Exploiting breath figure reversibility for in situ pattern modulation and hierarchical design.

The breath figure (BF) method employs condensation droplets as dynamic templates for patterning polymer films. In the classical approach, dropwise condensation and film solidification are simultaneously induced through solvent evaporation, leading to empirically derived patterns with limited predictability of the final design. Here we use the temporally arrested BF methodology, controlling condensation and polymerisation independently to create diverse BF patterns with varied pore size, arrangement and distribution. External temperature control enables us to further investigate and exploit the inherent reversibility of the phase change process that governs the pattern formation. We modulate the level of subcooling and superheating to achieve subsequent regimes of condensation and evaporation, permitting in situ regulation of the droplet growth and shrinkage kinetics. The full reversibility of the phase change processes joined with active photopolymerisation in the current approach thus allows arresting of predictable BF kinetics at intermediate stages, thereby accessing patterns with varied pore size and spacing for unchanged material properties and environmental conditions. This simultaneous active control over both the kinetics of phase change and polymer solidification offers affordable routes for the fabrication of diverse predictable porous surfaces; manufacture of monolithic hierarchical BF patterns are ultimately facilitated through the advanced control of the BF assembly using the method presented here.

Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement.

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10-30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18-65 mN/m) and substrate roughness (Rα ≃ 0-60 µm) on the kinetics of surface-induced crystallization through measurements of induction time (ti) under isothermal conditions. While ti is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ∼40 fold. For substrates with the lowest surface energy and longest ti, microscale surface roughness is found to enhance crystal nucleation, in particular for Rα ≥ 10 µm. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Growth of Myelin Figures from Parent Multilamellar Vesicles.

We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as dMLV ∝ t1/2, myelins grow linearly in time as LM ∝ t1, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium.

Lamellar-to-MLV transformation in SDS/octanol/brine examined by microfluidic-SANS and polarised microscopy.

The lamellar-to-multilamellar vesicle (MLV) transformation in a model surfactant system, sodium dodecyl sulfate (SDS), octanol and brine, is investigated under continuous and oscillatory microfluidic contraction-expansion flows, employing polarised optical microscopy and small angle neutron scattering (SANS), with sample volume probed down to ≃20 nL. We determine the lamellar-to-MLV transition requirements at varying flow velocity, oscillation amplitude, frequency, and number of oscillatory cycles. The spatio-temporal evolution of the hierarchical fluid structure is elucidated: lamellar sheets initially align with flow direction upon entering a constriction and then perpendicularly upon exiting; the formation of MLVs at the nanoscale is first observed by SANS within a few (<5) oscillatory cycles, followed by the gradual appearance of a regular (albeit not crystalline) MLV arrangement, at the micronscale, by optical microscopy after tens of cycles, under the conditions investigated. Once MLVs form under flow, these remain metastable for several days.

CO2-Driven diffusiophoresis for maintaining a bacteria-free surface.

Dissolution and dissociation of CO2 in an aqueous phase induce diffusiophoretic motion of suspended particles with a nonzero surface charge. We report CO2-driven diffusiophoresis of colloidal particles and bacterial cells in a circular Hele-Shaw geometry. Combining experiments and model calculations, we identify the characteristic length and time scales of CO2-driven diffusiophoresis in relation to system dimensions and CO2 diffusivity. The motion of colloidal particles driven by a CO2 gradient is characterized by measuring the average velocities of particles as a function of distance from the CO2 sources. In the same geometrical configurations, we demonstrate that the directional migration of wild-type V. cholerae and a mutant lacking flagella, as well as S. aureus and P. aeruginosa, near a dissolving CO2 source is diffusiophoresis, not chemotaxis. Such a directional response of the cells to CO2 (or an ion) concentration gradient shows that diffusiophoresis of bacteria is achieved independent of cell shape, motility and the Gram stain (cell surface structure). Long-time experiments suggest potential applications for bacterial diffusiophoresis to cleaning systems or anti-biofouling surfaces, by reducing the population of the cells near CO2 sources.

Toxigenic Clostridium difficile-mediated diarrhoea in hematopoietic stem cell transplantation in-patients: rapid diagnosis and efficient treatment.

Allogenic hematopoietic stem cell transplant (HSCT) recipients are susceptible to any kind of infectious agents including Clostridium difficile. We studied 86 allogenic-HSCT patients who faced diarrhoea while receiving antibiotics. DNA from stool samples were explored for the presence of C. difficile toxin genes (tcdA; tcdB) by multiplex real-time PCR. Results showed nine toxigenic C. difficile amongst which seven were positive for both toxins and two were positive for tcdB. Six of toxigenic C. difficile organisms harbouring both toxin genes were also isolated by toxigenic culture. Clostridium difficile infection was controlled successfully with oral Metronidazole and Vancomycin in the confirmed infected patients.

Phenotypical and Genotypical Comparison of Clostridium difficile Isolated from Clinical Samples: Homebrew DNA Fingerprinting versus Antibiotic Susceptibility Testing (AST) and Clostridial Toxin Genes.

BACKGROUND: Clostridium (Clostridioides) difficile is recognized as the major cause of healthcare antibiotic-associated diarrhea. We surveyed a molecular epidemiological correlation between the clinical isolates from two general hospitals in Iran through clustering toxigenic types and antibiotic susceptibility testing (AST) accuracy. METHODS: Study population included 460 diarrhoeic specimens from inpatients with a history of antibiotic therapy. All samples underwent enriched anaerobic culture, confirmed by detection of gluD gene with PCR. Toxin status and AST were assessed by the disk diffusion method (DDM) and minimal inhibitory concentrations (MICs) of metronidazole, vancomycin, and rifampin. C. difficile outbreak was analyzed through conventional PCR by tracing toxin genes and Homebrew pulsed-field gel electrophoresis (PFGE) for characterizing isolates within our healthcare systems. RESULTS: A total of 29 C. difficile strains were isolated by enriched anaerobic culture from the clinical samples. Among them, 22 (4.8%) toxigenic profiles yielded toxins A and B (tcdA, tcdB) and binary toxins (cdtA, cdtB). The minimum inhibitory concentration (MIC) was 18.1% and 9% for vancomycin and metronidazole, and all isolates were susceptible to rifampicin and its minimum inhibitory concentration was at <0.003 µg/mL. The most dominant toxigenic and antibiotic-resistant "pulsotype F" was detected through PFGE combined with multiple Clostridial toxigenic pattern and AST. CONCLUSIONS: DNA fingerprinting studies represent a powerful tool in surveying hypervirulent C. difficile strains in clinical settings. Resistance to vancomycin and metronidazole, as first-line antibiotics, necessitate accomplishment of proper control strategies and also prescription of tigecycline as a more appropriate option.

Orthogonal wave superposition of wrinkled, plasma-oxidised, polydimethylsiloxane surfaces.

We report a versatile approach to generate 2D dual-frequency patterns on soft substrates by superposition of 1D single-frequency wrinkles. Wave superposition is achieved by applying sequential orthogonal strains to elastomeric coupons, as opposed to the application of a (simultaneous) biaxial strain field. First, a 1D wrinkling pattern is induced by the well-known mechanical instability of a bilayer formed by oxygen plasma-oxidation of a (pre-strained) polydimethylsiloxane elastomer. The wrinkled surface formed upon strain release is then replicated to obtain a stress-free substrate, and stretched in the direction perpendicular to the first generation. Subsequent plasma exposure and mechanical relaxation (with independent process parameters) yield a prescribed second-generation wrinkling, whose profile and dependence on the first generation we examine in detail. By independently varying plasma oxidation and strain parameters in both directions, we demonstrate the formation of a wide array of topographies, including arrays of symmetric 2D checkerboard patterns with exceptional area coverage with respect to those formed by simultaneous 2D wrinkling. While the resulting topographies cannot be explained in terms of a simple orthogonal wave superposition, we show that, by accounting for the orthogonal prestrain experienced by the first wrinkling generation, the resulting 2D patterns can be readily calculated from 1D wrinkling behaviour.

Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt.

We investigate the shape, dimensions, and transformation pathways of micelles of linear sodium alkylbenzenesulfonate (NaLAS), a common anionic surfactant, in aqueous solution. Employing Small Angle Neutron Scattering (SANS) and surface tensiometry, we quantify the effects of surfactant concentration (0.6-15 wt%), temperature (5-40 °C) and added salt (≤0.35 M Na2SO4). Spherical micelles form at low NaLAS (≤2.6 wt%) concentration in water, and become elongated with increasing concentration and decreasing temperature. Addition of salt reduces the critical micelle concentration (CMC) and thus promotes the formation of micelles. At fixed NaLAS concentration, salt addition causes spherical micelles to grow into cylindrical micelles, and then multilamellar vesicles (MLVs), which we examine by SANS and cryo-TEM. Above a threshold salt concentration, the MLVs reach diameters of 100 s of nm to few µm, eventually causing precipitation. While the salt concentrations associated with the micelle-to-cylinder transformation increase only slightly with temperature, those required for the cylinder-to-MLV transformation exhibit a pronounced, linear temperature dependence, which we examine in detail. Our study establishes a solution structure map for this model anionic surfactant in water, quantifying the combined roles of concentration, temperature and salt, at practically relevant conditions.

Particle entrainment in dead-end pores by diffusiophoresis.

The transport of particulate matter to and from dead-end pores is difficult to achieve due to confinement effects. Diffusiophoresis is a phenomenon that results in the controlled motion of colloids along solute concentration gradients. Thus, by establishing an electrolyte concentration gradient within dead-end pores, it is possible to induce the flow of particles into and out of the pores via diffusiophoresis, as has been demonstrated recently. In this paper, we explain the pore-scale mechanism by which individual colloids are entrained in dead-end pores by diffusiophoresis. We flow particles past a series of dead-end pores in the presence of a solute concentration gradient. Our results reveal that particles execute pore-to-pore hops before ultimately being captured. We categorize an event as particle capture when the particle's trajectory terminates within the dead-end pore. Experiments and numerical simulations demonstrate that particle capture only occurs when flowing particles are positioned sufficiently close to the pore entry. Outside this capture region, the particles have insufficient diffusiophoretic velocities to induce capture and their dynamics are largely dominated by their free-stream advective velocities. We observe that the particles move closer to the device wall as they hop, thereby reducing the effect of flow advection and increasing that of diffusiophoresis. These results enhance our understanding of suspension dynamics in a driven system and have implications for the development, design, and optimization of diffusiophoretic platforms for drug delivery, cosmetics, and material recovery.

Erratum: Water-based Peeling of Thin Hydrophobic Films [Phys. Rev. Lett. 119, 154502 (2017)].

This corrects the article DOI: 10.1103/PhysRevLett.119.154502.

Water-Based Peeling of Thin Hydrophobic Films.

Inks of permanent markers and waterproof cosmetics create elastic thin films upon application on a surface. Such adhesive materials are deliberately designed to exhibit water-repellent behavior. Therefore, patterns made up of these inks become resistant to moisture and cannot be cleaned by water after drying. However, we show that sufficiently slow dipping of such elastic films, which are adhered to a substrate, into a bath of pure water allows for complete removal of the hydrophobic coatings. Upon dipping, the air-water interface in the bath forms a contact line on the substrate, which exerts a capillary-induced peeling force at the edge of the hydrophobic thin film. We highlight that this capillary peeling process is more effective at lower velocities of the air-liquid interface and lower viscosities. Capillary peeling not only removes such thin films from the substrate but also transfers them flawlessly onto the air-water interface.

Armoring confined bubbles in the flow of colloidal suspensions.

We study the process of coating the interface of a long gas bubble, which is translating in a horizontal circular capillary tube filled with a colloidal suspension. A typical elongated confined bubble is comprised of three distinct regions: a spherical front cap, a central body that is separated from the tube wall by a thin liquid film, and a spherical cap at the back. These three regions are connected by transitional sections. Particles gradually coat the bubble from the back to the front. We investigate the mechanisms that govern the initial accumulation of the particles and the growth of the particle-coated area on the interface of the bubble. We show that the initial accumulation of particles starts at the stable stagnation ring on the rear cap of the bubble, and the particles will completely coat the spherical cap at the back of the bubble before accumulating on the central body. Armoring the central interface of the bubble with particles thickens the liquid film around the bubble relative to that around the particle-free interface. This effect creates a rather sharp step on the interface of the bubble in the central region, which separates the armored region from the particle-free region. After the bubble is completely coated, the liquid film around the body of the bubble will adjust again to an intermediate thickness. We show that the three distinct thicknesses that the liquid film acquires during the armoring process can be well described analytically.

Bubble-Driven Detachment of Bacteria from Confined Microgeometries.

Moving air-liquid interfaces, for example, bubbles, play a significant role in the detachment and transport of colloids and microorganisms in confined systems as well as unsaturated porous media. Moreover, they can effectively prevent and/or postpone the development of mature biofilms on surfaces that are colonized by bacteria. Here we demonstrate the dynamics and quantify the effectiveness of this bubble-driven detachment process for the bacterial strain Staphylococcus aureus. We investigate the effects of interface velocity and geometrical factors through microfluidic experiments that mimic some of the confinement features of pore-scale geometries. Depending on the bubble velocity U, at least three different flow regimes are found. These operating flow regimes not only affect the efficiency of the detachment process but also modify the final distribution of the bacteria on the surface. We organize our results according to the capillary number, [Formula: see text], where µ and γ are the viscosity and the surface tension, respectively. Bubbles at very low velocities, corresponding to capillary numbers Ca < 5 × 10-5, exhibit detachment efficiencies of up to 80% at the early stage of bacterial adhesion. In contrast, faster bubbles at capillary numbers Ca > 10-3, have lower detachment efficiencies and cause significant nonuniformities in the final distribution of the cells on the substrate. This effect is associated with the formation of a thin liquid film around the bubble at higher Ca. In general, at higher bubble velocities bacterial cells in the corners of the geometry are less influenced by the bubble passage compared to the central region.

Protocol to perform pressurized blister tests on thin elastic films.

This work aims to identify common challenges in the preparation of the blister test devices designed for the measurement of the energy release rate for brittle thin films and to propose easy-to-implement solutions accordingly. To this end, we provide a step-by-step guide for fabricating a blister test device comprised of thin polystyrene films adhered to glass substrates. Thin films are first transferred from donor substrates to an air-water interface, which is then used as a platform to locate them on a receiver substrate. We embed a microchannel at the back of the device to evacuate the air trapped in the opening, through which the pressure is applied. We quantify the height and the radius of the blister to estimate the adhesion energy using the available expressions correlating the normal force and the moment with the shape of the blister. The present blister test provided an adhesion energy per unit area of G = 18±2 mJ/m^2 for polystyrene on glass, which is in good agreement with the measurement of G = 14±2 mJ/m^2 found in our independent cleavage test.

Temporally Arrested Breath Figure.

Since its original conception as a tool for manufacturing porous materials, the breath figure method (BF) and its variations have been frequently used for the fabrication of numerous micro- and nanopatterned functional surfaces. In classical BF, reliable design of the final pattern has been hindered by the dual role of solvent evaporation to initiate/control the dropwise condensation and induce polymerization, alongside the complex effects of local humidity and temperature influence. Herein, we provide a deterministic method for reliable control of BF pore diameters over a wide range of length scales and environmental conditions. To this end, we employ an adapted methodology that decouples cooling from polymerization by using a combination of initiative cooling and quasi-instantaneous UV curing to deliberately arrest the desired BF patterns in time. Through in situ real-time optical microscopy analysis of the condensation kinetics, we demonstrate that an analytically predictable self-similar regime is the predominant arrangement from early to late times O(10-100 s), when high-density condensation nucleation is initially achieved on the polymer films. In this regime, the temporal growth of condensation droplets follows a unified power law of D ∝ t. Identification and quantitative characterization of the scale-invariant self-similar BF regime allow fabrication of programmed pore size, ranging from hundreds of nanometers to tens of micrometers, at high surface coverage of around 40%. Finally, we show that temporal arresting of BF patterns can be further extended for selective surface patterning and/or pore size modulation by spatially masking the UV curing illumination source. Our findings bridge the gap between fundamental knowledge of dropwise condensation and applied breath figure patterning techniques, thus enabling mechanistic design and fabrication of porous materials and interfaces.

Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition.

Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0-60 °C), on the 20 wt% SDS micellar solutions with varying DDAO (⩽5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations.

A microfluidic-multiwell platform for rapid phase mapping of surfactant solutions.

Measurement of the phase behavior and (meta)stability of liquid formulations, including surfactant solutions, is required for the understanding of mixture thermodynamics, as well as their practical utilization. We report a microfluidic platform with a stepped temperature profile, imposed by a dual Peltier module, connected to an automated multiwell plate injector and optical setup, for rapid solution phase mapping. The measurement protocol is defined by the temperature step ΔT ≡ T1 - T2 (≲100 °C), volumetric flow rate Q ≡ ΔV/Δt (≲50 µl/min), which implicitly set the thermal gradient ΔT/Δt (≃0.1-50 °C/min), and measurement time (which must exceed the intrinsic timescale of the relevant phase transformation). Furthermore, U-shaped microchannels can assess the reversibility of such transformations, yielding a facile measurement of the metastable zone width of the phase diagram. By contrast with traditional approaches, the platform precisely controls the cooling and heating rates by tuning the flow rate, and the absolute temperature excursion by the hot and cold thermal profile, which remain stationary during operation, thus allowing the sequential and reproducible screening of large sample arrays. As a model system, we examined the transition from the micellar (L1) to the liquid crystalline lamellar phase (Lα), upon cooling, of aqueous solutions of sodium linear alkylbenzene sulfonate, a biodegradable anionic surfactant extensively employed in industry. Our findings are validated with quiescent optical microscopy and small angle neutron scattering data.

A Two-Step Approach for Diagnosing Glutamate Dehydrogenase Genes by Conventional Polymerase Chain Reaction from Clostridium difficile Isolates.

BACKGROUND Clostridium difficile is the major causative agent of nosocomial antibiotic-associated colitis. The gold standard for C. difficile detection is stool culture followed by cytotoxic assay, although it is laborious and time-consuming. We developed a screening test based on a two-step conventional polymerase chain reaction (PCR) approach to detect gluD, the glutamate dehydrogenase (GDH) enzyme gene, which is a marker for screening of C. difficile. Targeting gluD comparing to the conserved stable genetic element of pathogenicity locus (PaLoc), with an accessory gene of Cdd3, was an effective method for the detection of this pathogen from patients with enterocolitis. METHODS Fresh fecal samples of the patients who were clinically suspicious for antibiotic-associated colitis were collected. Stool specimens were cultured on the cycloserine-cefoxitin fructose agar (CCFA) in an anaerobic condition, following alcohol shock treatment and enrichment in Clostridium difficile Brucella broth (CDBB). On confirmed colonies, PCR was carried out for detection of PaLoc subsidiary gene, Cdd3, and toxicogenic genes, tcdA and tcdB. The gluD that is GDH gene detection was performed by conventional PCR on the extracted DNA from 578 fresh stool samples. RESULTS 57 (9.8%) strains of C. difficile were approved by conventional PCR for gluD and Cdd3 genes, in which 37 (6.4%) colonies had tcdA+/tcdB+ genotype, 2 (0.3%) tcdA+/tcdB-, 4 (0.7%) tcdA-/ tcdB+ and the remaining 14 (2.4%) colonies were tcdA and tcdB negative. CONCLUSION These results demonstrate that targeting gluD by PCR is quite promising for rapid detection of C. difficile from fresh fecal samples. Furthermore, the multiple-gene analysis for tcdA and tcdB assay proved a reliable approach for diagnosing of toxigenic strains among clinical samples.

Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS).

We report the spontaneous formation of multilamellar vesicles (MLVs) from low concentration (<30 wt%) aqueous micellar solutions of sodium linear alkylbenezene sulfonate (NaLAS) upon cooling, employing a combination of optical microscopy (OM), Small Angle Neutron Scattering (SANS), and Cryo-TEM. Upon cooling, MLVs grow from, and coexist with, the surfactant micelles, attaining diameters ranging from hundreds of nanometers to a few micrometers depending on the cooling rate, whilst the d-spacing of internal lamellae remains unchanged, at ≃ 3 nm. While microscale fluid and flow properties of the mixed MLVs and micellar phase depend on rate of cooling, the corresponding nanoscale structure of the surfactant aggregates, resolved by time-resolved SANS, remains unchanged. Our data indicate that the mixed MLV and micellar phases are in thermodynamic equilibrium with a fixed relative volume fraction determined by temperature and total surfactant concentration. Under flow, MLVs aggregate and consequently migrate away from the channel walls, thus reduce the overall hydrodynamic resistance. Our findings demonstrate that the molecular and mesoscopic structure of ubiquitous, low concentration NaLAS solutions, and in turn their flow properties, are dramatically influenced by temperature variation about ambient conditions.