Low-rate dynamic contact angles on polystyrene and the determination of solid surface tensions

Dependence of the Liquid Polarity in the Wetting of Rough Surface: An Effective Surface Tension Approach

Liquid and surface polarities play an important role in wetting phenomena, and this should still be true if the surface is rough. We analyze the wetting of high-polar and low-polar liquids on rough low-polar surfaces made of polystyrene. The experimental results are analyzed by the surface tension components (STC) and the equation-of-state (EQS) approaches. Both approaches predicted a clear increase of the contact angle (CA) with the surface roughness for high-polar liquids, but they failed for low-polar liquids: STC calculations produce the wrong tendency in the total solid surface energy, and EQS is not able to fit the data for these liquids. These results show that low-polar liquids show little dependence on the roughness of a low-polar surface, while high-polar liquids are very sensitive to it. As a consequence, the calculated CAs are close to experimental values only for the high-polar liquids, while there are great differences for low-polarity liquids. Both STC and EQS approaches are able to describe the apparent CAs on polystyrene rough surfaces by using effective surface and interfacial tensions, but their effectiveness is limited to high-polarity liquids.

Emulsion Polymerization Using an Amphiphilic Oligoether Ionic Liquid as a Surfactant

We investigate the use of an ionic liquid (IL) as a surfactant in emulsion polymerization (EP) reactions. ILs have been proposed as surfactants for micellar dispersions, emulsions, micro-emulsions and suspensions. Thus, it is important to acquire knowledge of the application of ILs in heterogeneous polymerizations. We selected the amphiphile cationic oligoether IoLiLyte C1EG™ as an IL for this purpose and compared its performance to that of the conventional surfactant dodecyl trimethyl ammonium bromide (DTAB) in the EP of methyl methacrylate and styrene. After we found the proper concentration range of the IL, this amphiphile showed similar polymerization rates to those observed with DTAB for both monomers. The evolution of monomer conversion and the final average diameter of formed polymeric particles were similar for both evaluated surfactants, demonstrating their capability to stabilize the EPs of the investigated monomers. We simulated the evolution of monomer conversion and particle size using a conventional model for emulsion polymerization, which showed good agreement with the experimental data, suggesting that the EP with this IL follows Smith-Ewart kinetics.

Fused Deposition Modeling of Microfluidic Chips in Transparent Polystyrene

Polystyrene (PS) is one of the most commonly used thermoplastic materials worldwide and plays a ubiquitous role in today's biomedical and life science industry and research. The main advantage of PS lies in its facile processability, its excellent optical and mechanical properties, as well as its biocompatibility. However, PS is only rarely used in microfluidic prototyping, since the structuring of PS is mainly performed using industrial-scale replication processes. So far, microfluidic chips in PS have not been accessible to rapid prototyping via 3D printing. In this work, we present, for the first time, 3D printing of transparent PS using fused deposition modeling (FDM). We present FDM printing of transparent PS microfluidic channels with dimensions as small as 300 µm and a high transparency in the region of interest. Furthermore, we demonstrate the fabrication of functional chips such as Tesla-mixer and mixer cascades. Cell culture experiments showed a high cell viability during seven days of culturing, as well as enabling cell adhesion and proliferation. With the aid of this new PS prototyping method, the development of future biomedical microfluidic chips will be significantly accelerated, as it enables using PS from the early academic prototyping all the way to industrial-scale mass replication.

Formation of Hierarchical Porous Films with Breath-Figures Self-Assembly Performed on Oil-Lubricated Substrates

Hierarchical honeycomb patterns were manufactured with breath-figures self-assembly by drop-casting on the silicone oil-lubricated glass substrates. Silicone oil promoted spreading of the polymer solution. The process was carried out with industrial grade polystyrene and polystyrene with molecular mass M w = 35 , 000 g m o l . Both polymers gave rise to patterns, built of micro and nano-scaled pores. The typical diameter of the nanopores was established as 125 nm. The mechanism of the formation of hierarchical patterns was suggested. Ordering of the pores was quantified with the Voronoi tessellations and calculation of the Voronoi entropy. The Voronoi entropy for the large scale pattern was S v o r = 0.6 - 0.9 , evidencing the ordering of pores. Measurement of the apparent contact angles evidenced the Cassie-Baxter wetting regime of the porous films.

Determining the Surface Tension of Two-Dimensional Nanosheets by a Low-Rate Advancing Contact Angle Measurement

Because of their atomic thinness, two-dimensional (2D) nanosheets need be bound to a substrate or be dispersed in material in various applications. The surface tension (ST) of a 2D nanosheet is critical for analyzing the physicochemical interactions between 2D nanosheets and other materials. To date, the determination of the ST of 2D nanosheets has relied mainly on the contact angle (CA) method. However, because of the difficulty in measuring the thermodynamically significant Young?s CA, which is the only meaningful CA that can be used to determine the ST, significant differences exist in reported STs of 2D nanosheets. In this study, we obtained such unique Young?s CAs on graphene, boron nitride, molybdenum disulfide, and tungsten disulfide nanosheets by a low-rate advancing contact angle measurement using a rigorously designed experimental setup. By interpreting the CA with Neumann?s equation of state, we determined the STs of these four nanosheets to be 29.7 ? 0.6, 30.9 ? 0.7, 27.8 ? 0.7, and 29.1 ? 0.8 mJ/m², respectively. The surface energies of these 2D nanosheets were estimated to be in the range 95?120 mJ/m² by considering the contribution of ST and surface entropy. The accuracy of these determined STs was validated by the exfoliation and dispersion of 2D nanosheets in liquids with a series of STs. The study may have important implications for understanding the physicochemical interactions between 2D nanosheets and other materials and the development of 2D nanosheet-based devices.

Alternative SiO2 Surface Direct MDCK Epithelial Behavior

The mechanical interactions of cells are mediated through adhesive interactions. In this study, we examined the growth, cellular behavior, and adhesion of MDCK epithelial cells on three different SiO₂ substrates: amorphous glass coverslips and the silicon oxide layers that grow on ⟨111⟩ and ⟨100⟩ wafers. While compositionally all three substrates are almost similar, differences in surface energy result in dramatic differences in epithelial cell morphology, cell-cell adhesion, cell-substrate adhesion, actin organization, and extracellular matrix (ECM) protein expression. We also observe striking differences in ECM protein binding to the various substrates due to the hydrogen bond interactions. Our results demonstrate that MDCK cells have a robust response to differences in substrates that is not obviated by nanotopography or surface composition and that a cell's response may manifest through subtle differences in surface energies of the materials. This work strongly suggests that other properties of a material other than composition and topology should be considered when interpreting and controlling interactions of cells with a substrate, whether it is synthetic or natural.

The measurement of the surface energy of solids using a laboratory drop tower

This work presents a technique for the study and measurement of the interfacial energies of solid-liquid-gas systems. The instrument and the evaluation method for the measurements obtained by it, allow the analysis of the energy changes of sessile drops submitted to microgravity. A mathematical model based on the thermodynamic of wetting is applied to evaluate the interfacial energies as a function of the drop shape changes due to the effect of the release of gravitation during the experiment. The presented model bases on the thermodynamic equilibrium of the interfaces and not on the balance of bi-dimensional tensors on the contour line. For this reason, the model does not follow Young's equation as the current surface wetting characterization techniques usually do.

Measurement of contact angles in a simulated microgravity environment generated by a large gradient magnetic field

The contact angle is an important parameter that is essential for studying interfacial phenomena. The contact angle can be measured using commercially available instruments. However, these well-developed instruments may not function or may be unsuitable for use in some special environments. A simulated microgravity generated by a large gradient magnetic field is such an environment in which the current measurement instruments cannot be installed. To measure the contact angle in this environment, new tools must be designed and manufactured to be compatible with the size and physical environment. In this study, we report the development and construction of a new setup that was specifically designed for use in a strong magnetic field to measure the contact angle between a levitated droplet and a solid surface. The application of the setup in a large gradient magnetic field was tested, and the contact angles were readily measured.

Identification of Dewetting Stages and Preparation of Single Chain Gold Nanoparticle Rings by Colloidal Lithography

Massively parallel nanoparticle assembly was carried out by means of colloidal lithographic experiments over a silicon substrate supported (sub)microparticle Langmuir-Blodgett monolayer, using high purity aqueous solution of PEGylated gold nanoparticles. The size of the polystyrene template particles in the monolayer was varied between 608 nm and 2.48 μm, while gold nanoparticles with diameters between 18 and 65 nm were used. Thanks to the PEGylation of the gold nanoparticles, they could be used as tracer objects to follow the drying process. In this way, different dewetting stages could be identified in the confined space between and underneath the template polystyrene spheres. Depending on the concentration of the nanoparticles, the presented approach allows the preparation of single-particle width necklace structures composed of gold particles. At the same time, the high purity of the substrate as well as of the evolved particle rings is preserved and unwanted particle deposition on the substrate surface is minimized.

Asymmetric deformation of swollen microspheres on a water surface

Fabrication of anisotropic particles simply by assembly of swollen spheres on a water surface and evaporation of the swelling agent.

Relationship between Segmental Relaxation of Polystyrene Films and Stick-Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces

A novel method was previously reported for detecting the glass transition of thin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick-slip behavior on their surfaces during dynamic wetting of liquid droplets. In the present study, the frequency dependence of the stick-slip behavior is investigated. The results show that the stick-slip behavior of liquid dynamic wetting on PS films is dependent on the contact line velocity, which is related to the deformation frequency of the PS surface during the moving liquid front. The stick-slip behavior was revealed to be determined by a dimensionless parameter (ξ), which is the ratio of the PS segmental relaxation time (τα) and the characteristic time (τc) for PS surface deformation near the droplet contact line. When ξ is close to 1 (τα ≈ τc), the Δθ (jumping angle), a scale of the stick-slip behavior, reaches a maximum. This correlation between Δθ and ξ demonstrates that the stick-slip behavior is related to the energy dissipation caused by the PS α-relaxation process, and the peak temperature (or frequency) in Δθ corresponds to the α-relaxation temperature (time) of the polymer. These results strongly demonstrate that the utilization of the stick-slip behavior is a creditable method, similar to dynamic viscoelastic measurement, for probing the glass transition and segmental relaxation of thin polymer films.

A theory for the surface tensions and contact angles of hydrogen-bonding liquids

The surface tensions of non-hydrogen-bonding, organic liquids can be accurately calculated from their electromagnetic properties, using an approximate form of the Lifshitz theory. A simple extension of this approach to the calculation of the surface tensions of hydrogen-bonding liquids is proposed. It is shown that the higher surface tensions of hydrogen-bonding liquids can be accounted for, with reasonable accuracy, by the increase in dispersion due to the shortened distance of approach between hydrogen-bonded atoms. Similar considerations allow calculations of contact angles on several low-energy solid surfaces in terms of molecular and electromagnetic properties. In accordance with well-known experimental observations, the calculated contact angles of both hydrogen-bonding and non-hydrogen-bonding liquids on the same low-energy surface nearly follow a single, smooth pattern.

Rapid spectrophotometric method for determining surface free energy of microalgal cells

Microalgae are one of the most promising renewable energy sources with environmental sustainability. The surface free energy of microalgal cells determines their biofouling and bioflocculation behavior and hence plays an important role in microalgae cultivation and harvesting. To date, the surface energetic properties of microalgal cells are still rarely studied. We developed a novel spectrophotometric method for directly determining the surface free energy of microalgal cells. The principles of this method are based on analyzing colloidal stability of microalgae suspensions. We have shown that this method can effectively differentiate the surface free energy of four microalgal strains, i.e., marine Chlorella sp., marine Nannochloris oculata, freshwater autotrophic Chlorella sp., and freshwater heterotrophic Chlorella sp. With advantages of high-throughput and simplicity, this new spectrophotometric method has the potential to evolve into a standard method for measuring the surface free energy of cells and abiotic particles.

Liquid polystyrene: a room-temperature photocurable soft lithography compatible pour-and-cure-type polystyrene

Materials matter in microfluidics. Since the introduction of soft lithography as a prototyping technique and polydimethylsiloxane (PDMS) as material of choice the microfluidics community has settled with using this material almost exclusively. However, for many applications PDMS is not an ideal material given its limited solvent resistance and hydrophobicity which makes it especially disadvantageous for certain cell-based assays. For these applications polystyrene (PS) would be a better choice. PS has been used in biology research and analytics for decades and numerous protocols have been developed and optimized for it. However, PS has not found widespread use in microfluidics mainly because, being a thermoplastic material, it is typically structured using industrial polymer replication techniques. This makes PS unsuitable for prototyping. In this paper, we introduce a new structuring method for PS which is compatible with soft lithography prototyping. We develop a liquid PS prepolymer which we term as "Liquid Polystyrene" (liqPS). liqPS is a viscous free-flowing liquid which can be cured by visible light exposure using soft replication templates, e.g., made from PDMS. Using liqPS prototyping microfluidic systems in PS is as easy as prototyping microfluidic systems in PDMS. We demonstrate that cured liqPS is (chemically and physically) identical to commercial PS. Comparative studies on mouse fibroblasts L929 showed that liqPS cannot be distinguished from commercial PS in such experiments. Researchers can develop and optimize microfluidic structures using liqPS and soft lithography. Once the device is to be commercialized it can be manufactured using scalable industrial polymer replication techniques in PS--the material is the same in both cases. Therefore, liqPS effectively closes the gap between "microfluidic prototyping" and "industrial microfluidics" by providing a common material.

Orienting block copolymer microdomains with block copolymer brushes

A simple, rapid, and robust technique for controlling the self-assembly of block copolymers (BCPs) with a large segmental interaction parameter, χ, is described using a surface modified with anchored BCP brushes. End-functionalized poly(styrene-b-ethylene oxide)s (PS-b-PEOs), where the fraction of PS (f(PS)) was varied, end-functionalized neat PS, and end-functionalized neat PEO were end-grafted onto Si substrates modifying the surface with polymer brushes. Thin films of cylinder-forming PS-b-PEO were prepared on modified Si substrates and thermally annealed. When neat PS and PEO were used as the anchored brushes, the microdomains of the PS-b-PEO oriented parallel to the substrate upon thermal annealing due to the preferential interactions of one block to the anchored brushes. However, when end-functionalized PS-b-PEOs were used to modify the substrate, hexagonally packed cylindrical PEO microdomains oriented normal to the substrate, having long-range lateral ordering, were obtained over a very wide range of f(PS) (0.32 to 0.77).

Comparison of the relaxation of sessile drops driven by harmonic and stochastic mechanical excitations

Currently, there is no conclusive evidence regarding the global equilibrium condition of vibrated drops. However, it is well-known that vibration of sessile drops effectively reduces the contact angle hysteresis. In this work, applying a recent methodology for evaluating the most-stable contact angle, we examined the impact of the type of excitation signal (random signal versus periodical signal) on the values of the most-stable contact angle for polymer surfaces. Using harmonic signals, the oscillation frequency affected the postvibration contact angle. Instead, the white noise signal enabled sessile drops to relax regardless of their initial configuration. In spite of that, the values of most-stable contact angle obtained with different signals mostly agreed. We concluded that not only the amount of relaxation can be important for relaxing a sessile drop but also the rate of relaxation. Together with receding contact angle, most-stable contact angle, measured with the proposed methodology, was able to capture the thermodynamic changes of "wetted" polymer surfaces.

Effect of the polymer nature on the structural organization of lipid/polymer particle assemblies

The nano-organized LipoParticle assemblies, consisting of polymer particles coated with lipid layers, are investigated with the aim of evidencing the impact of the particle chemical nature on their physicochemical behavior. To this end, these colloidal systems are elaborated from anionic submicrometer poly(styrene) (P(St)) or poly(lactic acid) (PLA) particles, and lipid mixtures composed of zwitterionic 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and cationic 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP). As revealed by various experimental techniques, such as quasielastic light scattering, zeta potential measurements, transmission electron microscopy, and 1H NMR spectroscopy, the features of both LipoParticle systems are similar when cationic lipid formulations (DPPC/DPTAP mixtures) are used. This result emphasizes the major role of electrostatic interactions as driving forces in the assembly elaboration process. Conversely, the assemblies prepared only with the zwitterionic DPPC lipid are strongly dependent on the particle chemical nature. The structural characteristics of the assemblies based on PLA particles are not controlled and correspond to aggregates, contrary to P(St) particles. To understand this specific phenomenon, and to consequently improve the final organization of these assemblies which are potentially of great interest in biotechnology and biomedicine, numerous investigations are carried out such as the studies of the impact of the ionic strength and the pH of the preparation media, as well as the presence of ethanol (involved in the PLA particle synthesis) or the mean size of the lipid vesicles. From the resulting data and according to the nature of spherical solid support, hydrophobic effects, hydrogen bonds, or dipole-dipole interactions would also appear to influence the LipoParticle elaboration in the case of zwitterionic lipid formulation.

Contact angle of water on polystyrene thin films: effects of CO(2) environment and film thickness

We examine the contact angle of water droplets on polystyrene (PS) thin films of varying thicknesses supported by silicon wafers under both air and pressurized carbon dioxide (CO2) environments. At 23 degrees C, the contact angle is found to increase upon increasing CO2 pressure in the vapor regime and then levels off in the liquid CO2 regime. A macroscopic model based on Young's equation and the geometric-mean method for interfacial tensions, and long-range van der Waals interactions, correctly predicts the trends and the magnitude of the contact angle dependence on pressure, although deviations occur at high CO2 activities. The contact angle was also found to depend on film thickness, h, when h was comparable to or smaller than 50 nm. Specifically, the contact angle decreases with decreasing PS film thickness. This behavior could be accounted for with the use of a model that incorporates the effects of film thickness, CO2 pressure, and the long-range van der Waals potential.

Anisotropic wettability on imprinted hierarchical structures

A series of two-level hierarchical structures on polystyrene (PS) and poly(methyl methacrylate) (PMMA) were fabricated using sequential nanoimprinting lithography (NIL). The hierarchical structures consist of micrometer and sub-micrometer scale grating imprinted with varying orientations. Through water contact angle measurements, these surface hierarchical structures showed a wide range of anisotropic wettabilities on PMMA and PS, with PMMA having an anisotropic wettability from 6 degrees to 54 degrees and PS having an anisotropic wettability from 8 degrees to 32 degrees. At the same time, the water contact angle of PMMA and PS can be tuned to nearly 120 degrees without modifying the surface chemistry. A tunable anisotropic wettability is beneficial for applications where controlling the direction of liquid flow is important, such as in microfluidic devices.

The influence of flotation agent concentration on the wettability and flotability of polystyrene

The fundamental flotation process is the formation of a flocculant by air bubbles and solid particles in an aqueous solution. The behavior of plastic particles is significantly influenced by the wettability of the plastics. In this article the reciprocal relationship between the flotability and wettability of polystyrene was studied at different concentrations of flotation agents, particularly terpineol, polyethylene glycol dodecyl ether, tannic acid, and calcium lignosulfonate. The conclusions obtained demonstrate the dissimilar action of flotation depressants, what means different adhesion mechanisms on a plastic surface.

Ellipsometric study of adsorption on nanopatterned block copolymer substrates

We report ellipsometrically obtained adsorption isotherms for a carefully chosen test liquid on block copolymer films of Kraton G1650, compared with adsorption isotherms on homogeneous films of the constituent polymers. Standard atomic force microscopy images imply the outer surface of Kraton G1650 is chemically patterned on the nanoscale, but this could instead be a reflection of structure buried beneath a 10 nm layer of the lower energy component. Our test liquid was chosen on the basis that it did not dissolve in either component and in addition that it was nonwetting on the lower energy polymer while forming thick adsorbed films on pure substrates of the higher energy component. Our ellipsometry data for Kraton G1650 rule out the presence of segregation by the lower energy constituent to the outer surface, implying a mixed surface consistent with Cassie's law. We discuss implications of our findings and related work for the outer surface structures of block copolymer films.

Surface energy of ethylene-co-1-butene copolymers determined by contact angle methods

Wilhelmy plate measurements of contact angles with a series of test liquids are used to calculate the surface energies of two poly(ethylene-co-1-butene) random copolymers. Results from five methods of calculation are reported: one-liquid (Good-Girifalco and Neumann), two-liquid (harmonic mean and geometric mean), and three-liquid (Lifshitz-van der Waals acid-base) methods. We find that all five methods are sensitive to the choice of test liquids used for contact angle measurements, as previously reported, but consistent results are obtained if recommended combinations of liquids are used. The mean results of the three-liquid acid-base method are judged to be the most reliable and informative, leading to surface energies of 30.8 mJ/m2 for poly(ethylene-co-1-butene) copolymer composed of 92 mol% ethylene and 30.2 mJ/m2 for copolymer composed of 88 mol% ethylene.

Experimental Study on Contact Angle Patterns: Liquid Surface Tensions Less Than Solid Surface Tensions

The interpretation of contact angles in terms of solid surface tensions is not trivial. In the past, we and others have postulated that contact angles should be measured with liquid of surface tension larger than the anticipated solid surface tension, i.e., gamma(lv)>gamma(sv). This has recently been disputed. It is also not entirely obvious how to proceed experimentally since gamma(sv) is not known initially. Typically, one starts with a liquid of high gamma(lv) (such as water) and goes lower. We have stopped in the past when the contact angles became small. A question arises as to what would happen if we would go on. Contact angles of liquids with gamma(lv) less than or near gamma(sv) were measured on eight polymer-coated solid surfaces. The experimental contact angle patterns for gamma(lv)<gamma(sv) and for gamma(lv)>gamma(sv) were compared. Results suggest that contact angle interpretation in terms of solid surface tensions requires contact angles to be measured for gamma(lv)>gamma(sv) because the Young equation is not applicable for gamma(lv)<gamma(sv). Thus contact angle approaches that disregard this requirement are questionable. Copyright 2000 Academic Press.