The nanoscale protrusions of different morphologies on wing surfaces of four cicada species were examined under an environmental scanning electron microscope (ESEM). The water contact angles (CAs) of the wing surfaces were measured along with droplet adhesion values using a high-sensitivity microelectromechanical balance system. The water CA and adhesive force measurements obtained were found to relate to the nanostructuring differences of the four species. The adhesive forces in combination with the Cassie-Baxter and Wenzel approximations were used to predict wetting states of the insect wing cuticles. The more disordered and inhomogeneous surface of the species Leptopsalta bifuscata demonstrated a Wenzel type wetting state or an intermediate state of spreading and imbibition with a CA of 81.3° and high adhesive force of 149.5 µN. Three other species (Cryptotympana atrata, Meimuna opalifer and Aola bindusara) exhibited nanostructuring of the form of conically shaped protrusions, which were spherically capped. These surfaces presented a range of high adhesional values; however, the CAs were highly hydrophobic (C. atrata and A. bindusara) and in some cases close to superhydrophobic (M. opalifer). The wetting states of A. bindusara, C. atrata and M. opalifer (based on adhesion and CAs) are most likely represented by the transitional region between the Cassie-Baxter and Wenzel approximations to varying degrees.
Hierarchically structured flower leaves (petals) of many plants are superhydrophobic, but water droplets do not roll-off when the surfaces are tilted. On such surfaces water droplets are in the “Cassie impregnating wetting state”, which is also known as the “petal effect”. By analyzing the petal surfaces of different species, we discovered interesting new wetting characteristics of the surface of the flower of the wild pansy (Viola tricolor). This surface is superhydrophobic with a static contact angle of 169° and very low hysteresis, i.e., the petal effect does not exist and water droplets roll-off as from a lotus (Nelumbo nucifera) leaf. However, the surface of the wild pansy petal does not possess the wax crystals of the lotus leaf. Its petals exhibit high cone-shaped cells (average size 40 µm) with a high aspect ratio (2.1) and a very fine cuticular folding (width 260 nm) on top. The applied water droplets are in the Cassie–Baxter wetting state and roll-off at inclination angles below 5°. Fabricated hydrophobic polymer replicas of the wild pansy were prepared in an easy two-step moulding process and possess the same wetting characteristics as the original flowers. In this work we present a technical surface with a new superhydrophobic, low adhesive surface design, which combines the hierarchical structuring of petals with a wetting behavior similar to that of the lotus leaf.
anti-adhesive; petal effect; petal structures; polymer replication; superhydrophobic
Background: Controlled transport of microdroplets is a topic of interest for various applications. It is well known that liquid droplets move towards areas of minimum contact angle if placed on a flat solid surface exhibiting a gradient of contact angle. This effect can be utilised for droplet manipulation. In this contribution we describe how controlled droplet movement can be achieved by a surface pattern consisting of cones and funnels whose length scales are comparable to the droplet diameter.
Results: The surface energy of a droplet attached to a cone in a symmetry-preserving way can be smaller than the surface energy of a freely floating droplet. If the value of the contact angle is fixed and lies within a certain interval, then droplets sitting initially on a cone can gain energy by moving to adjacent cones.
Conclusion: Surfaces covered with cone-shaped protrusions or cavities may be devised for constructing “band-conveyors” for droplets. In our approach, it is essentially the surface structure which is varied, not the contact angle. It may be speculated that suitably patterned surfaces are also utilised in biological surfaces where a large variety of ornamentations and surface structuring are often observed.
microdroplets; microfluidics; surface; surface energy; surface structures
This paper describes the use of capillary pressure to initiate and control the rate of spontaneous liquid-liquid flow through microfluidic channels. In contrast to flow driven by external pressure, flow driven by capillary pressure is dominated by interfacial phenomena and is exquisitely sensitive to the chemical composition and geometry of the fluids and channels. A step-wise change in capillary force was initiated on a hydrophobic SlipChip by slipping a shallow channel containing an aqueous droplet into contact with a slightly deeper channel filled with immiscible oil. This action induced spontaneous flow of the droplet into the deeper channel. A model predicting the rate of spontaneous flow was developed based on the balance of net capillary force with viscous flow resistance, using as inputs the liquid-liquid surface tension, the advancing and receding contact angles at the three-phase aqueous-oil-surface contact line, and the geometry of the devices. The impact of contact angle hysteresis, the presence or absence of a lubricating oil layer, and adsorption of surface-active compounds at liquid-liquid or liquid-solid interfaces were quantified. Two regimes of flow spanning a 104-fold range of flow rates were obtained and modeled quantitatively, with faster (mm/s) flow obtained when oil could escape through connected channels as it was displaced by flowing aqueous solution, and slower (micrometer/s) flow obtained when oil escape was mostly restricted to a μm-scale gap between the plates of the SlipChip (“dead-end flow”). Rupture of the lubricating oil layer (reminiscent of a Cassie-Wenzel transition) was proposed as a cause of discrepancy between the model and the experiment. Both dilute salt solutions and complex biological solutions such as human blood plasma could be flowed using this approach. We anticipate that flow driven by capillary pressure will be useful for design and operation of flow in microfluidic applications that do not require external power, valves, or pumps, including on SlipChip and other droplet- or plug-based microfluidic devices. In addition, this approach may be used as a sensitive method of evaluating interfacial tension, contact angles and wetting phenomena on chip.
A macroscopic evaporating water droplet with suspended particles on a solid surface will form a ring-like structure at the pinned contact line due to induced capillary flow. As the droplet size shrinks, the competition between the time scales of the liquid evaporation and the particle movement may influence the resulting ring formation. When the liquid evaporates much faster than the particle movement, coffee ring formation may cease. Here, we experimentally show that there exists a lower limit of droplet size, Dc, for the successful formation of a coffee ring structure. When the particle concentration is above a threshold value, Dc can be estimated by considering the collective effects of the liquid evaporation and the particle diffusive motion within the droplet. For suspended particles of size ~100 nm, the minimum diameter of the coffee ring structure is found to be ~10 µm.
Structural DNA nanotechnology utilizes DNA molecules as programmable information-coding polymers to create higher order structures at the nanometer scale. An important milestone in structural DNA nanotechnology was the development of scaffolded DNA origami in which a long single-stranded viral genome (scaffold strand) is folded into arbitrary shapes by hundreds of short synthetic oligonucleotides (staple strands). The achievable dimensions of the DNA origami tiles units are currently limited by the length of the scaffold strand. Here we demonstrate a strategy referred to as ‘super-origami’ or ‘origami of origami’ to scale up DNA origami technology. First, this method uses a collection of bridge strands to pre-fold a single stranded DNA scaffold into a loose framework. Subsequently, pre-formed individual DNA origami tiles are directed onto the loose framework so that each origami tile serves as a large staple. Using this strategy, we demonstrate the ability to organize DNA origami nanostructures into larger spatially addressable architectures.
Structural DNA nanotechnology; self-assembly; DNA origami; scale up
Wetting properties of biomaterials, in particular nanomaterials, play an important role, as these influence interactions with biological elements, such as proteins, bacteria, and cells. In this study, the wetting phenomenon of titanium substrates coated with selenium nanoparticles was studied using experimental and mathematical modeling tools. Importantly, these selenium-coated titanium substrates were previously reported to increase select protein adsorption (such as vitronectin and fibronectin), to decrease bacteria growth, and increase bone cell growth. Increased selenium nanoparticle coating density resulted in higher contact angles but remained within the hydrophilic regime. This trend was found in disagreement with the Wenzel model, which is widely used to understand the wetting properties of rough surfaces. The trend also did not fit well with the Cassie–Baxter model, which was developed to understand the wetting properties of composite surfaces. A modified wetting model was thus proposed in this study, to understand the contributing factors of material properties to the hydrophilicity/hydrophobicity of these nanostructured selenium-coated surfaces. The analysis and model created in this study can be useful in designing and/or understanding the wetting behavior of numerous biomedical materials and in turn, biological events (such as protein adsorption as well as bacteria and mammalian cell functions).
hydrophilicity, hydrophobicity, Wenzel model, Cassie; Baxter model, free energy, implant material, proteins, cells, bacteria
A water drop on a superhydrophobic surface that is pinned by wire loops can be reproducibly cut without formation of satellite droplets. Drops placed on low-density polyethylene surfaces and Teflon-coated glass slides were cut with superhydrophobic knives of low-density polyethylene and treated copper or zinc sheets, respectively. Distortion of drop shape by the superhydrophobic knife enables a clean break. The driving force for droplet formation arises from the lower surface free energy for two separate drops, and it is modeled as a 2-D system. An estimate of the free energy change serves to guide when droplets will form based on the variation of drop volume, loop spacing and knife depth. Combining the cutting process with an electrofocusing driving force could enable a reproducible biomolecular separation without troubling satellite drop formation.
Various techniques exerting mechanical stress on cells have been developed to investigate cellular responses to externally controlled stimuli. Fundamental mechanotransduction processes, how applied physical forces are converted into biochemical signals, have often been examined by transmitting such forces through cells and probing its pathway at cellular levels. In fact, many cellular biomechanics studies have been performed by trapping (or immobilizing) individual cells, either attached to solid substrates or suspended in liquid media. In that context, we demonstrated two-dimensional acoustic trapping, where a lipid droplet of 125 μm in diameter was directed transversely towards the focus (or the trap center) similar to that of optical tweezers. Under the influence of restoring forces created by a 30 MHz focused ultrasound beam, the trapped droplet behaved as if tethered to the focus by a linear spring. In order to apply this method to cellular manipulation in the Mie regime (cell diameter > wavelength), the availability of sound beams with its beamwidth approaching cell size is crucial. This can only be achieved at a frequency higher than 100 MHz. We define ultrasound beams in the frequency range from 100 MHz to a few GHz as ultrasound microbeams because the lateral beamwidth at the focus would be in the micron range (reviewer #1). Hence a zinc oxide (ZnO) transducer that was designed and fabricated to transmit a 200 MHz focused sound beam was employed to immobilize a 10 μm human leukemia cell (K-562) within the trap. The cell was laterally displaced with respect to the trap center by mechanically translating the transducer over the focal plane. Both lateral displacement and position trajectory of the trapped cell were probed in a two-dimensional space, indicating that the retracting motion of these cells was similar to that of the lipid droplets at 30 MHz. The potential of this tool for studying cellular adhesion between white blood cells and endothelial cells was discussed, suggesting its capability as a single cell manipulator.
Avian pulmonary capillaries differ from those of mammals in three important ways. The blood-gas barrier is much thinner, it is more uniform in thickness, and the capillaries are far more rigid when their transmural pressure is altered. The thinness of the barrier is surprising because it predisposes the capillaries to stress failure. A possible mechanism for these differences is that avian pulmonary capillaries, unlike mammalian, are supported from the outside by air capillaries, but the details of the support are poorly understood. To clarify this we studied the blood and air capillaries in chicken lung using transmission electron microscopy (EM) and two relatively new techniques that allow 3D visualization: electron tomography and serial block-face scanning EM. These studies show that the pulmonary capillaries are flanked by epithelial bridges composed of two extremely thin epithelial cells with large surface areas. The junctions of the bridges with the capillary walls show thickening of the epithelial cells and an accumulation of extracellular matrix. Collapse of the pulmonary capillaries when the pressure outside them is increased is apparently prevented by the guy wire-like action of the epithelial bridges. The enlarged junctions between the bridges and the walls could provide a mechanism that limits the hoop stress in the capillary walls when the pressure inside them is increased. The support of the pulmonary capillaries may also be explained by an interdependence mechanism whereby the capillaries are linked to a rigid assemblage of air capillaries. These EM studies show the supporting structures in greater detail than has previously been possible, particularly in 3D, and they allow a more complete analysis of the mechanical forces affecting avian pulmonary capillaries.
epithelial bridges; epithelial plates; stress failure; electron tomography; serial block-face scanning electron microscopy
We present a simple bench-top technique to produce centimeter long concentration gradients in biomaterials incorporating soluble, material, and particle gradients. By patterning hydrophilic regions on a substrate, a stripe of prepolymer solution is held in place on a glass slide by a hydrophobic boundary. Adding a droplet to one end of this “pre-wet” stripe causes a rapid capillary flow that spreads the droplet along the stripe to generate a gradient in the relative concentrations of the droplet and pre-wet solutions. The gradient length and shape are controlled by the pre-wet and droplet volumes, stripe thickness, fluid viscosity and surface tension. Gradient biomaterials are produced by crosslinking gradients of prepolymer solutions. Demonstrated examples include a concentration gradient of cells encapsulated in three dimensions (3D) within a homogeneous biopolymer and a constant concentration of cells encapsulated in 3D within a biomaterial gradient exhibiting a gradient in cell spreading. The technique employs coated glass slides that may be purchased or custom made from tape and hydrophobic spray. The approach is accessible to virtually any researcher or student and should dramatically reduce the time required to synthesize a wide range of gradient biomaterials. Moreover, since the technique employs passive mechanisms it is ideal for remote or resource poor settings.
This account reviews the use of templates, fabricated by patterning paper, for the delivery of aqueous solutions of reactants (predominantly, ions) in the preparation of structured, thin materials (e.g., films of ionotropic hydrogels). In these methods, a patterned sheet of paper transfers an aqueous solution of reagent to a second phase—either solid or liquid—brought into contact with the template; this process can form solid structures with thicknesses that are typically ≤1.5 mm. The shape of the template and the pattern of a hydrophobic barrier on the paper control the shape of the product, in its plane, by restricting the delivery of the reagent in two dimensions. The concentration of the reagents, and the duration that the template remains in contact with the second phase, control growth in the third dimension (i.e., thickness). The method is especially useful in fabricating shaped films of ionotropic hydrogels (e.g., calcium alginate) by controlling the delivery of solutions of multivalent cations to solutions of anionic polymers. The templates can also be used to direct reactions that generate patterns of solid precipitates within sheets of paper. This review examines applications of the method for: (i) patterning bacteria in two dimensions within a hydrogel film, (ii) manipulating hydrogel films and sheets of paper magnetically, and (iii) generating dynamic 3-D structures (e.g., a cylinder of rising bubbles of O2) from sheets of paper with 2-D patterns of a catalyst (e.g., Pd0) immersed in appropriate reagents (e.g., 1% H2O2 in water).
Vapour–liquid–solid route and its variants are routinely used for scalable synthesis of semiconducting nanowires, yet the fundamental growth processes remain unknown. Here we employ atomic-scale computations based on model potentials to study the stability and growth of gold-catalysed silicon nanowires. Equilibrium studies uncover segregation at the solid-like surface of the catalyst particle, a liquid AuSi droplet, and a silicon-rich droplet–nanowire interface enveloped by heterogeneous truncating facets. Supersaturation of the droplets leads to rapid one-dimensional growth on the truncating facets and much slower nucleation-controlled two-dimensional growth on the main facet. Surface diffusion is suppressed and the excess Si flux occurs through the droplet bulk which, together with the Si-rich interface and contact line, lowers the nucleation barrier on the main facet. The ensuing step flow is modified by Au diffusion away from the step edges. Our study highlights key interfacial characteristics for morphological and compositional control of semiconducting nanowire arrays.
The vapour–liquid–solid method is used to produce semiconducting nanowires but the fundamental processes involved are poorly understood. Wang et al. use atomic-scale simulations to elucidate the mechanisms involved in the growth and stability of gold-catalysed silicon nanowires.
We demonstrate that the assembly of an amphiphilic polyamine on the interfaces of micrometer-sized droplets of a thermotropic liquid crystal (LC) dispersed in aqueous solutions can be used to facilitate the immobilization of LC droplets on chemically functionalized surfaces. Polymer 1 was designed to contain both hydrophobic (alkyl-functionalized) and hydrophilic (primary and tertiary amine-functionalized) side chain functionality. The assembly of this polymer at the interfaces of aqueous dispersions of LC droplets was achieved by spontaneous adsorption of polymer from aqueous solution. Polymer adsorption triggered transitions in the orientational ordering of the LCs, as observed by polarized light and bright-field microscopy. We demonstrate that the presence of polymer 1 on the interfaces of these droplets can be exploited to immobilize LC droplets on planar solid surfaces through covalent bond formation (e.g., for surfaces coated with polymer multilayers containing reactive azlactone functionality) or through electrostatic interactions (e.g., for surfaces coated with multilayers containing hydrolyzed azlactone functionality). Characterization of immobilized LC droplets by polarized, fluorescence, and laser scanning confocal microscopy revealed the general spherical shape of the polymer-coated LC droplets to be maintained after immobilization, and that immobilization led to additional ordering transitions within the droplets that was dependent on the nature of the surfaces with which they were in contact. Polymer 1-functionalized LC droplets were not immobilized on polymer multilayers treated with poly(ethylene imine) (PEI). We demonstrate that the ability to design surfaces that promote or prevent the immobilization of polymer-functionalized LC droplets can exploited to pattern the immobilization of LC droplets on surfaces. The results of this investigation provide the basis of an approach that could be used to tailor the properties of dispersed LC emulsions and to immobilize these droplets on functional surfaces of interest in a broad range of fundamental and applied contexts.
In specialized capillary beds such as the kidney glomerulus, the sheet-like structure of the basement membrane in conjunction with opposing monolayers of endothelium and epithelium form the functioning filtration unit of the kidney. Using a novel cross-linking method on a collagen substrate, we have created a novel hydrogel scaffold to substitute for the basement membrane. Using a simple casting method to create thin films of the hydrogel scaffold (1–5μm), the scaffolds were suitable for long-term static culture, and supported cell attachment and long term cell viability similar to a standard type I collagen substrate. Bulk diffusion and protein permeability of the hydrogel scaffold were evaluated, in addition to its use in a perfusion chamber where it withstood hydraulic pressures typical for glomerular capillaries. This system thus provided a suitable cell substrate for the co-culture of renal epithelial podocytes and endothelial cells in a device that replicates the geometry of the in vivo juxtaposition of the two cell types in relation to their basement membrane.
kidney; podocyte; endothelium; extracellular matrix; collagen; glomerular filtration barrier
For screening the conditions for a reaction by using droplets (or plugs) as microreactors, the composition of the droplets must be indexed. Indexing here refers to measuring the concentration of a solute by addition of a marker, either internal or external. Indexing may be performed by forming droplet pairs, where in each pair the first droplet is used to conduct the reaction, and the second droplet is used to index the composition of the first droplet. This paper characterizes a method for creating droplet pairs by generating alternating droplets, of two sets of aqueous solutions in a flow of immiscible carrier fluid within PDMS and glass microfluidic channels. The paper also demonstrates that the technique can be used to index the composition of the droplets, and this application is illustrated by screening conditions of protein crystallization. The fluid properties required to form the steady flow of the alternating droplets in a microchannel were characterized as a function of the capillary number Ca and water fraction. Four regimes were observed. At the lowest values of Ca, the droplets of the two streams coalesced; at intermediate values of Ca the alternating droplets formed reliably. At even higher values of Ca, shear forces dominated and caused formation of droplets that were smaller than the cross-sectional dimension of the channel; at the highest values of Ca, coflowing laminar streams of the two immiscible fluids formed. In addition to screening of protein crystallization conditions, understanding of the fluid flow in this system may extend this indexing approach to other chemical and biological assays performed on a microfluidic chip.
Many insects cling to vertical and inverted surfaces with pads that adhere by nanometre-thin films of liquid secretion. This fluid is an emulsion, consisting of watery droplets in an oily continuous phase. The detailed function of its two-phasic nature has remained unclear. Here we show that the pad emulsion provides a mechanism that prevents insects from slipping on smooth substrates. We discovered that it is possible to manipulate the adhesive secretion in vivo using smooth polyimide substrates that selectively absorb its watery component. While thick layers of polyimide spin-coated onto glass removed all visible hydrophilic droplets, thin coatings left the emulsion in its typical form. Force measurements of stick insect pads sliding on these substrates demonstrated that the reduction of the watery phase resulted in a significant decrease in friction forces. Artificial control pads made of polydimethylsiloxane showed no difference when tested on the same substrates, confirming that the effect is caused by the insects’ fluid-based adhesive system. Our findings suggest that insect adhesive pads use emulsions with non-Newtonian properties, which may have been optimized by natural selection. Emulsions as adhesive secretions combine the benefits of ‘wet’ adhesion and resistance against shear forces.
insect adhesion; friction; tribology; biomechanics; emulsion
Creating surfaces capable of resisting liquid-mediated adhesion is extremely difficult due to the strong capillary forces that exist between surfaces. Land snails use this to adhere to and traverse across almost any type of solid surface of any orientation (horizontal, vertical or inverted), texture (smooth, rough or granular) or wetting property (hydrophilic or hydrophobic) via a layer of mucus. However, the wetting properties that enable snails to generate strong temporary attachment and the effectiveness of this adhesive locomotion on modern super-slippy superhydrophobic surfaces are unclear. Here we report that snail adhesion overcomes a wide range of these microscale and nanoscale topographically structured non-stick surfaces. For the one surface which we found to be snail resistant, we show that the effect is correlated with the wetting response of the surface to a weak surfactant. Our results elucidate some critical wetting factors for the design of anti-adhesive and bio-adhesion resistant surfaces.
The ability of Lepidoptera, or butterflies and moths, to drink liquids from rotting fruit and wet soil, as well as nectar from floral tubes, raises the question of whether the conventional view of the proboscis as a drinking straw can account for the withdrawal of fluids from porous substrates or of films and droplets from floral tubes. We discovered that the proboscis promotes capillary pull of liquids from diverse sources owing to a hierarchical pore structure spanning nano- and microscales. X-ray phase-contrast imaging reveals that Plateau instability causes liquid bridges to form in the food canal, which are transported to the gut by the muscular sucking pump in the head. The dual functionality of the proboscis represents a key innovation for exploiting a vast range of nutritional sources. We suggest that future studies of the adaptive radiation of the Lepidoptera take into account the role played by the structural organization of the proboscis. A transformative two-step model of capillary intake and suctioning can be applied not only to butterflies and moths but also potentially to vast numbers of other insects such as bees and flies.
Lepidoptera; capillarity; flexible microfluidics; permeability; plateau instability; porous materials
DNA origami involves the folding of long single-stranded DNA into designed structures with the aid of short staple strands; such structures may enable the development of useful nanomechanical DNA devices. Here we develop versatile sensing systems for a variety of chemical and biological targets at molecular resolution. We have designed functional nanomechanical DNA origami devices that can be used as 'single-molecule beacons', and function as pinching devices. Using 'DNA origami pliers' and 'DNA origami forceps', which consist of two levers ~170 nm long connected at a fulcrum, various single-molecule inorganic and organic targets ranging from metal ions to proteins can be visually detected using atomic force microscopy by a shape transition of the origami devices. Any detection mechanism suitable for the target of interest, pinching, zipping or unzipping, can be chosen and used orthogonally with differently shaped origami devices in the same mixture using a single platform.
DNA origami involves the folding of long single-stranded DNA into designed structures that may aid the development of useful nanomechanical DNA devices. In this study, DNA origami pliers and forceps are shown to undergo conformational changes on single-molecule binding.
Living cells communicate and cooperate to produce the emergent properties of tissues. Synthetic mimics of cells, such as liposomes, are typically incapable of cooperation and therefore cannot readily display sophisticated collective behavior. Here, we print tens of thousands of picoliter aqueous droplets that become joined by single lipid bilayers to form a cohesive material with cooperating microcompartments. 3D structures can be built with heterologous droplets in software-defined arrangements. The droplet networks can be functionalized with membrane proteins to allow rapid electrical communication along a specific path. The networks can also be programmed by osmolarity gradients to fold into otherwise unattainable designed structures. Printed droplet networks might be used as tissue engineering substrates, interfaced with tissues, or developed as mimics of living tissue.
ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.
ZnO; Nanowires; Microscopy
We report observations of spontaneous formation of micrometer-sized water droplets within micrometer-thick films of a range of different oils (isotropic and nematic 4-cyano-4’-pentylbiphenyl (5CB), and silicone, olive and corn oil) that are supported on glass substrates treated with octadecyltrichlorosilane (OTS) and immersed under water. Confocal imaging was used to determine that the water droplets nucleate and grow at the interface between the oils and OTS-treated glass with a contact angle of ~130°. A simple thermodynamic model based on macroscopic interfacial energetic arguments consistent with the contact angle of 130°, however, fails to account for the spontaneous formation of the water droplets. ζ-potential measurements performed with OTS-treated glass (− 59.0 ± 16.4 mV) and hydrophobic monolayers formed on gold films (2.0 ± 0.7 mV), when combined with the observed absence of droplet formation under films of oil supported on the latter surfaces, suggest that the charge of the oil-solid interface promotes partitioning of water to the interfacial region. The hydrophobic nature of the OTS-treated glass promotes dewetting of water accumulated in the interfacial region into droplets (a thin film of water is seen to form on bare glass). The inhibitory effect on droplet formation of both salt (NaCl) and sucrose (0.1mM to 500mM) added to the aqueous phase was similar, indicating that both solutes lower the chemical potential of the bulk water (osmotic effect) sufficiently to prevent partitioning of the water to the interface between the oil and supporting substrates. These results suggest that charged, hydrophobic surfaces can provide routes to spontaneous formation of surface-supported, water-in-oil emulsions.
emulsions; interfaces; spontaneous emulsification; water droplets; interfacial droplets; octadecyltrichlorosilane (OTS); monolayers; liquid crystal; isotropic oil
Some fluoro-substituted liquid crystals mixed with arachidic acid in monolayers formed at air-liquid (Langmuir films) and air-solid substrate (Langmuir-Blodgett films) interfaces were investigated. Molecular organization in Langmuir films was determined on the basis of the analysis of the shape of the surface pressure-mean molecular area isotherm and observations made by means of a Brewster angle microscope. It was found that in the compression process the liquid crystal molecules are pushed out towards the top of the first monolayer being in direct contact with the subphase. Langmuir films were transferred onto the quartz substrates at various surface pressures and mono- and multilayered Langmuir-Blodgett films were obtained. The films were characterized using electronic absorption measurements. The conditions for obtaining the homeotropic orientation of the liquid crystal molecules were determined.
fluoro-substituted liquid crystal; Langmuir film; Langmuir-Blodgett film; Brewster angle microscopy; electronic absorption
Lipoprotein lipase (LPL)-deficient mice have been created by gene targeting in embryonic stem cells. At birth, homozygous knockout pups have threefold higher triglycerides and sevenfold higher VLDL cholesterol levels than controls. When permitted to suckle, LPL-deficient mice become pale, then cyanotic, and finally die at approximately 18 h of age. Before death, triglyceride levels are severely elevated (15,087 +/- 3,805 vs 188 +/- 71 mg/dl in controls). Capillaries in tissues of homozygous knockout mice are engorged with chylomicrons. This is especially significant in the lung where marginated chylomicrons prevent red cell contact with the endothelium, a phenomenon which is presumably the cause of cyanosis and death in these mice. Homozygous knockout mice also have diminished adipose tissue stores as well as decreased intracellular fat droplets. By crossbreeding with transgenic mice expressing human LPL driven by a muscle-specific promoter, mouse lines were generated that express LPL exclusively in muscle but not in any other tissue. This tissue-specific LPL expression rescued the LPL knockout mice and normalized their lipoprotein pattern. This supports the contention that hypertriglyceridemia caused the death of these mice and that LPL expression in a single tissue was sufficient for rescue. Heterozygous LPL knockout mice survive to adulthood and have mild hypertriglyceridemia, with 1.5-2-fold elevated triglyceride levels compared with controls in both the fed and fasted states on chow, Western-type, or 10% sucrose diets. In vivo turnover studies revealed that heterozygous knockout mice had impaired VLDL clearance (fractional catabolic rate) but no increase in transport rate. In summary, total LPL deficiency in the mouse prevents triglyceride removal from plasma, causing death in the neonatal period, and expression of LPL in a single tissue alleviates this problem. Furthermore, half-normal levels of LPL cause a decrease in VLDL fractional catabolic rate and mild hypertriglyceridemia, implying that partial LPL deficiency has physiological consequences.