetting lengths is observed.

Smart Control for Water Droplets on Temperature and Force Dual-Responsive Slippery Surfaces

Responsive slippery lubricant-infused porous surfaces (SLIPSs), featuring excellent liquid repelling/sliding capabilities in response to external stimuli, have attracted great attention in smart droplet manipulations. However, most of the reported responsive SLIPSs function under a single stimulus. Here, we report a kind of smart slippery surface capable of on-demand control between sliding and pinning for water droplets via alternately freezing/thawing the stretchable polydimethylsiloxane sheet in different strains. Diverse parameters are quantified to investigate the critical sliding volume of the droplet, including lubricant infusion amount, laser-scanning power, and pillar spacing. By virtue of the cooperation of temperature and force fields acting on the SLIPS, we demonstrate the intriguing applications including controllable chemical reaction and on-demand electrical circuit control. We envision that this dual-responsive surface should provide more possibilities in smart control of microscale droplets, especially in active vaccine-involved biochemical microreactions where a lower temperature is highly favored.

Patterning a Superhydrophobic Area on a Facile Fabricated Superhydrophilic Layer Based on an Inkjet-Printed Water-Soluble Polymer Template

An elaborated surface with a superhydrophilic area and a superhydrophobic area was fabricated by inkjet printing a water-soluble polymer template on a superhydrophilic layer. Titanate was used to generate the superhydrophilic layer with an in situ reaction. A water-soluble polymer template was inkjet printed on the facile fabricated superhydrophilic layer. Superhydrophobic treatment was carried out on the inkjet-printed surface with perfluorinated molecules. A superhydrophilic-superhydrophobic patterned surface (SSPS) was obtained by washing out the water-soluble polymer template. Various patterns of SSPS were fabricated with the different water-soluble polymer templates. Then, adhesion and deposition of water droplets were studied on the SSPS with the different wetting abilities on the surface. Meanwhile, a microreaction with a microfluidic chip was realized on the SSPS. In this work, systematic research on fabricating an SSPS based on a facile fabricated superhydrophilic layer with an inkjet-printed water-soluble polymer template is presented. It will have great potential for patterning materials, fabricating devices, and researching interfaces, such as microdroplet self-removal, analyte enrichment, and liquid-liquid interface reaction.

Magnetic actuation and deformation of a soft shuttle

Here, we describe the magnetic actuation of soft shuttles for open-top microfluidic applications. The system is comprised of two immiscible liquids, including glycerol as the soft shuttle and a suspension of iron powder in sucrose solution as the magnetic drop. Permanent magnets assembled on 3D printed motorized actuators were used for the actuation of the magnetic drop, enabling the glycerol shuttle to be propelled along customized linear, circular, and sinusoidal paths. The dynamics of the hybrid shuttle-magnetic drop system was governed by the magnetic force, the friction at the interface of the shuttle and the substrate, and the surface tension at the interface of the shuttle and the magnetic drop. Increasing the magnetic force leads to the localized deformation of the shuttle and eventually the full extraction of the magnetic drop. The versatility of the system was demonstrated through the propelling of the shuttle across a rough surface patterned with microfabricated barriers as well as taking advantage of the optical properties of the shuttle for the magnification and translation of microscale characters patterned on a planar surface. The integration of the system with current electrowetting actuation mechanisms enables the highly controlled motion of the magnetic drop on the surface of a moving shuttle. The simplicity, versatility, and controllability of the system provide opportunities for various fluid manipulation, sample preparation, and analysis for a range of chemical, biochemical, and biological applications.

How Does Chemistry Influence Liquid Wettability on Liquid-Infused Porous Surface?

Design of Nepenthes pitcher-inspired slippery liquid-infused porous surface (SLIPS) appeared as an important avenue for various potential and practically relevant applications. In general, hydrophobic base layers were infused with selected liquid lubricants for developing chemically inert SLIPS. Here, in this current study, an inherently hydrophilic (soaked beaded water droplet with ∼20° within a couple of minutes), porous and thick (above 200 μm) polymeric coating, loaded with readily chemically reactive acrylate moieties yielded a chemically reactive SLIPS, where residual acrylate groups in the synthesized hydrophilic and porous interface rendered stability to the infused lubricants. The chemically reactive SLIPS is capable of reacting with the solution of primary amine-containing nucleophiles in organic solvent through 1,4-conjugate addition reaction, both in the presence (referred as "in situ" modification) and absence (denoted as pre-modification) of lubricated phase in the porous polymeric coating. Such amine reactive SLIPS was further extended to (1) examining the impact of different chemical modifications on the performance of SLIPS and (2) developing a spatially selective and "in situ" postmodification with primary amine-containing nucleophiles through 1,4-conjugate addition reaction. Moreover, the chemically reactive SLIPS was capable of sustaining various physical abrasions and prolonged (minimum 10 days) exposure to complex and harsh aqueous phases, where infused lubricants protect the residual acrylate groups from harsh aqueous exposures. Such, principle will be certainly useful for spatially selective covalent immobilization of water-insoluble functional molecules/polymers directly from organic solvents, which would be of potential interest for various applied and fundamental contexts.

Life and death of liquid-infused surfaces: a review on the choice, analysis and fate of the infused liquid layer

We review the rational choice, the analysis, the depletion and the properties imparted by the liquid layer in liquid-infused surfaces – a new class of low-adhesion surface.

Droplet on Soft Shuttle: Electrowetting-on-Dielectric Actuation of Small Droplets

Here, we introduce the novel concept of a "soft shuttle" for transportation, manipulation, and diffusion studies of small liquid droplets using electrowetting on the dielectric mechanism. This method enables manipulation of droplets several times smaller than the electrode size and, importantly, minimizes evaporation, contamination, and exposure of the sample to high voltages. We demonstrate various modes of droplet loading, transporting, and unloading. Using advanced imaging processing techniques, we obtained detailed information about the shuttle and droplet centroids. Furthermore, varying water concentration on the soft shuttle allows for modulation of the diffusion kinetics of samples into the shuttle, which also can be controlled with soft shuttle actuation velocity. We believe that this novel approach for the manipulation of droplets will advance the field of droplet-based open microfluidics and can be potentially useful for applications in biotechnology, diagnostics, or analytical chemistry.

Beetle-Inspired Hierarchical Antibacterial Interface for Reliable Fog Harvesting

The microdroplets in fog flow have been considered as an important resource for supplying fresh drinking water. Most of the reported works of fog collection focus on the water-collecting ability rather than the environmental reliability of selected materials. In this work, a beetle-inspired hierarchical fog-collecting interface based on the antibacterial needle-array (ABN) and hydrophilic/hydrophobic cooperative structure is displayed. The hydrophilic ABN is coated with zwitterionic carboxybetaine (CB) brushes that endow the fog collector with a long-term cleaning in harsh environment. Due to its strong affinity to water molecules, the tilted needles with a CB coating can facilitate the capture of fog and the rapid delivery of condensed water driven by gravity. After being transported to the connected hydrophobic sheet, the collected droplets can be rapidly detached and stored in the container, achieving a high fog-harvesting rate. Furthermore, CB-patterned channels are integrated on the hydrophobic sheet for the pathway-controlled water delivery. The CB coating is able to efficiently resist bacterial adhesion and contamination during fog harvesting, protecting the device from microbiological corrosion. The current design provides a promising method to incorporate antibacterial ability into fog collectors, which offer great opportunity to develop water harvesters for real-world applications.

Combining the geometry of folded paper with liquid-infused polymer surfaces to concentrate and localize bacterial solutions

Point-of-care (POC) detection and diagnostic platforms provide critical information about health and safety conditions in austere and resource-limited settings in which medical, military, and disaster relief operations are conducted. In this work, low-cost paper materials commonly used in POC devices are coated with liquid-infused polymer surfaces and folded to produce geometries that precisely localize complex liquid samples undergoing concentration by evaporation. Liquid-infused polymer surfaces were fabricated by infusing silicone-coated paper with a chemically compatible polydimethylsiloxane oil to create a liquid overlayer. Tests on these surfaces showed no remaining bacterial cells after exposure to a sliding droplet containing a concentrated solution of Escherichia coli or Staphylococcus aureus, while samples without a liquid layer showed adhesion of both microdroplets and individual bacterial cells. Folding of the paper substrates with liquid-infused polymer surfaces into several functional 3D geometries enabled a clean separation and simultaneous concentration of a liquid containing rhodamine dye into discrete, predefined locations. When used with bacteria, which are known for their ability to adhere to nearly any surface type, functional geometries with liquid-infused polymer surfaces concentrated the cells at levels significantly higher than geometries with dry control surfaces. These results show the potential of synergistically combining paper-based materials with liquid-infused polymer surfaces for the manipulation and handling of complex samples, which may help the future engineering of POC devices.

Separation of Floating Oil Drops Based on Drop-Liquid Substrate Interfacial Tension

Though various strategies exist for the transport of oil drops suspended on a liquid substrate, selective manipulation of different kinds of drops based on their respective characteristics remains a challenge. In practice, it is possible to have multiple drops having different wetting states with the liquid substrate, whose separation is desired. In this work, we exploit curvature-induced capillary forces for the selective manipulation (transport as well as separation) of oil droplets based on their interfacial tension (IFT) with the underlying liquid substrate. To demonstrate this, we have selected two oils having different IFTs with the aqueous liquid substrate and tuned their curvature-induced capillary interaction (inward or outward from the source) by controlled addition of the surfactant. We experimentally realize three droplet manipulation regimes: repulsion, attraction, and separation regime. In the repulsion and attraction regimes, both the drops behave in a similar manner. Strikingly, in the separation regime, drops can be effectively separated based on their IFT; low IFT droplets are attracted toward the source, while high IFT droplets do the reverse.

Facile Actuation of Organic and Aqueous Droplets on Slippery Liquid-Infused Porous Surfaces for the Application of On-Chip Polymer Synthesis and Liquid-Liquid Extraction

Digital microfluidics employs water-repellant surfaces to exquisitely manipulate droplets of water for chemical analysis. However, the actuation and manipulation of organic droplets is still relatively unexplored as it is significantly more difficult to synthesize organic-repellent surfaces compared to water-repellent surfaces. Here, we present the fabrication of slippery liquid-infused porous surfaces (SLIPS) based on a porous polymer monolithic approach. The synthesized SLIPS were able to repel organic liquids such as hexane and methanol with a contact angle of 42.1 ± 0.4° and 69.0 ± 1.8°, respectively, as well as water with a contact angle of 115.8 ± 0.8°. More importantly for digital microfluidic applications, the sliding angle of liquids tested was between 4° and 6°. As a result, droplets containing magnetically susceptible material could be facilely manipulated on the SLIPS surface. A systematic actuation study was carried out to explore how actuation parameters including speed, paramagnetic particle (PMP) concentrations, and droplet volume impacted the outcomes (droplet actuation, disengagement, and PMP extraction). Two different applications were used to demonstrate the utility of actuating organic droplets on SLIPS surfaces including on-chip liquid-liquid extractions of natural products (NPs) from marine bacteria and droplet-based polymer synthesis with different polymerization conditions. Both applications employ an aqueous droplet and organic droplet interface at which either phase transfer or a chemical reaction is carried out. Two NPs (prodigiosin from Pseudoalteromonas rubra and violacein from Pseudoalteromonas luteoviolacea) were extracted, from aqueous droplets containing the bacteria, into butanol droplets and characterized with matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS). Nylon 6,6 was synthesized on-chip via magnetic actuation of organic droplets containing adipoyl chloride and hexamethylamine. Relative intensities of the characteristic polymer masses suggest that droplet-based microfluidic synthesis on slips can be used to probe reaction conditions. The compatibility of SLIPS with both aqueous and organic solutions opens up a wider number of droplet-based sample preparation protocols and chemical transformations.