Wetting of porous thin films exhibiting large contact angles.

Porous solid films that promote large apparent contact angles are interesting systems since their wetting properties are dependent on both the surface structure and water penetration into the film. In this study, a parahydrophobic coating is made by sequential dip coating of titanium dioxide nanoparticles and stearic acid on polished copper substrates. The apparent contact angles are determined using the tilted plate method, and it is found that the liquid-vapor interaction decreases and water droplets are more likely to move off the film when the number of coated layers increases. Interestingly, it is found that under some conditions, the front contact angle can be smaller than the back contact angle. Scanning electron microscopy observations demonstrate that the coating process led to the formation of hydrophilic TiO2 nanoparticle domains and hydrophobic stearic acid flakes that allows heterogeneous wetting. By monitoring the electrical current through the water droplet to the copper substrate, it is found that the water drops penetrate the coating layer to make direct contact with the copper surface with a time delay and magnitude that depends on the coating thickness. This additional penetration of water into the porous film enhances the adhesion of the droplet to the film and provides a clue to understand the contact angle hysteresis.

Ion Concentration Influences the Charge Transfer Due to a Water-Air Contact Line Moving over a Hydrophobic Surface: Charge Measurements and Theoretical Models.

A metal electrode covered by an inert, hydrophobic polymer surface is dipped into water, and the charge transfer was measured as a function of ion concentration for different chlorides, sulfates, and nitrates. A generic behavior is observed wherein the charge transfer first increases and then decreases as the ion concentration increases. However, for acids, the charge transfer decreases monotonously with concentration and even reverses polarity. Two different models, both in which the charge transfer is attributed to removal of ions from the electrical double layer as the contact line passes by, are discussed and shown to provide possible explanations of the experimental data.

Influence of Salt Concentration on Charge Transfer When a Water Front Moves across a Junction between a Hydrophobic Dielectric and a Metal Electrode.

An energy-harvesting device based on water moving across the junction between a hydrophobic dielectric and a metal electrode is demonstrated. The charge transfer due to contact electrification as the junction is dipped vertically into water is investigated. Experiments combined with finite element simulations reveal how the electrode voltage changes during the dipping process. Moreover, the charge transfer observed for a range of salt concentrations is studied, and it is found that there exists an optimal salt concentration which allows maximum charge transfer. It is suggested that these results can be understood because of the additional charge removal from the diffuse electrical double layer at the hydrophobic surface. It is demonstrated that by tuning the salt concentration, one can harvest more than 3 times the electrical power as compared with pure water.

The Influence of Microscale Surface Roughness on Water-Droplet Contact Electrification.

When water comes in contact with a hydrophobic fluoropolymer, a triboelectric charge tends to form on the surface. Here, it is investigated how the triboelectric charge formed upon contact with water drops depends on the microscale surface statistics of the polymer. In particular, it is found that the transition to a superhydrophobic fakir state results in a considerable reduction in triboelectric contact charge, due to a reduced liquid?solid contact area. Thus, when processing charge-sensitive electronic systems one may want to utilize such surfaces promoting reduced tribocharging. This also has implications for energy harvesting purposes, where one may collect electrical energy by letting water droplets move on the polymer with an interdigitated current-collecting electrode on its back side. In such a situation, it is observed that the surfaces promoting the superhydrophobic fakir state give rise to larger water droplet velocities and smaller collected charge, which explains the need for careful assessment of surface treatment before applying microstructured polymers for water droplet energy harvesting.

Contact electrification and energy harvesting using periodically contacted and squeezed water droplets.

We investigate the contact electrification occurring when a small water droplet resting on a metal electrode is brought periodically in contact with a hydrophobic film of fluorinated ethylene propylene. It is found that the maximum current increases with the drop volume according to a power law. The time scale for the contact current to develop is consistent with that required for a droplet to spread and is, therefore, longer than the time required to form the electric double layer. Adding salt into the water does reduce the contact current but not entirely, which suggests that any remaining water layer cannot entirely neutralize the charges developed upon contact. With an average power of 0.7 µW and a peak power near 5 µW at a frequency of 5 Hz, a 200 µL droplet of pure water can be used to light up a light-emitting diode.

Pyranine-induced self-assembly of colloidal structures using poly(allylamine-hydrochloride).

Complexes of dyes and polyelectrolytes have found widespread use in a variety of functional materials and interfaces. Here it is found that upon mixing the anionic dye pyranine and a cationic polyelectrolyte, poly(allylamine-hydrochloride), two different colloidal structures may form. Above a certain concentration of anionic dye, crosslinking of the polyelectrolyte is initiated, and the formation of sheet-like colloidal structures was observed. Addition of hydroxyl ions resulted in the formation of micron-sized spherical colloids. It was also found that the colloidal shape transition was accompanied by a significant red-shift in the fluorescence emission. Combining fluorescence measurements with studies of the particle size with time, it was found that red-shift was related to the crosslinking of the dye and the polyelectrolyte, and was not influenced significantly by the aggregation and particle growth. Further information about the colloidal behavior and stability was obtained by letting droplets dry and follow the kinetics of this process. It was found that the particles collapsed near the contact line and formed a ring deposit, in agreement with previous studies. However, unlike previous studies, the thickness of the ring deposit did not grow significantly with time, due to the peculiar process of formation found here.

Adsorption of bovine serum albumin on polyelectrolyte-coated glass substrates: applications to colloidal lithography.

The adsorption of bovine serum albumin (BSA) labeled with fluorescein isothiocyanate (FITC) on polyelectrolyte-coated glass substrates was investigated using fluorescence microscopy. Glass substrates may inhibit adsorption of proteins due to electrostatic repulsion. However, when the substrate is modified with a thin film of positively charged polyelectrolytes, proteins can be adsorbed due to the attractive electrostatic interactions. In this study, poly(allylamine-hydrochloride) (PAH) molecules, which have positively charged amino groups at pH 7, were used to generate a positively charged layer on the glass substrate. A surfactant, sodium dodecyl sulphate (SDS), was used to alter the glass-protein interaction and subsequently modulate the coverage of adsorbed protein. We applied this technique to control the heterogeneously charged microscopic patterns of biomolecules created when the adsorption of protein is done in conjunction with colloidal lithography.

Magnetically tunable optical absorbance in a colloidal system.

We study the optical absorbance from a magnetically arranged colloidal structure, and investigate the possibility of creating a magnetically controlled optical sensor using this system. The colloids form chains when exposed to an external magnetic field, which tend to collapse and form a more random particle arrangement when the field is removed. We show that a small magnetic field is able to change the sensor's reflection coefficient by more than 30%, and investigate in detail the relaxation mechanism when the field is turned off.

Influence of salt on colloidal lithography of albumin.

We investigate the influence of salt on colloidal lithography of biomolecular patterns. Albumin labeled with fluorescein isothiocyanate (FITC) was adsorbed on polyelectrolyte-coated glass substrates covered by negatively charged colloids using fluorescence microscopy. After removing the colloids, a well-defined albumin pattern remains, and we study how the pattern changes upon adding salt to the protein solution. The proposed method is simple and cheap and can be used to create stable one- and two-dimensional biomolecular arrays.

Field-induced adsorption exclusion of particles from one-dimensional nanomagnets.

I study the adsorption of paramagnetic colloids to one-dimensional nanomagnets. It is found that in the absence of external magnetic fields the colloids tend to adsorb to the nanomagnet by arranging themselves in a nearly close-packed formation, whereas in an external field some of the colloids are repelled due to dipolar interactions. I develop a theory for this phenomenon and show that it is in agreement with experimental data.

Order-disorder transition in a quasi-two-dimensional colloidal system.

A 2D colloidal system governed by repulsive dipolar forces tends to form a more ordered system when the interaction strength between the particles increases. Here we report an order-disorder transition of the colloidal system followed by chain formation upon increasing the dipolar interactions and show that the critical field scales with the density of colloids. Our system can do this by changing its dimensionality and therefore exhibits novel behavior that could help us understand colloidal ordering phenomena.

Spectral density of polychromatic electromagnetic waves.

We investigate theoretically how the vectorial nature of polychromatic electromagnetic fields results in polarization components with different spectral characteristics, thus leading to redshifts, blueshifts, and spectral distributions with multiple peaks. We discuss how these effects can be used to design spatially localized spectra with tailored spectral densities.

Colloidal optomagnetic dimmer.

We demonstrate a colloidal optomagnetic dimmer based on the interaction between micrometer-sized paramagnetic colloidal spheres and a magnetic film. The colloidal particles undergo Brownian motion, which when exposed to light results in characteristic intensity fluctuations, and we demonstrate that weak magnetic fields that are typically 200 A/m (2.5 G) can be used to control both the average intensity and the intensity fluctuations. The system can be used as a colloidal optical dimmer in microfluidic systems.

Optical transfer function of three-dimensional display systems.

I investigate the optical transfer function of three-dimensional display systems. Moreover, I obtain an average sampled modulation transfer function describing discrete, sampled display systems and show that in the proper limit of geometrical optics it is equivalent to the shift-invariant optical transfer function. I apply the theory to describe holographic stereograms and discuss the effects of amplitude and phase filters on the optical resolution.

Diffusion and cluster formation in one-dimensional systems with attractive interactions.

We study cluster formation in a finite one-dimensional model system where the particles experience long-range attractive forces. The particles are first placed in equidistant positions by a repulsive potential, which then is turned off, and only a weak long-range potential acts between the particles. It is shown that the mean-square deviation in distance between the colloids at first increases due to normal Brownian motion, followed by a crossover to anomalous diffusion governed by the long-range forces. Moreover, we also found that the subsequent cluster formation could be described by a Poisson distribution. The results presented here may help us understand diffusion and cluster formation in one-dimensional systems.

Strongly focused polarized light pulse.

We investigate theoretically the electric field of a focused light pulse carrying an inhomogeneous polarization distribution. It is found that the spectra of the polarization components are in general different, thus leading to a spatial spectral distribution that differs from the scalar case.

Physical mechanisms of rehydration in Polypodium polypodioides, a resurrection plant.

Resurrection plants have an amazing ability to withstand water drought. Here we investigate experimentally the rapidity of such revivals using the resurrection fern (Polypodium polypodioides) as a model example. Upon drying, the leaves of the resurrection fern fold into a thin cylindrical shell, thus protecting the photosynthetic area from light. In the dry state the fern looks dead, but will quickly come back once exposed to water by unfolding the cylindrical shell into a nearly planar sheet. We investigate here the mass and radius of curvature of the cylindrical shell as a function of time after rehydration and develop a phenomenological model to describe the observed phenomena. In particular, we demonstrate that the mass of the rehydrating plant follows a simple kinetic relationship, whereas the unfolding is governed by a more complex nonlinear constitutive relationship between the water uptake and the induced strain.

Monolayer to bilayer transition in a dipolar system.

We study the transition from a one-dimensional magnetic dipolar monolayer to a bilayer as it is compressed beyond the close-packed condition. The pressure in a close-packed monolayer is found to be nearly independent of the number of dipoles. In the case of weak dipolar interactions, our experimental results indicate that the bilayer formation is governed by short-range steric and electrostatic repulsion, whereas for strong dipolar interactions the bilayer formation is governed by long-range dipolar repulsion.

Colloidal rings in a liquid mixture.

We investigate the self-assembly of colloidal particles on microscopic decane droplets in water and show that, by use of paramagnetic colloids, it is possible to assemble ringlike structures that can be controlled with a magnetic field. Moreover, the use of paramagnetic colloids allows us to determine the attractive forces between the colloids located at the three-phase contact line between decane, water, and air. The attractive force is in the femtonewton range and is attributed to capillary interactions due to interface deformations. When the liquid emulsion dries on a glass slide, we observe solid deposits in the form of microscopic rings of varying diameters.

Self-assembly of colloidal pyramids in magnetic fields.

We study routes toward the construction of 2D colloidal pyramids. We find that magnetic beads may self-assemble into pyramids near a nonmagnetic 1D boundary as long as the number of beads in the pyramid does not exceed 10. We have also found that a strong magnetic field gradient could act as a boundary, thus assisting the self-assembly of magnetic colloids in water, and have observed the formation of stable microscopic pyramids within a certain magnetic field range. Our results indicate that colloidal pyramids can be formed in a number of ways by utilizing external fields.