Confining analyte droplets on visible Si pillars for improving reproducibility and sensitivity of SALDI-TOF MS

Analytical and Bioanalytical Chemistry - We present a universal method to efficiently improve reproducibility and sensitivity of surface-assisted laser desorption/ionization time of flight mass...     

Evaporation of water droplets on polymer surfaces

The evaporation of water droplets on polymer surfaces was investigated by using a digital image analysis technique. There were three distinct stages in the water evaporation process: a constant contact area mode, a constant contact angle mode, and a mixed mode that is independent of both the initial quantity of water droplets and the hydrophobic properties of the polymer surfaces. The physical factors influencing the first and second transitions in the evaporation process were found to be the attainment of the receding angle on the polymer surfaces and the Marangoni instability in the evaporating water droplets, which result from the concentration gradient of contaminants. This study also provides qualitative information about the microfluid flows inside the evaporating water droplets and the morphology of drying stains on polymer surfaces. The contaminants were found to be concentrated at the perimeter of the stains, in agreement with the observed outward microfluid flow in the mixed mode of the evaporation process.     

Wicking and spreading of water droplets on nanotubes

Recently, there has been intensive research on the use of nanotechnology to improve the wettability of solid surfaces. It is well-known that nanostructures can improve the wettability of a surface, and this is a very important safety consideration in regard to the occurrence of boiling crises during two-phase heat transfer, especially in the operation of nuclear power plant systems. Accordingly, there is considerable interest in wetting phenomena on nanostructures in the field of nuclear heat transfer. Much of the latest research on liquid absorption on a surface with nanostructures indicates that liquid spreading is generated by capillary wicking. However, there has been comparatively little research on how capillary forces affect liquid spreading on a surface with nanotubes. In this paper, we present a visualization of liquid spreading on a zircaloy surface with nanotubes, and establish a simple quantitative method for measuring the amount of water absorbed by the nanotubes. We successfully describe liquid spreading on a two-dimensional surface via one-dimensional analysis. As a result, we are able to postulate a relationship between liquid spreading and capillary wicking in the nanotubes.     

Suspension polymerization of thermotropic monomer: Formation of droplets or beads

Ordered beads were obtained by the suspension copolymerization of amesogenic methacrylate-based monomer and methacrylic acid. The stability of these particles and the liquid crystal organization inside the particles depend mainly on the time of polymerization and on the crosslinking. Suspended droplets of monomers or polymers can be obtained as well as solid spheres. The solid particles exhibit ordered microstructures with a liquid crystal configuration that has a variable form depending on polymerization time and observation temperature. Bead characteristics such as morphology, size and polydispersity, and porosity are discussed and compared with those of non-mesomorphic beads. The mesomorphic beads are smaller and their porosity lower. The presence of a crosslinker during the polymerization process leads to more mechanically stable particles with retention of polymorphism.     

Suspension polymerization of thermotropic monomer: Formation of droplets or beads

Ordered beads were obtained by the suspension copolymerization of amesogenic methacrylate-based monomer and methacrylic acid. The stability of these particles and the liquid crystal organization inside the particles depend mainly on the time of polymerization and on the crosslinking. Suspended droplets of monomers or polymers can be obtained as well as solid spheres. The solid particles exhibit ordered microstructures with a liquid crystal configuration that has a variable form depending on polymerization time and observation temperature. Bead characteristics such as morphology, size and polydispersity, and porosity are discussed and compared with those of non-mesomorphic beads. The mesomorphic beads are smaller and their porosity lower. The presence of a crosslinker during the polymerization process leads to more mechanically stable particles with retention of polymorphism.     

Sliding behavior of water droplets on flat polymer surface

Flat films of methyl methacrylate-fluoroalkyl methacrylate copolymers were prepared, and their hydrophobicity was investigated. It was revealed that the F concentration directly affects the static hydrophobicity on the flat polymer surface in a systematic manner. Furthermore, the sliding behavior of a water droplet on these surfaces depends on the static hydrophobicity; the sliding motion changes from constant velocity to constant acceleration with an increase in the water contact angle.     

Molecular dynamics simulations of water droplets on polymer surfaces

Molecular dynamics simulations were used to study the wetting of polymer surfaces with water. Contact angles of water droplets on crystalline and two amorphous polyethylene (PE) and poly(vinyl chloride) (PVC) surfaces were extracted from atomistic simulations. Crystalline surfaces were produced by duplicating the unit cell of an experimental crystal structure, and amorphous surfaces by pressing the bulk polymer step by step at elevated temperature between two repulsive grid surfaces to a target density. Different-sized water droplets on the crystalline PE surface revealed a slightly positive line tension on the order of 10(-12)-10(-11) N, whereas droplets on crystalline PVC did not yield a definite line tension. Microscopic contact angles produced by the simple point charge (SPC) water model were mostly a few degrees smaller than those produced by the extended SPC model, which, as the model with lowest bulk energy, presents an upper boundary for contact angles. The macroscopic contact angle for the SPC model was 94 degrees on crystalline PVC and 113 degrees on crystalline PE. Amorphicity of the surface increased the water contact angle on PE but decreased it on PVC, for both water models. If the simulated contact angles on crystalline and amorphous surfaces are combined in proportion to the crystallinity of the polymer in question, simulated values in relatively good agreement with measured values are obtained.     

Dynamic effects of bouncing water droplets on superhydrophobic surfaces

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water repellent properties. Superhydrophobic surfaces may be generated by the use of hydrophobic coating, roughness, and air pockets between solid and liquid. Dynamic effects, such as the bouncing of a droplet, can destroy the composite solid-air-liquid interface. The relationship between the impact velocity of a droplet and the geometric parameters affects the transition from the solid-air-liquid interface to the solid-liquid interface. Therefore, it is necessary to study the dynamic effect of droplets under various impact velocities. We studied the dynamic impact behavior of water droplets on micropatterned silicon surfaces with pillars of two different diameters and heights and with varying pitch values. A criterion for the transition from the Cassie and Baxter regime to the Wenzel regime based on the relationship between the impact velocity and the parameter of patterned surfaces is proposed. The trends are explained based on the experimental data and the proposed transition criterion. For comparison, the dynamic impact behavior of water droplets on nanopatterned surfaces was investigated. The wetting behavior under various impact velocities on multiwalled nanotube arrays also was investigated. The physics of wetting phenomena for bouncing water droplet studies here is of fundamental importance in the geometrical design of superhydrophobic surfaces.     

Functional bionetworks from nanoliter water droplets

We form networks from aqueous droplets by submerging them in an oil/lipid mixture. When the droplets are joined together, the lipid monolayers surrounding them combine at the interface to form a robust lipid bilayer. Various protein channels and pores can incorporate into the droplet-interface bilayer (DIB), and the application of a potential with electrodes embedded within the droplets allows ionic currents to be driven across the interface and measured. By joining droplets in linear or branched geometries, functional bionetworks can be created. Although the interfaces between neighboring droplets comprise only single lipid bilayers, the structures of the networks are long-lived and robust. Indeed, a single droplet can be "surgically" excised from a network and replaced with a new droplet without rupturing adjacent DIBs. Networks of droplets can be powered with internal "biobatteries" that use ion gradients or the light-driven proton pump bacteriorhodopsin. Besides their interest as coupled protocells, the droplets can be used as devices for ultrastable bilayer recording with greatly reduced electrolyte volume, which will permit their use in rapid screening applications.     

Self-organization of bidisperse colloids in water droplets

Most of the colloidal clusters have been produced from oil-in-water emulsions with identical microspheres dispersed in oil droplets. Here, we present new types of binary colloidal clusters from phase-inverted water-in-oil emulsions using various combinations of two different colloids with several size ratios: monodisperse silica or polystyrene microspheres for larger particles and silica or titania nanoparticles for smaller particles. Obviously, a better understanding of how finite groups of different colloids self-organize in a confined geometry may help us control the structure of matter at multiple length scales. In addition, since aqueous dispersions have much better phase stability, we could produce much more diverse colloidal materials from water-in-oil emulsions rather than from oil-in-water emulsions. Interestingly, the configurations of the large microspheres were not changed by the presence of the small particles. However, the arrangement of the smaller particles was strongly dependent on the nature of the interparticle interactions. The experimentally observed structural evolutions were consistent with the numerical simulations calculated using Surface Evolver. These clusters with nonisotropic structures can be used as building blocks for novel colloidal structures with unusual properties or by themselves as light scatterers, diffusers, and complex adaptive matter exhibiting emergent behavior.     

Deformation of water droplets on solid surface in electric field

The purpose of this paper is to analyze the deformation of water droplets on a solid surface under electric stress. A mathematical model making it possible to simulate the axisymmetric as well as non-axisymmetric deformations of droplets is developed. According to this model, the droplet deformation depends on several parameters such as the volume and the number of droplets, the conductivity and the permittivity of droplets, their proximity to one another, the surface of the solid material, and the location of each droplet on the dielectric surface. The results of the simulation show the disturbance of the background field through the presence of a single or multiple droplets. An experimental study is also achieved by considering one to three droplets aligned simultaneously on a dielectric smooth surface between two electrodes subjected to AC voltages. The influence of the background field and the droplet location regarding the electrodes on the deformation of water droplets are evidenced.     

Wetting behavior of water droplets on hydrophobic microtextures of comparable size

The wetting behavior of water droplets on periodically structured hydrophobic surfaces was investigated. The effect of structure geometry, roughness, and relative pore fraction on the contact angles was investigated experimentally for droplets of size comparable to the size of the structures. It was found that surface geometry may induce a transition from groove-filling and Wenzel-like behavior to nonfilling of surface grooves and consequential Cassie-Baxter behavior. Numerical calculations of the free energy of these systems suggest that the equilibrium behavior is in line with the experimental observations. The observations may serve as guidelines for the design of surfaces with the desired wetting behavior.     

Wetting of structured hydrophobic surfaces by water droplets

Super-hydrophobic surfaces may arise due to an interplay between the intrinsic, relatively high, contact angle of the more or less hydrophobic solid surface employed and the geometric features of the solid surface. In the present work, this relationship was investigated for a range of different surface geometries, making use of surface free energy minimization. As a rule, the free energy minima (and maxima) occur when the Laplace and Young conditions are simultaneously fulfilled. Special effort has been devoted to investigating the free energy barriers present between the Cassie-Baxter (heterogeneous wetting) and Wenzel (homogeneous wetting) modes. The predictions made on the basis of the model calculations compare favorably with experimental results presented in the literature.     

Dynamic behavior of water droplets on solid surfaces with pillar-type nanostructures

In the present study, we investigated the static and dynamic behavior of water droplets on solid surfaces featuring pillar-type nanostructures by using molecular dynamics simulations. We carried out the computation in two stages. As a result of the first computational stage, an initial water cube reached an equilibrium state at which the water droplet showed different shapes depending on the height and the lateral and gap dimensions of the pillars. In the second computational stage, we applied a constant body force to the static water droplet obtained from the first computational stage and evaluated the dynamic behavior of the water droplet as it slid along the pillar-type surface. The dynamic behavior of the water droplet, which could be classified into three different groups, depended on the static state of the water droplet, the pillar characteristics (e.g., height and the lateral and gap dimensions of the pillars), and the magnitude of the applied body force. We obtained the advancing and receding contact angles and the corresponding contact angle hysteresis of the water droplets, which helped classify the water droplets into the three different groups.     

Surface tension of water droplets upon homogeneous droplet nucleation in water vapor

A method has been proposed for determining interfacial free energy from the data of molecular dynamics simulation. The method is based on the thermodynamic integration procedure and is distinguished by applicability to both planar interfaces and those characterized by a high curvature. The workability of the method has been demonstrated by the example of determining the surface tension for critical nuclei of water droplets upon condensation of water vapor. The calculation has been performed at temperatures of 273–373 K and a pressure of 1 atm, thus making it possible to determine the temperature dependence of the surface tension for water droplets and compare the results obtained with experimental data and the simulation results for a “planar” vapor–liquid interface.     

Influence of substrate heating on the evaporation dynamics of pinned water droplets

The dynamics of evaporating water droplets deposited on a heated substrate is investigated numerically. Droplets are pinned with a contact line radius of R = 1 mm. Evaporative mass flow and convection occurring inside the droplets are studied for different heating substrate sizes L S and heating temperatures T S. A simplified model neglecting hydrodynamics in air and evaporative cooling and assuming droplets to be spherical caps is simulated with a finite element method. A toruslike convective cell appears inside the droplets as evaporation takes place. For L S/ R > 1, the contact line is warmer than the apex of the droplets, and convection generates a downstream flow in the vicinity of the symmetry axis of the droplets. For L S/ R < 1, it is the apex that is warmer. Convection then generates an upstream flow. The overall evaporation time is described. It slows when L S/ R > 1.     

Interfacial tension controlled fusion of individual femtolitre droplets and triggering of confined chemical reactions on demand

This paper describes stepwise on-demand generation and fusion of femtolitre aqueous droplets based on interfacial tension. Sub-millisecond reaction times from droplet fusion were demonstrated, as well as a reversible chemical toggle switch based on alternating fusion of droplets containing acidic or basic solution, monitored with the pH-dependent emission of fluorescein.     

Shape-gradient composite surfaces: water droplets move uphill

The approach of water droplets self-running horizontally and uphill without any other forces was proposed by patterning the shape-gradient hydrophilic material (i.e., mica) to the hydrophobic matrix (i.e., wax or low-density polyethylene (LDPE)). The shape-gradient composite surface is the best one to drive water droplet self-running both at the high velocity and the maximal distance among four different geometrical mica/wax composite surfaces. The driving force for the water droplets self-running includes: (1) the great difference in wettability of surface materials, (2) the low contact angle hysteresis of surface materials, and (3) the space limitation of the shape-gradient transportation area. Furthermore, the average velocity and the maximal distance of the self-running were mainly determined by the gradient angle (alpha), the droplet volume, and the difference of the contact angle hysteresis. Theoretical analysis is in agreement with the experimental results.     

Hydrophobic and ionic interactions in nanosized water droplets

A number of situations such as protein folding in confined spaces, lubrication in tight spaces, and chemical reactions in confined spaces require an understanding of water-mediated interactions. As an illustration of the profound effects of confinement on hydrophobic and ionic interactions, we investigate the solvation of methane and methane decorated with charges in spherically confined water droplets. Free energy profiles for a single methane molecule in droplets, ranging in diameter (D) from 1 to 4 nm, show that the droplet surfaces are strongly favorable as compared to the interior. From the temperature dependence of the free energy in D = 3 nm, we show that this effect is entropically driven. The potentials of mean force (PMFs) between two methane molecules show that the solvent separated minimum in the bulk is completely absent in confined water, independent of the droplet size since the solute particles are primarily associated with the droplet surface. The tendency of methanes with charges (M(q+) and M(q-) with q(+) = |q(-)| = 0.4e, where e is the electronic charge) to be pinned at the surface depends dramatically on the size of the water droplet. When D = 4 nm, the ions prefer the interior whereas for D < 4 nm the ions are localized at the surface, but with much less tendency than for methanes. Increasing the ion charge to e makes the surface strongly unfavorable. Reflecting the charge asymmetry of the water molecule, negative ions have a stronger preference for the surface compared to positive ions of the same charge magnitude. With increasing droplet size, the PMFs between M(q+) and M(q-) show decreasing influence of the boundary owing to the reduced tendency for surface solvation. We also show that as the solute charge density decreases the surface becomes less unfavorable. The implications of our results for the folding of proteins in confined spaces are outlined.     

On the solvation of ions in small water droplets

The solvations of positively and negatively charged model ions in water droplets have been studied using Monte Carlo simulations performed with a polarizable intermolecular potential function model. Special focus has been placed on the position of the ion in the water droplet. It was found that the sign of the ionic charge is of minor importance but an increased ionic charge localizes the ion to the central regions of the droplet, whereas a large polarizability and a large ionic radius favor locations close to the surface of the water droplet.