Application of Surface Wettability to Control Spreading of an Impacting Droplet.

While the simplest outcome of a normal impact on a flat stationary solid surface is radially symmetric spreading, it is important to note that asymmetric spreading can intrinsically occur with a tangential velocity along the surface. However, no previous attempt has been made to restore the symmetry of a lamella that intrinsically spreads asymmetrically. Adjusting the lamella's asymmetric shape to a symmetric one is achieved in this work by varying wettability to affect the receding velocity of the contact line, according to the Taylor-Culick theory. Here we theoretically and practically show how restoring the symmetry can be achieved. Theoretically we built a framework to map the needed receding velocity at every given point of the contact line to allow for symmetry to be restored, and then this framework was applied to generate a wetting map that shows how at each local the wettability of the surface needs to be defined. Simulated results confirmed the effectiveness of our framework and identified the envelope of its applicability. Next, to apply the idea experimentally, the wetting map was transformed to a single wettability contrast area dubbed the "patch". Experimental results showed the effectiveness of the patch design in correcting the asymmetric spreading lamella for water droplets impacting a surface for the following Weber number conditions: Wen ≤ 300, Wet ≤ 300, and 0.51 ≤ Wen/Wet ≤ 2.04.

Experimental Investigation of Dropwise Condensation Shedding by Shearing Airflow in Microgravity Using Different Surface Coatings.

The shedding kinematics of water droplets in a condensation environment when exposed to aerodynamic forces in microgravity was studied. Understanding the shedding of droplets from a surface is a critical part of the dropwise condensation process for improving heat transfer. Because gravity as a droplet removal technique is not available in space, the use of airflow to shed droplets is considered for condensing heat exchangers in environmental control and life support systems. Surface coatings affect drop adhesion, and here, four different surfaces (PMMA, PS, PTFE, and SHS) and various droplet sizes (80, 60, and 40 µL) were used to understand the above phenomenon. It was found that the critical velocity to shed a droplet in microgravity was up to 8% lower than that in normal gravity. Also, the effect of the droplet size was investigated for both microgravity and normal gravity; the shedding velocity was lower for microgravity, and it decreased as droplet size increased. Increasing the hydrophobicity of the coating decreased the critical velocity for shedding. Finally, the droplet was found to detach from superhydrophobic surfaces in microgravity. The detachment of droplets from the substrate will hamper the condensation process that can produce a larger fresh area; also, detachment of droplets and entrainment in airflow counter the concept of removing moisture from the air in a dehumidification process.

Inkjet printing on hydrophobic surfaces: Controlled pattern formation using sequential drying.

Inkjet-printed micro-patterns on hydrophobic surfaces have promising applications in the fabrication of microscale devices such as organic thin-film transistors. The low wettability of the surface prevents the inkjet-printed droplets from spreading, connecting to each other, and forming a pattern. Consequently, it is challenging to form micro-patterns on surfaces with low wettability. Here, we propose a sequential printing and drying method to form micro-patterns and control their shape. The first set of droplets is inkjet-printed at a certain spacing and dried. The second set of droplets is printed between these dry anchors on the surface with low wettability. As a result, a stable bridge on the surface with low wettability forms. This printing method is extended to more complicated shapes such as triangles. By implementing an energy minimization technique, a simple model was devised to predict the shape of the inkjet-printed micro-patterns while confirming that their equilibrium shape is mainly governed by surface tension forces. The gradient descent method was utilized with parametric boundaries to emulate droplet pinning and wettability of the anchors and to prevent convergence issues from occurring in the simulations. Finally, the energy minimization based simulations were used to predict the required ink to produce dry lines and triangles with smooth edges.

Receding Phase and Rebound Behavior for Drop Impact onto an Ultrathin Film.

Experimentally, the recoil phase leading to rebound behavior for drop impact onto ultrathin oil-covered solid surfaces was studied. It was found that the oil film can rupture during the impact process when the contact angle between the drop liquid and substrate is smaller than 90°. Due to such rupture, the substrate wettability of the substrate can affect the behavior of the drop impact. The rupture of the oil film can be promoted by an increase in impact Weber number (We) and a decrease in the film viscosity and thickness. The effect of We, oil viscosity, and film thickness on the rebound behavior of the drop was also investigated. For low-viscous oil films (5 cSt), it was shown that the smooth and circular edge of the liquid lamella is the key parameter affecting the level of rebound. The smooth rim of the lamella can cause an elongated rebound, while a lamella with a jagged rim will result in a stout rebound. For the impact cases onto oil films with medium and high viscosity, the effects of the film viscosity become more important; the rebound type can be suppressed due to the viscous dispassion. We have also shown that silicone oil can cloak the daughter drops generated in the rebound process for the first time. Due to the existence of the oil, the daughter drops do not merge even when they make contact in the air.

Droplet Control Based on Pinning and Substrate Wettability.

Pinning of liquid droplets on solid substrates is ubiquitous and plays an essential role in many applications, especially in various areas such as microfluidics and biology. Although pinning can often reduce the efficiency of various applications, a deeper understanding of this phenomenon can actually offer possibilities for technological exploitation. Here, by means of molecular dynamics simulation, we identify the conditions that lead to droplet pinning or depinning and discuss the effects of key parameters in detail, such as the height of the physical pinning barrier and the wettability of the substrates. Moreover, we describe the mechanism of barrier crossing by the droplet upon depinning, identify the driving force of this process, and, also, elucidate the dynamics of the droplet. Not only does our work provide a detailed description of the pinning and depinning processes but also it explicitly highlights how both processes can be exploited in nanotechnology applications to control the droplet motion. Hence, we anticipate that our study will have significant implications for the nanoscale design of substrates in micro- and nanoscale systems and will assist with assessing pinning effects in various applications.

Energy of a Drop Required to Break a Liquid Film.

An external disturbance can destabilize and break a liquid film on a nonwettable surface. Previous studies focused on evaluating critical film thickness for a spontaneous breakup, but the required energy has been unknown. We experimentally found that the energy of a drop to break a liquid film is an order of magnitude more than that predicted by a free energy balance. Here, we show how to evaluate the energy needed to rupture a liquid film by considering the formation of a crater with a critical size.

Spray-On Nanocomposite Coatings: Wettability and Conductivity.

Nanocomposite coatings, i.e., a combination of nanocompounds, and a polymer matrix together with suitable additives and solvents is a very versatile method for producing multifunctional coatings. Some of the most desired coating properties have a high repellency to liquids (e.g., superhydrophobic and/or superoleophobic) and electrical and thermal conductivities. From a practical perspective, coatings that can be sprayed are very suitable for large-scale production, conformity, and reduced time and cost. Carbon-based, metallic, and ceramic are the three groups of nanocompounds commonly used to formulate spray-on nanocomposite coatings. In this invited feature article, we discuss the applications, advantages, and challenges of using such nanocompounds to produce coatings with good water repellency or/and elevated electrical or/and thermal conductivities. We also discuss the role of additives and solvents briefly in relation to the properties of the coatings. Important spraying parameters, such as stand-off distance and its influence on the final coating properties, will also be examined. Our overall aim is to provide a guideline for the production of practical multifunctional nanocomposites utilizing carbon-based, metallic, or ceramic nanoparticles or nanofibers that covers both aspects of in-air wettability and conductivity under one umbrella.

Behavior of a Liquid Bridge between Nonparallel Hydrophobic Surfaces.

When a liquid bridge is formed between two nonparallel identical surfaces, it can move along the surfaces. Literature indicates that the direction of bridge movement is governed by the wettability of surfaces. When the surfaces are hydrophilic, the motion of the bridge is always toward the cusp (intersection of the plane of the two bounding surfaces). On the other hand, the movement is hitherto thought to be always pointing away from the cusp when the surfaces are hydrophobic. In this study, through experiments, numerical simulations, and analytical reasoning, we demonstrate that for hydrophobic surfaces, wettability is not the only factor determining the direction of the motion. A new geometrical parameter, i.e., confinement (cf), was defined as the ratio of the distance of the farthest contact point of the bridge to the cusp, and that of the closest contact point to the cusp. The direction of the motion depends on the amount of confinement (cf). When the distance between the surfaces is large (resulting in a small cf), the bridge tends to move toward the cusp through a pinning/depinning mechanism of contact lines. When the distance between the surfaces is small (large cf), the bridge tends to move away from the cusp. For a specific system, a maximum cf value (cfmax) exists. A sliding behavior (i.e., simultaneous advancing on the wider side and receding on the narrower side) can also be seen when a liquid bridge is compressed such that the cf exceeds the cfmax. Contact angle hysteresis (CAH) is identified as an underpinning phenomenon that together with cf fundamentally explains the movement of a trapped liquid between two hydrophobic surfaces. If there is no CAH, however, i.e., the case of ideal hydrophobic surfaces, the cf will be a constant; we show that the bridge slides toward the cusp when it is stretched, while it slides away from the cusp when it is compressed (note sliding motion is different from motion due to pinning/depinning mechanism of contact lines). As such, the displacement is only related to geometrical parameters such as the amount of compression (or stretching) and the dihedral angle between the surfaces.

Application of 3D printing for fabrication of superhydrophobic surfaces with reversible wettability.

Control of surface wettability is needed in many applications. The potential use of 3D printing technology to gain control over wettability remains largely unexplored. In this paper, Fused Deposition Molding (FDM) 3D printing technology was utilized to print polylactic acid (PLA) microplate array structures to generate superhydrophobic surfaces with reversable wetting properties. This was achieved by spraying polydimethylsiloxane (PDMS) and silica (SiO2) solutions, over microplate surfaces. Anisotropic wetting properties were also achieved based on the surface structure design. Due to the shape memory properties of PLA, the morphology of the microplate arrays could be switched between the original upright shape and deformed shape. Through alternating pressing and heating treatments, the microplate arrays showed anisotropic wettability switching. The difference between the contact angle (CA) and sliding angle (SA) of water droplets on the original surface parallel to and perpendicular to the microplate array direction were ΔCA = 7° and ΔSA = 3° respectively, and those on the surface of the deformed microplate array were ΔCA = 7° and ΔSA = 21°, respectively. This process enabled reversible alteration in the wetting behavior of water droplets on the original and deformed surfaces between sliding and sticking states. PLA-based shape memory anisotropic superhydrophobic surfaces with tunable adhesion were successfully applied to rewritable platforms, micro droplet reaction platforms, and gas sensing.

Microfluidic Droplet-Generation Device with Flexible Walls.

Controlling droplet sizes is one of the most important aspects of droplet generators used in biomedical research, drug discovery, high-throughput screening, and emulsion manufacturing applications. This is usually achieved by using multiple devices that are restricted in their range of generated droplet sizes. In this paper, a co-flow microfluidic droplet-generation device with flexible walls was developed such that the width of the continuous (C)-phase channel around the dispersed (D)-phase droplet-generating needle can be adjusted on demand. This actuation mechanism allowed for the adjustment of the C-phase flow velocity, hence providing modulated viscous forces to manipulate droplet sizes in a single device. Two distinct droplet-generation regimes were observed at low D-phase Weber numbers, i.e., a dripping regime at high- and medium-channel widths and a plug regime at low-channel widths. The effect of channel width on droplet size was investigated in the dripping regime under three modes of constant C-phase flow rate, velocity, and Capillary number. Reducing the channel width at a constant C-phase flow rate had the most pronounced effect on producing smaller droplets. This effect can be attributed to the combined influences of the wall effect and increased C-phase velocity, leading to a greater impact on droplet size due to the intensified viscous force. Droplet sizes in the range of 175-913 µm were generated; this range was ~2.5 times wider than the state of the art, notably using a single microfluidic device. Lastly, an empirical model based on Buckingham's Pi theorem was developed to predict the size of droplets based on channel width and height as well as the C-phase Capillary and Reynolds numbers.

Folding characteristics of membranes in capillary origami.

A study was conducted to understand the effects of membrane shape, thickness, contact angle, surface tension and large deflection on capillary origami. For experiments, square and triangular membranes made of PDMS with various thicknesses and sizes were used to encapsulate different liquids. Models for membranes under pure bending were developed using the energy balance between interfacial energies (liquid-vapor, solid-liquid and solid-vapor energies) and bending energy evaluated by a small-deflection and a large-deflection assumptions. This paper is the first study to consider the large deflection for membranes as well as to include the terms for the wettability of the membrane and its shape. The developed models evaluated an important characteristic length, i.e., elasto-capillary length (LEC), which is proportional to the critical length (Lcritical) below which membranes cannot be closed to encapsulate liquid. The experimental results showed that the large-deflection model can estimate Lcritical more accurately in terms of membrane shape, thickness, contact angle and surface tension for liquids with similar properties to water than the small-deflection model. The developed models should be further improved to extend the applicability to liquids with low surface tension and low contact angle.

Recent advances in biosensors for real time monitoring of pH, temperature, and oxygen in chronic wounds.

Chronic wounds are among the major healthcare issues affecting millions of people worldwide with high rates of morbidity, losses of limbs and mortality. Microbial infection in wounds is a severe problem that can impede healing of chronic wounds. Accurate, timely and early detection of infections, and real time monitoring of various wound healing biomarkers related to infection can be significantly helpful in the treatment and care of chronic wounds. However, clinical methodologies of periodic assessment and care of wounds require physical visit to wound care clinics or hospitals and time-consuming frequent replacement of wound dressing patches, which also often adversely affect the healing process. Besides, frequent replacements of wound dressings are highly expensive, causing a huge amount of burden on the national health care systems. Smart bandages have emerged to provide in situ physiochemical surveillance in real time at the wound site. These bandages integrate smart sensors to detect the condition of wound infection based on various parameters, such as pH, temperature and oxygen level in the wound which reduces the frequency of changing the wound dressings and its associated complications. These devices can continually monitor the healing process, paving the way for tailored therapy and improved quality of patient's life. In this review, we present an overview of recent advances in biosensors for real time monitoring of pH, temperature, and oxygen in chronic wounds in order to assess infection status. We have elaborated the recent progress in quantitative monitoring of several biomarkers important for assessing wounds infection status and its detection using smart biosensors. The review shows that real-time monitoring of wound status by quantifying specific biomarkers, such as pH, temperature and tissue oxygenation to significantly aid the treatment and care of chronic infected wounds.

Innovative Solid Slippery Coating: Uniting Mechanical Durability, Optical Transparency, Anti-Icing, and Anti-Graffiti Traits.

Slippery coatings, such as the slippery liquid-infused porous surface (SLIPS), have gained significant attention for their potential applications in anti-icing and anti-fouling. However, they lack durability when subjected to mechanical impact. In this study, we have developed a robust slippery coating by blending polyurethane acrylate (PUA) with methyltriethoxysilane (MTES) and perfluoropolyether (PFPE) in the solvent of butyl acetate. The resulting mixture is homogeneous and allows for uniform coating on various substrates using a drop coating process followed by drying at 160 °C for 3 h. The cured coating exhibits excellent water repellency (contact angle of ~108° and sliding angle of ~8°), high transparency (average visible transmittance of ~90%), exceptional adherence to the substrate (5B rating according to ASTMD 3359), and remarkable hardness (4H on the pencil hardness scale). Moreover, the coating is quite flexible and can be folded without affecting its wettability. The robustness of the coating is evident in its ability to maintain a sliding angle below 25° even when subjected to abrasion, water jetting, high temperature, and UV irradiation. Due to its excellent nonwetting properties, the coating can be employed in anti-icing, anti-graffiti, and anti-sticking applications. It effectively reduces ice adhesion on aluminum substrates from approximately 217 kPa to 12 kPa. Even after 20 cycles of icing and de-icing, there is only a slight increase in ice adhesion, stabilizing at 40 kPa. The coating can resist graffiti for up to 400 cycles of writing with an oily marker pen and erasing with a tissue. Additionally, the coating allows for easy removal of 3M tape thereon without leaving any residue.

Oxygen releasing patches based on carbohydrate polymer and protein hydrogels for diabetic wound healing: A review.

Diabetic wounds are among the major healthcare challenges, consuming billions of dollars of resources and resulting in high numbers of morbidity and mortality every year. Lack of sufficient oxygen supply is one of the most dominant causes of impaired healing in diabetic wounds. Numerous clinical and experimental studies have demonstrated positive outcomes as a result of delivering oxygen at the diabetic wound site, including enhanced angiogenesis, antibacterial and cell proliferation activities. However, prolonged and sustained delivery of oxygen to improve the wound healing process has remained a major challenge due to rapid release of oxygen from oxygen sources and limited penetration of oxygen into deep skin tissues. Hydrogels made from sugar-based polymers such as chitosan and hyaluronic acid, and proteins such as gelatin, collagen and hemoglobin have been widely used to deliver oxygen in a sustained delivery mode. This review presents an overview of the recent advances in oxygen releasing hydrogel based patches as a therapeutic modality to enhance diabetic wound healing. Various types of oxygen releasing wound healing patch have been discussed along with their fabrication method, release profile, cytocompatibility and in vivo results. We also briefly discuss the challenges and prospects related to the application of oxygen releasing biomaterials as wound healing therapeutics.

Novel SLIPS based on the photo-thermal MOFs with enhanced anti-icing/de-icing properties.

A photo-thermal anti-icing/de-icing SLIPS coating is designed based on porous light-responsive MOFs. Due to the strong light absorption and high light-thermal conversion, the as-synthetic SCMOFs exhibited prolonged freezing delay time and depressed water crystallization point under light irradiation. Meantime, the SCMOFs exhibit good deicing properties. With the irradiation, the half-melted ice slips off quickly.

Durable Superhydrophobic Wood via One-Step Immersion in Composite Silane Solution.

A superhydrophobic coating endows pristine hydrophilic wood with excellent water/moisture repellency and thus prolongs its service life. Generally, the superhydrophobic coating on wood is fabricated by a two-step process in which the nanoparticles are first introduced onto the surface and then modified by low-surface-energy molecules. Herein, for the first time, we have fabricated the superhydrophobic wood via a one-step process free of nanoparticles by immersing the pristine hydrophilic wood, such as pine, balsawood, and basswood, into a composite silane solution of hexadecyltrimethoxysilane and methyltrimethoxysilane. The wood remains superhydrophobic or highly hydrophobic after long-term exposure to mechanical damage (such as abrading, knife-cutting, and tape-peeling), chemical damage (such as immersion in acid, alkali, or ethanol), and environmental impacting (such as UV irradiation and low/high-temperature exposure).

Water and mildew proof SiO2 & ZnO/silica sol superhydrophobic composite coating on a circuit board.

In order to improve the waterproof and mildew resistance of electronic equipment, a superhydrophobic coating was prepared on a circuit board. First, hexadecyl trimethoxysilane was used to modify the nano silica and nano zinc oxide particles, and then the modified nanoparticles were mixed with the silica sol. Then the superhydrophobic coating was prepared on the surface of the printed circuit board by a spraying process. The preparation technology and physical and chemical properties of the coating were studied. The contact angle of the final sample can reach 169.47°, the sliding angle can reach 1.2°, it has good acid and alkali corrosion resistance, resistance to NaCl, self-cleaning performance and antimildew performance.

Motion of a liquid bridge between nonparallel surfaces.

Bulk motion of a liquid bridge between two nonparallel identical solid surfaces undergoing multiple loading cycles (compressing and stretching) was investigated numerically and experimentally. The effects of the following governing parameters were studied: the dihedral angle between the two surfaces (ψ), the amount of compressing and stretching (Δh), and wettability parameters i.e. the advancing contact angle (θa) and Contact Angle Hysteresis (CAH). Experiments were done using various combinations of ψ, Δh and on surfaces with different wettabilities to understand the effect of each parameter individually. Additionally, a numerical model using Surface Evolver software was developed to augment the experimental data and extract information about the shape of the bridge. An empirical function was proposed and validated to calculate the minimum amount of Δh needed to initiate the bulk motion (i.e. to overcome the initial lag of the motion in response to the compressing of the bridge), at a given dihedral angle ψ. The effect of governing parameters on magnitude and precision of the motion was investigated. The magnitude of the motion was found to be increased by increasing ψ and Δh, and/or by decreasing θa and CAH. We demonstrated the possibility of modulating the precision of the motion with θa. Additionally, it was shown that the magnitude of the motion (in one loading cycle) increases after each loading cycle, if the contact lines depin only on the narrower side of the bridge during compressing and only on the wider side during stretching (asymmetric depinning). Whereas, depinning on both sides of the bridge (symmetric depinning) reduced the magnitude of bridge motion in each cycle under cyclic loading. A larger ψ was found to convert symmetric depinning into asymmetric depinning. These findings not only enhance the understanding of bridge motion between nonparallel surfaces, but also are beneficial in controlling magnitude, precision, and lag of the motion in practical applications.

Stability of a liquid bridge between nonparallel hydrophilic surfaces.

Formation of liquid bridges between two solid surfaces is frequently observed in industry and nature, e.g. in printing applications. When the two solid surfaces are not parallel (with a dihedral angle ψ between them), an interesting phenomenon emerges: if ψ exceeds a critical angle (denoted as ψc) the bridge is no longer stable, and propels itself toward the cusp of the surfaces. In this work we performed, for the first time, a systematic study on the parameters influencing ψc by combining experimental, theoretical, and numerical investigations. It was shown that ψc is determined by the advancing contact angle (θa) and Contact Angle Hysteresis (CAH) of the surfaces: it increases as θa or CAH increases, and these two parameters have a nonlinear and interdependent influence on ψc. Based on our quantitative results, an empirical equation is presented to predict the critical angle, ψc=f(θa,CAH) in closed analytical form. This equation can be used to calculate ψc for bridges formed by moving down a pre-tilted surface towards a sessile drop on a stationary lower surface, or bridges between initially parallel surfaces which the top surface tilts after bridge formation.

Investigation of a hybrid method of soft tissue graft fixation for anterior cruciate ligament reconstruction.

To increase knee stability following anterior cruciate ligament (ACL) reconstruction, development of increasingly stronger and stiffer fixation is required. This study assessed the initial pullout force, stiffness of fixation, and failure modes for a novel hybrid fixation method combining periosteal and direct fixation using porcine femoral bone. A soft tissue graft was secured by combining both an interference screw and an EndoButton (Smith and Nephew Endoscopy, Andover, MA). The results were compared with the traditional direct fixation method using a titanium interference screw. Twenty porcine hindlimbs were divided into two groups. Specimens were loaded in line with the bone tunnel on a materials testing machine. Maximum pullout force of the hybrid fixation (588+/-37 N) was significantly greater than with an interference screw alone (516+/-37 N). The stiffness of the hybrid fixation (52.1+/-12.8 N/mm) was similar to that of screw fixation (56.5+/-10.2 N/mm). Graft pullout was predominant for screw fixation, whereas a combination of graft pullout and graft failure was seen for hybrid fixation. These results indicate that initial pullout force of soft tissue grafts can be increased by using the suggested novel hybrid fixation method.