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Inspired by the superhydrophobic effect displayed in nature, we set out to mimic the interplay between the chemistry and physics in the lotus leaf to see if the same design principle can be applied to control wetting and adhesion between toners and inks on various printing surfaces. Since toners and inks are organic materials, superoleophobicity has become our design target. In this work, we report the design and fabrication of a model superoleophobic surface on silicon wafer. The model surface was created by photolithography, consisting of texture made of arrays of ∼3 μm diameter pillars, ∼7 μm in height with a center-to-center spacing of 6 μm. The surface was then made oleophobic with a fluorosilane coating, FOTS, synthesized by the molecular vapor deposition technique with tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane. Contact angle measurement shows that the surface exhibits super repellency toward water and oil (hexadecane) with a water and hexadecane contact angles at 156° and 158°, respectively. Since the sliding angles for both liquids are also very small (∼10°), we conclude that the model surface is both superhydrophobic and superoleophobic. By comparing with the contact angle data of the bare silicon surfaces (both smooth and textured), we also conclude that the superoleophobicity is a result of both surface texturing and fluorination. Results from investigations of the effects of surface modification and pillar geometry indicate that both surface oleophobicity and pillar geometry are contributors to the superoleophobicity. More specifically, we found that superoleophobicity can only be attained on our model textured surface when the flat surface coating has a relatively high oleophobicity (i.e., with a hexadecane contact angle of >73°). SEM examination of the pillars with higher magnification reveals that the side wall in each pillar is not smooth; rather it consists of a ∼300 nm wavy structure (due to the Bosch etching process) from top to bottom. Comparable textured surfaces with (a) smooth straight side wall pillars and (b) straight side wall pillars with a 500 nm re-entrant structure made of SiO(2) were fabricated and the surfaces were made oleophobic with FOTS analogously. Contact angle data indicate that only the textured surfaces with the re-entrant pillar structure are both superoleophobic and superhydrophobic. The result suggests that the wavy structure at the top of each pillar is the main geometrical contributor to the superoleophobic property observed in the model surface.

The absorption and the steady-state fluorescence emission of a class of donor-acceptor-donor (D-A-D) molecules, bis-[4-(dimethylamino)phenyl]squaraine and its derivatives (1-19), have been studied. Squaraines generally exhibit intense solution absorption in the red (e ~3 X 105 cm"1 M"1). All substituents studied in this work exert a bathochromic effect on the absorption. The effect is small and is attributable to the minor involvement of the donor group in the S0 -*• S, excitation. In conjunction with emission data, we are able to show that the bathochromic shift originates from the solute-solvent complex and that the complexation constant increases as the D-A-D charge-transfer character of squaraine increases. Electronic and steric factors affecting the D-A-D character and subsequently the absorption maximum are discussed in terms of the solute-solvent complex model. Multiple-emission bands are observed in the fluorescence spectra of 1-19. Three bands, a, ß, and y, are identified from their typical Stokes shifts. Controlled experiments showed that the multiple-emission bands are intrinsic emissive properties of squaraines. Results from mixed-solvent experiments, solvent-effect studies, and temperature-effect studies show that squaraines form solute-solvent complexes in organic solvents. A photophysical model to account for the multiple emission is proposed. Excitation of squaraine in solution results in two excited states, namely, the excited state of free squaraine and the excited state of the solute-solvent complex. These two excited states emit photons to give the «-emission and the /3-emission. Rotational relaxation(s) (around the C-C bond between the phenyl ring and the four-membered ring of squaraine) is (are) shown to be the major radiationless decay process(es) of these two excited states. As a result, a twisted relaxed excited state is generated. This relaxed excited state can undergo a rotational relaxation to the ground state or emit a photon to give the -emission. The effect of structural changes on the multiple emission is discussed.

The synthesis of several amphiphilic squaraine dyes and a study of their aggregation behavior and photophysics are reported. The several different squaraines are found to give spectrally blue-shifted aggregates in aqueous and mixed aqueous-organic solution and in microheterogeneous media (bilayer vesicles). While in some cases an intermediate dimer can be detected in the monomer to aggregate conversion process, in others direct conversion of monomer to aggregate is observed. The aggregation number can be determined together with the equilibrium constant and thermodynamic parameters for some of the squaraines in different environments. In several cases the aggregation number is found to be ca. 4. The finding of a strong induced circular dichroism signal when the aggregate (but not dimer or monomer) is generated in the presence of a chiral host (or counterion) suggests that the aggregate is chiral. From these results and molecular simulations indicating that an extended monolayer of some of the squaraines adopts a glide or herringbone lattice we propose a chiral "pinwheel" structure for the unit aggregate and suggest that extended aggregate structures or crystals may be a mosaic of these unit aggregates. In contrast to the monomers, which are strongly fluorescent, the squaraine dimers and aggregates are nonfluorescent and have extremely short exciton lifetimes, as indicated by transient spectroscopy.

The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (α) and advancing (θA) and receding (θR) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to solution coated polymer films to chemical vapor deposition polymerized polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and two natural surfaces derived from the front and back side of the rose petal. Static water contact angle data suggest that these surfaces vary from hydrophilic with θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces versus α, (cos θR – cos θA), and the contact angle hysteresis (θA – θR) all yield scattered plots, indicating that there is little correlation between θ and α, (cos θR – cos θA) and (θA – θR). Since the later three parameters have been mentioned to relate to adhesion semiempirically between a liquid droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. In this work, we study both the wetting and adhesion forces between water and these 20 surfaces on a microelectromechanical balance (tensiometer). When the water drop first touches the surface, the attractive force during this wetting step was measured as the “snap-in” force. The adhesion force between the water drop and the surface was measured as the “pull-off” force when the water drop separates (retracts) from the surface. The snap-in force is shown to decrease monotonously as θA decreases and becomes zero when θA is >150°. The very good correlation is not unexpected due to the similarity between the wetting and the “snap-in” process. The analysis of the pull-off force data is slightly more complicated, and we found that the quality of the water–surface separation depends on the surface “adhesion”. For surfaces that show strong adhesion with water, there is always a small drop of water left behind after the water droplet is pulled off from the surface. Despite this complication, we plot the pull-off force versus α, (cos θR – cos θA) and (θA – θR), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle θR. Specifically, pull-off force decreases monotonically as θR increases, suggesting that θR is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between θR and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when θR is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design...

The spectroscopic and electrochemical properties of a series of p-iV^ZV-dialkylaminobenzylidenemalononitriles derivatives (1, 2, and 3) have been studies. These materials exhibit a strong intramolecular charge-transfer absorption. In this series the conformational freedom of the electron-donating group (R2N) was progressively restricted by structurally anchoring the nitrogen atom to the aromatic ring by alkyl chains. Molecular restriction of rotation of the NR2 group had a profound effect on the nonradiative decay rate and the excited state dipole moment. However, it had little effect on the ground state dipole moment. The most pronounced change in dipole moment ( µ) on electronic excitation was observed when the NR2 group was free to rotate. The rate of internal rotation of the dialkylamino group was estimated to be 2 X 10n s'1.

Previously, we reported the creation of a fluorosilane (FOTS) modified pillar array silicon surface comprising ~3-μm-diameter pillars (6 μm pitch with ~7 μm height) that is both superhydrophobic and superoleophobic, with water and hexadecane contact angles exceeding 150° and sliding angles at ~10° owing to the surface fluorination and the re-entrant structure in the side wall of the pillar. In this work, the effects of surface texturing (pillar size, spacing, and height) on wettability, contact angle hysteresis, and "robustness" are investigated. We study the static, advancing, and receding contact angles, as well as the sliding angles as a function of the solid area fraction. The results reveal that pillar size and pillar spacing have very little effect on the static and advancing contact angles, as they are found to be insensitive to the solid area fraction from 0.04 to ~0.4 as the pillar diameter varies from 1 to 5 μm and the center-to-center spacing varies from 4.5 to 12 μm. On the other hand, sliding angle, receding contact angle, and contact angle hysteresis are found to be dependent on the solid area fraction. Specifically, receding contact angle decreases and sliding angle and hysteresis increase as the solid area fraction increases. This effect can be attributable to the increase in pinning as the solid area fraction increases. Surface Evolver modeling shows that water wets and pins the pillar surface whereas hexadecane wets the pillar surface and then penetrates into the side wall of the pillar with the contact line pinning underneath the re-entrant structure. Due to the penetration of the hexadecane drop into the pillar structure, the effect on the receding contact angle and hysteresis is larger relative to that of water. This interpretation is supported by studying a series of FOTS pillar array surfaces with varying overhang thickness. With the water drop, the contact line is pinned on the pillar surface and very little overhang thickness effect was observed. On the other hand, the hexadecane drop is shown to wet the pillar surface and the side wall of the overhang. It then pins at the lower edge of the overhang structure. A plot of the thickness of the overhang as a function of the static, advancing, and receding contact angles and sliding angle of hexadecane reveals that static, advancing, and receding contact angles decrease and sliding angle increases as the thickness of the overhang increases. A larger overhang effect is observed with octane due to its lower surface tension. The robustness of the pillar array surface against external pressure induced wetting and abrasion was modeled. Surface Evolver simulation (with the hexadecane drop) indicates that wetting breakthrough pressure as high as ~70 kPa is achievable with 0.5-μm-diameter pillar array FOTS surfaces. Mechanical modeling shows that bending of the pillars is the key failure by abrasion, which can be avoided with a short pillar structure. The path to fabricate a superoleophobic surface that can withstand the external force equivalent of a...