Total exfoliation of graphite in molten salts.

The exfoliation of graphite to graphene nanoplatelets (GnP) in a molten salt medium is investigated in this study. It is shown that this mechanical force-free process yielded a large-sized GnP product (>15 microns) with a low defect density. The effect of the surface tension of the molten salt on graphite exfoliation efficiency was investigated for a series of alkali chloride salts (CsCl, KCl, NaCl and eutectic NaCl-KCl) at 850 °C. It was demonstrated that the produced GnP could be completely and easily separated from the salt. Molten salt with the lowest value of surface tension (CsCl) displayed the highest wettability of the graphitic layers and hence facilitated total exfoliation of the graphite to GnP. The exfoliation of graphite in molten salts is applicable in the thermal energy storage field, as well as in exfoliation of other layered materials. Herein, it is demonstrated that the thermal conductivity of the GnP-CsCl composite is enhanced by ∼300% compared to the neat salt.

Stages That Lead to Drop Depinning and Onset of Motion.

In this paper, we consider drops that are subjected to a gradually increasing lateral force and follow the stages of the motion of the drops. We show that the first time a drop slides as a whole is when the receding edge of the drop is pulled by the advancing edge (the advancing edge drags the receding edge). The generality of this phenomenon includes sessile and pendant drops and spans over various chemically and topographically different cases. Because this observation is true for both pendant and sessile cases, we exclude hydrostatic pressure as its reason. Instead, we explain it in terms of the wetting adaptation and interfacial modulus, that is, the difference in the energies of the solid interface at the advancing and receding edges. At the receding edge, a slight motion exposes to the air a recently wetted solid surface whose molecules had reoriented to the liquid and will take time to reorient back to the air. This results in a high surface energy at the solid-air interface which pulls on the triple line, that is, inhibits the motion of the receding edge. On the other hand, at the advancing edge, a slight advancement does not change the nature of the solid interfacial molecules outside the drop, and the advancing side's sliding can continue. Moreover, the solid molecules under the drop at the advancing edge take time to reorient, and hence, their configuration is not yet adapted for the liquid and therefore not adapted for retention of the advancing edge. Therefore, in sliding-drop experiments, the advancing edge moves before the receding one, typically a few times before the receding edge moves. For the same reason, the last motion of the receding edge usually happens as a result of the advancing edge pulling on it.

Open Problems in Wetting Phenomena: Pinning Retention Forces.

We review existing explanations for drop pinning and the origin of the force required to initiate the sliding of a drop on a solid surface (depinning). Theories that describe these phenomena include de Gennes', Marmur's, Furmidge's, the related Furmidge-Extrand's, and Tadmor's theory. These theories are all well cited but generally do not address each other, and usually papers that cite one of them ignore the others. Here, we discuss the advantages and disadvantages of these theories and their applicability to different experimental systems. Thus, we link different experimental systems to the theories that describe them best. We describe the force laws that can be deduced should these theories be united and the major open problems that remain. We describe a physical meaning that can be extracted from retention force measurements, specifically, the interfacial modulus that describes the tendency of a solid to conform to the liquid. This has implications for various wetting phenomena such as adhesion robustness, drug penetration into biological tissues, and solid robustness/resilience versus solid degradation over time as a result of its contact with a liquid.

Comment on "Comparison of the Lateral Retention Forces on Sessile, Pendant, and Inverted Sessile Drops".

Tadmor et al.'s 2009 PRL article shows experiments of pendant drops with ∼30% higher retention forces than their sessile analogues. A recent article (de la Madrid, R. et al. Langmuir 2019, 35, 2871) seemingly explains this result theoretically using a drastically different experimental system that shows a ∼3% higher force that exceeds the scatter in three out of four data points. The differences between the two experimental systems might have allowed the two theories to coexist, but Tadmor's theory, which can explain both, allows an understanding of the solid-liquid interaction, which the newer theory lacks.

The Influence of Gravity on Contact Angle and Circumference of Sessile and Pendant Drops has a Crucial Historic Aspect.

Normally, pendant drops adapt contact angles that are closer to 90° than their sessile analogues. This is due to the drop's weight that pulls the pendant drop and straightens its contact angles. In this paper, we show a case in which the opposite happens: sessile drops that adapt contact angles that are closer to 90° than their pendant analogues. To achieve these peculiar states, one needs to increase the effective gravity on the drops and then relax it again to 1 g. Apparently, this and other phenomena depend not only on the direction of the gravitational force but also on the drop's history. We show that the drop's contact angle (and resultant area) is affected by two types of histories: short-term history and long-term history. For example, if we gradually increase the effective gravity on the drop, decrease it back to 1 g, and then repeat this cycle again and again, we see that the first cycle is drastically different, whereas other cycles approach a plateau in their behavior. In addition to drop's history, we explain these observations in terms of volume conservation, drop contact area, and pinning effect. This study may be generalized for other body forces such as electrical and magnetic or accelerating systems.

Droplets Sliding down a Vertical Surface under Increasing Horizontal Forces.

We have investigated the retention forces of liquid drops on rotating, vertical surfaces. We considered two scenarios: in one, a horizontal, centrifugal force pushes the drop toward the surface (?pushed drop? case), and in the other, a horizontal, centrifugal force pulls the drop away from the surface (?pulled drop? case). Both drops slide down as the centrifugal force increases, although one expects that the pushed drop should remain stuck to the surface. Even more surprising, when the centrifugal force is low, the pushed drop moves faster than the pulled drop, but when the centrifugal force is high, the pushed drop moves much slower than the pulled drop. We explain these results in terms of interfacial modulus between the drop and the surface.

Wrinkling number and force of a particle raft in compression.

A particle raft is formed by a layer of small particles floating on a water surface, which has a higher load bearing capacity than pure water. In the present work, we have made a comprehensive study on the wrinkling number and force of the particle raft in planar compression. The wrinkling number during the whole loading process is measured, accompanied with snapshots on the morphologies of the particle raft. The force-displacement curve is given based on the loading system, which has been validated by the numerical simulation. Moreover, the experiment and theoretical results both show that the equivalent Young's modulus is dependent upon the loading displacement. Finally, the maximum wrinkling number of the raft has been analyzed by the scaling law, which agrees well with the experimental result. These findings have deepen our understandings on the mechanical properties of soft materials, which also hold implications on drug delivery, chemical engineering, micro-fluidics, environment protection, petroleum exploitation, mineral flotation, etc.

Why Drops Bounce on Smooth Surfaces.

It is shown that introducing gravity in the energy minimization of drops on surfaces results in different expressions when minimized with respect to volume or with respect to contact angle. This phenomenon correlates with the probability of drops to bounce on smooth surfaces on which they otherwise form a very small contact angle or wet them completely. Theoretically, none of the two minima is stable: the drop should oscillate from one minimum to the other as long as no other force or friction will dissipate the energy. Experimentally, smooth surfaces indeed show drops that bounce on them. In some cases, they bounce after touching the solid surface, and in some cases they bounce from a nanometric air, or vacuum film. The bouncing energy can be stored in the interfaces: liquid-air, liquid-solid, and solid-air. The lack of a single energy minimum prevents a simple convergence of the drop's shape on the solid surface, and supports its bouncing back to the air. Therefore, the lack of a simple minimum described here supports drop bouncing on hydrophilic surfaces such as that reported by Kolinski et al. Our calculation shows that the smaller the surface tension, the bigger the difference between the contact angles calculated based on the two minima. This agrees with experimental finding where reducing the surface tension, for example, by adding surfactants, increases the probability for bouncing of the drops on smooth surfaces.

Solid-Liquid Work of Adhesion.

We establish a tool for direct measurements of the work needed to separate a liquid from a solid. This method mimics a pendant drop that is subjected to a gravitational force that is slowly increasing until the solid-liquid contact area starts to shrink spontaneously. The work of separation is then calculated in analogy to Tate's law. The values obtained for the work of separation are independent of drop size and are in agreement with Dupré's theory, showing that they are equal to the work of adhesion.

Misconceptions in wetting phenomena.

In a recent paper ('t Mannetje, D.; Banpurkar, A.; Koppelman, H.; Duits, M. H. G.; van den Ende, D.; Mugele, F. Electrically Tunable Wetting Defects Characterized by a Simple Capillary Force Sensor. Langmuir 2013, 29, 9944-9949), there are a few misconceptions regarding the interpretations of theories emanating from Shanahan and de Gennes in describing centrifugal adhesion balance (CAB) experiments, making their results seemingly contradictory to the theory. These are clarified here. We show that their results, if interpreted correctly, do not contradict the theories mentioned above.

Gels That Serve as Mucus Simulants: A Review.

Mucus is a critical part of the human body's immune system that traps and carries away various particulates such as anthropogenic pollutants, pollen, viruses, etc. Various synthetic hydrogels have been developed to mimic mucus, using different polymers as their backbones. Common to these simulants is a three-dimensional gel network that is physically crosslinked and is capable of loosely entrapping water within. Two of the challenges in mimicking mucus using synthetic hydrogels include the need to mimic the rheological properties of the mucus and its ability to capture particulates (its adhesion mechanism). In this paper, we review the existing mucus simulants and discuss their rheological, adhesive, and tribological properties. We show that most, but not all, simulants indeed mimic the rheological properties of the mucus; like mucus, most hydrogel mucus simulants reviewed here demonstrated a higher storage modulus than its loss modulus, and their values are in the range of that found in mucus. However, only one mimics the adhesive properties of the mucus (which are critical for the ability of mucus to capture particulates), Polyvinyl alcohol-Borax hydrogel.

On the role of the three-phase contact line in surface deformation.

Viscoelastic braking theories developed by Shanahan and de Gennes and by others predict deformation of a solid surface at the solid-liquid-air contact line. This phenomenon has only been observed for soft smooth surfaces and results in a protrusion of the solid surface at the three-phase contact line, in agreement with the theoretical predictions. Despite the large (enough to break chemical bonds) forces associated with it, this deformation was not confirmed experimentally for hard surfaces, especially for hydrophobic ones. In this study we use superhydrophobic surfaces composed of an array of silicon nanostructures whose Young modulus is 4 orders of magnitude higher than that of surfaces in earlier recorded viscoelastic braking experiments. We distinguish between two cases: when a water drop forms an adhesive contact, albeit small, with the apparent contact angle θ < 180° and when the drop-surface adhesion is such that the conditions for placing a resting drop on the surface cannot be reached (i.e., θ = 180°). In the first case we show that there is a surface deformation at the three-phase contact line which is associated with a reduction in the hydrophobicity of the surface. For the second case, however, there cannot be a three-phase contact line associated with a drop in contact with the surface, and indeed, if we force-place a drop on the surface by holding it with a needle, no deformation is detected, nor is there a reduction in the hydrophobic properties of the surface. Yet, if we create a long horizontal three-phase contact line by partially immersing the superhydrophobic substrate in a water bath, we see a localized reduction in the hydrophobic properties of the surface in the region where the three-phase contact line used to be. The SEM scan of that region shows a narrow horizontal stripe where the nanorods are no longer there, and instead there is only a shallow structure that is lower than the nanorods height and resembles fused or removed nanorods. Away from that region, either on the part of the surface which was exposed to bulk water or the part which was exposed to air, no change in the hydrophobic properties of the surface is observed, and the SEM scan confirms that the nanorods seem intact in both regions.

A first order-based model for the kinetics of formation of Pickering emulsions.

MOTIVATION AND BACKGROUND: Many physical systems are composed of two immiscible fluids containing solid particles whose role is to emulsify the two fluids. Such emulsions are called Pickering emulsions (PE). The present study introduces a theoretical framework for a first order kinetics of the creation of such emulsions and continues to verify the model experimentally using water and oil where water is the majority, or continuous, phase and oil is the minority, or dispersed, phase. These are referred to as O/W emulsions. The motivation for choosing this O/W system is to study the applicability of Pickering emulsions in marine environment and the role these emulsions can play in the cleaning of oil spills. As opposed to the use of surfactants which may be toxic to wildlife, the solid particles used to stabilize PEs are generally non-toxic. Theoretical and experimental methods are employed, as outlined below: THEORETICAL MODEL: A theoretical model based on first order kinetics is constructed. Unlike classic first order kinetics, our reaction is not chemical nor is it of 1:1 stoichiometry, but its time dependence is similar to that of first order. This behavior is a function of various system-specific parameters such as the energies of the different interfaces in the system, the solid particles' size, the densities of the components of the system, and the rate at which the system is agitated. The rate of formation of PEs is found to be proportional to 1-e-kt, where t is the time from the moment the system's components were introduced and k is a constant whose proportionality we describe analytically as a function of the various parameters in the system. EXPERIMENTAL FINDINGS: Our experimental results show exceptionally good agreement with the model, and it is shown that for the specific system tested (water, sand, light fraction petroleum), we get full emulsification of the 100ml system with 5 ml petroleum and 50 g sand within about 30s. This result is encouraging for studies that consider the use of such a system for the cleaning of oil spills.

Drops retracting while forming a rim.

HYPOTHESIS: Surfactant laden droplets may spread faster on a substrate and, subsequently, retract (spontaneously reversing the drop's direction). We hypothesize that this Marmur-Lelah type retraction can be explained by a de Gennes-type triple line fluctuation expression that is modified to represent our anisotropic surfactant adsorption. This explanation requires that the retraction originates inner to the triple line (not at the triple line itself). EXPERIMENTS: Drops of oil with surfactant (mainly tetradecane with octadecylamine) were allowed to spread on freshly cleaved mica surfaces at various concentrations and recorded with a high-speed camera. FINDINGS: At low concentrations, the subsequent retraction left a rim of liquid at the triple line location of maximal spreading whose thickness was inversely related to the surfactant concentration. This indeed agrees with de Gennes' triple line fluctuation expression that is modified to fit the anisotropic adsorption of the surfactant during the spreading period, which induces a solid-liquid interfacial energy gradient. According to this explanation, the rim may exist also at high surfactant solution concentrations, however, there can be a limit at which the surfactant solution concentration is so high that the rim's thickness is practically zero.

Measurement of lateral adhesion forces at the interface between a liquid drop and a substrate.

A novel instrument allows for the first time measurements of the lateral adhesion forces at a solid-liquid interface, f(parallel), in a way that is decoupled from the normal forces, f(perpendicular). We use it to measure how f(parallel) between a drop and a surface is influenced by different f(perpendicular) and different histories of drop resting periods on the surface prior to sliding, t(rest). The variation of f(parallel) with t(rest) is similar for different f(perpendicular) and always plateaus as t(rest)-->infinity. We show that the f(parallel) plateau value is higher when f(perpendicular) is lower. This seemingly counterintuitive result is in agreement with recent theories.

A Novel Technique Enables Quantifying the Molecular Interaction of Solvents with Biological Tissues.

The pharmaceutical industry uses various solvents to increase drug penetrability to tissues. The solvent's choice affects the efficacy of a drug. In this paper, we provide an unprecedented means of relating a solvent to a tissue quantitatively. We show that the solvents induce reorientation of the tissue surface molecules in a way that favors interaction and, therefore, penetrability of a solvent to a tissue. We provide, for the first time, a number for this tendency through a new physical property termed Interfacial Modulus (Gs). Gs, which so far was only predicted theoretically, is inversely proportional to such interactions. As model systems, we use HeLa and HaCaT tissue cultures with water and with an aqueous DMSO solution. The measurements are done using Centrifugal Adhesion Balance (CAB) when set to effective zero gravity. As expected, the addition of DMSO to water reduces Gs. This reduction in Gs is usually higher for HaCaT than for HeLa cells, which agrees with the common usage of DMSO in dermal medicine. We also varied the rigidities of the tissues. The tissue rigidity is not expected to relate to Gs, and indeed our results didn't show a correlation between these two physical properties.

As-placed contact angles for sessile drops.

As-placed contact angle is the contact angle a drop adapts as a result of its placement on a surface. As expected, the as-placed contact angle, thetaAP, of a sessile drop on a horizontal surface decreases with the drop size due to the increase in hydrostatic pressure. We present a theoretical prediction for thetaAP which shows that it is a unique function of the advancing contact angle, thetaA, drop size, and material properties (surface tensions and densities). We test our prediction with published and new data. The theory agrees with the experiments. From the relation of the as-placed contact angle to drop size the thermodynamic equilibrium contact angle is also calculated.

Controlling arbitrary humidity without convection.

In this paper we show a way that allows for the first time to induce arbitrary humidity of desired value for systems without convective flow. To enable this novelty we utilize a semi-closed environment in which evaporation is not completely suppressed. In this case, the evaporation rate is determined both by the outer (open) humidity and by the inner (semi-closed) geometry including the size/shape of the evaporating medium and the size/shape of the semi-closure. We show how such systems can be used to induce desired humidity conditions. We consider water droplet placed on a solid surface and study its evaporation when it is surrounded by other drops, hereon "satellite" drops and covered by a semi-closed hemisphere. The main drop's evaporation rate is proportional to its height, in agreement with theory. Surprisingly, however, the influence of the satellite drops on the main drop's evaporation suppression is not proportional to the sum of heights of the satellite drops. Instead, it shows proportionality close to the satellite drops' total surface area. The resultant humidity conditions in the semi-closed system can be effectively and accurately induced using different satellite drops combinations.

Fourier transform infrared-probed O(3P) microreactor: demonstration with ethylene reactions in argon matrix.

To demonstrate the development of an oxygen atom microreactor in the form of liquid-helium-cooled solid argon matrix deposited on an infrared (IR) window, the oxidation of ethylene by mobile O atoms has been investigated. O atom diffusion through the argon matrix is confirmed and used to examine ethylene-oxygen atom reactions. In a bench-scale matrix isolation system probed with a Fourier transform infrared (FT-IR) spectrometer, matrices of solid Ar at 8-10 K doped with NO2 and ethylene have been prepared on a ZnSe window within an evacuated cryostat. The matrices have been photolyzed using 350-450 nm photons, and the reaction products resulting from the reaction of O(3P), one of the photolysis products of NO2, with ethylene have been identified using FT-IR and a Gaussian 98W simulation program. These products include oxirane, acetaldehyde, ethyl nitrite radical, and ketene. The temperature effect in the range of 10-30 K on the products formed has also been investigated. The reaction mechanisms are discussed and the viability of the solid Ar matrix being a low temperature microreactor to examine reaction mechanisms of mobile oxygen atoms is elaborated.

Additional attraction between surfactant-coated surfaces.

A simple model that shows an additional attraction between solvated surfactant-coated systems is developed. The model simply calculates the van der Waals attraction between the solvated surfactant layers. This attraction was previously neglected as it was expected to have a small energetic contribution. This is indeed the case; however, despite the small energetic contribution the force is large. In other words, although the expression that we get is small in energy, it is large in force. This is particularly important for surface force balance measurements, where using the developed expression, some apparent discrepancies between measured and theoretical values may now have a possible explanation, and especially those associated with surfactant-coated surfaces. We apply the new expression to a given system, and compare with the experimental results.