Research

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16 pages, 1566 KiB

Open AccessFeature PaperArticle

Spreading of Micrometer-Sized Droplets under the Influence of Insoluble and Soluble Surfactants: A Numerical Study

by Thomas Antritter, Peter Hachmann, Tatiana Gambaryan-Roisman, Bernhard Buck and Peter Stephan

Colloids Interfaces 2019, 3(3), 56; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3030056 - 09 Aug 2019

Cited by 1 | Viewed by 3773

Wetting and spreading of surfactant solutions play an important role in many technical applications. In printing processes, the size of individual droplets is typically on the order of a few tens of microns. The purpose of this study is to develop a better [...] Read more.

Wetting and spreading of surfactant solutions play an important role in many technical applications. In printing processes, the size of individual droplets is typically on the order of a few tens of microns. The purpose of this study is to develop a better understanding of the interaction between spreading and surfactant transport on these small length and related time scales. Therefore, numerical simulations based on the volume-of-fluid method including Marangoni stresses and transport of an insoluble or soluble surfactant are performed. The results for an insoluble surfactant show competing effects of Marangoni flow on the one hand, and a decreasing surfactant concentration as the droplet spreads on the other hand. Even in the case of a soluble surfactant, adsorption and desorption could only partly mitigate these effects, demonstrating the importance of the sorption kinetics for fast, small scale wetting processes. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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14 pages, 3489 KiB

Open AccessFeature PaperArticle

One- and Two-Dimensional NMR in Studying Wood–Water Interaction at Moisturizing Spruce. Anisotropy of Water Self-Diffusion

by Victor V. Rodin

Colloids Interfaces 2019, 3(3), 54; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3030054 - 02 Aug 2019

Cited by 3 | Viewed by 3791

Abstract

This paper examines how wetting the surface of wood affects characteristics of wood materials. An important question is how moisturizing wood has an effect on diffusion parameters of water, which will change conditions of the technological treatment of material. A fibrous structure of [...] Read more.

This paper examines how wetting the surface of wood affects characteristics of wood materials. An important question is how moisturizing wood has an effect on diffusion parameters of water, which will change conditions of the technological treatment of material. A fibrous structure of wood can result in different diffusivities of water in the perpendicular direction and along the wood fibers. The work explores how 1- and 2-dimensional NMR with pulsed field gradients (PFG) highlights an anisotropic diffusion of water when moisturizing spruce wood. The methods applied: T₂-relaxation (CPMG) measurements with the application of inverse Laplace transform (ILT), cross-relaxation experiments (Goldman–Shen pulse sequence), 1D PFG NMR on oriented wood pieces or applying gradients in various orientation, and 2D diffusion-diffusion correlation spectroscopy (DDCOSY) with two pairs of colinear gradient pulses. The results showed anisotropic restricted diffusion correlating the size of tracheid cells. The experimental 2D diffusion-diffusion correlation maps were compared with model calculations based on parameters of 2D experiment on spruce and the theory of 2D DDCOSY with ILT. Moisturizing spruce wood resulted in anisotropic diffusion coefficient which can be monitored in 2D NMR to discover different diffusion coefficients of water along the axis of wood fibers and in orthogonal direction. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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13 pages, 5384 KiB

Open AccessFeature PaperArticle

Superspreading on Hydrophobic Substrates: Effect of Glycerol Additive

by Nina M. Kovalchuk, Jacques Dunn, Jack Davies and Mark J. H. Simmons

Colloids Interfaces 2019, 3(2), 51; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3020051 - 31 May 2019

Cited by 11 | Viewed by 8234

Abstract

The spreading of solutions of three trisiloxane surfactants on two hydrophobic substrates, polyethylene and polyvinylidenefluoride, was studied with the addition of 0–40 mass % of glycerol. It was found that all the surfactant solutions spread faster than silicone oil of the same viscosity, [...] Read more.

The spreading of solutions of three trisiloxane surfactants on two hydrophobic substrates, polyethylene and polyvinylidenefluoride, was studied with the addition of 0–40 mass % of glycerol. It was found that all the surfactant solutions spread faster than silicone oil of the same viscosity, confirming the existence of a mechanism which accelerates the spreading of the surfactant solutions. For the non-superspreading surfactant, BT-233, addition of glycerol improved the spreading performance on polyvinylidenefluoride and resulted in a transition from partial to complete wetting on polyethylene. The fastest spreading was observed for BT-233 at a concentration of 2.5 g/L, independent of glycerol content. For the superspreading surfactants, BT-240 and BT-278, the concentration at which the fastest spreading occurs systematically increased with concentration of glycerol on both substrates from 1.25 g/L for solutions in water to 10 g/L for solutions in 40% glycerol/water mixture. Thus, the surfactant equilibration rate (and therefore formation of surface tension gradients) and Marangoni flow are important components of a superspreading mechanism. De-wetting of the solutions containing glycerol, once spread on the substrates, resulted in the formation of circular drop patterns. This is in contrast to the solely aqueous solutions where the spread film shrank due to evaporation, without any visible traces being left behind. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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7 pages, 238 KiB

Open AccessArticle

Anomalously Slow Dewetting of Colloidal Particles at a Liquid/Gas Interface

by Alexander Nepomnyashchy

Colloids Interfaces 2019, 3(1), 26; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3010026 - 08 Feb 2019

Viewed by 2495

Abstract

We consider the anomalously slow dewetting of colloidal particles adsorbed at a liquid/gas interface. The particles move in the vertical direction under the action of a regular capillary force and an irregular force caused by defects (roughness and chemical heterogeneity). The particle diffusion [...] Read more.

We consider the anomalously slow dewetting of colloidal particles adsorbed at a liquid/gas interface. The particles move in the vertical direction under the action of a regular capillary force and an irregular force caused by defects (roughness and chemical heterogeneity). The particle diffusion is modeled by a random walk over a potential minima with jump rates determined by the Arrhenius law. The averaged particle motion is found under the assumption of Gaussian distributions for characteristic properties of spatial heterogeneities. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

9 pages, 1790 KiB

Open AccessArticle

Wetting and Spreading of Commercially Available Aqueous Surfactants on Porous Materials

by Phillip Johnson, Toby Routledge, Anna Trybala, Mauro Vaccaro and Victor Starov

Colloids Interfaces 2019, 3(1), 14; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3010014 - 21 Jan 2019

Cited by 12 | Viewed by 3305

Abstract

The wetting properties of aqueous solutions of a commercially available surfactant at various concentrations on porous media are investigated using the KRUSS DSA100 shape analyzer and the ADVANCED software to process the data. Time evolution of both the contact angle and drop base [...] Read more.

The wetting properties of aqueous solutions of a commercially available surfactant at various concentrations on porous media are investigated using the KRUSS DSA100 shape analyzer and the ADVANCED software to process the data. Time evolution of both the contact angle and drop base diameter at each surfactant concentration after deposition were monitored. Three different porous substrates (sponges) were examined. The sponges used were a car sponge, dish sponge and audio sponge. The sponges were investigated both dry and at different degrees of saturation, that is, the amount of water absorbed into the sponge. It was found that pure distilled water droplets deposited on the dry porous media showed non-wetting. However, if droplets of surfactant solutions were deposited, then a change to a complete wetting case was found at all surfactant concentrations used. It has been observed that for all sponges, no matter the degree of saturation, they display a minimum contact angle after which the droplet is rapidly absorbed into the porous media. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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15 pages, 4018 KiB

Open AccessArticle

Facile Fabrication of Multifunctional ZnO Urchins on Surfaces

by Abinash Tripathy, Patryk Wąsik, Syama Sreedharan, Dipankar Nandi, Oier Bikondoa, Bo Su, Prosenjit Sen and Wuge H. Briscoe

Colloids Interfaces 2018, 2(4), 74; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids2040074 - 14 Dec 2018

Cited by 6 | Viewed by 4801

Abstract

Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed,

μ

m-long, tapered nanoneedles radiating from the [...] Read more.

Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed,

μ

m-long, tapered nanoneedles radiating from the urchin center. The ZnO urchin structures were successfully formed on several different substrates with high surface density and coverage, including silicon (Si), glass, polydimethylsiloxane (PDMS), and copper (Cu) sheets, as well as Si seeded with ZnO nanocrystals. Time-resolved SEM revealed growth kinetics of the ZnO nanostructures on Si, capturing the emergence of “infant” urchins at the early growth stage and subsequent progressive increases in the urchin nanoneedle length and density, whilst the spiky nanoneedle morphology was retained throughout the growth.

ε

-Zn(OH)₂ orthorhombic crystals were also observed alongside the urchins. The crystal structures of the nanostructures at different growth times were confirmed by synchrotron X-ray diffraction measurements. On seeded Si substrates, a two-stage growth mechanism was identified, with a primary growth step of vertically aligned ZnO nanoneedle arrays preceding the secondary growth of the urchins atop the nanoneedle array. The antibacterial, anti-reflective, and wetting functionality of the ZnO urchins—with spiky nanoneedles and at high surface density—on Si substrates was demonstrated. First, bacteria colonization was found to be suppressed on the surface after 24 h incubation in gram-negative Escherichia coli (E. coli) culture, in contrast to control substrates (bare Si and Si sputtered with a 20 nm ZnO thin film). Secondly, the ZnO urchin surface, exhibiting superhydrophilic property with a water contact angle

~ 0 °

, could be rendered superhydrophobic with a simple silanization step, characterized by an apparent water contact angle

θ

of 159° ± 1.4° and contact angle hysteresis

Δ θ

< 7°. The dynamic superhydrophobicity of the surface was demonstrated by the bouncing-off of a falling 10

μ L

water droplet, with a contact time of 15.3 milliseconds (ms), captured using a high-speed camera. Thirdly, it was shown that the presence of dense spiky ZnO nanoneedles and urchins on the seeded Si substrate exhibited a reflectance R < 1% over the wavelength range λ = 200–800 nm. The ZnO urchins with a unique morphology fabricated via a simple route at room temperature, and readily implementable on different substrates, may be further exploited for multifunctional surfaces and product formulations. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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Review

Jump to: Research

17 pages, 2081 KiB

Open AccessReview

Dependency of Contact Angles on Three-Phase Contact Line: A Review

by H. Yildirim Erbil

Colloids Interfaces 2021, 5(1), 8; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids5010008 - 01 Feb 2021

Cited by 31 | Viewed by 5219

Abstract

The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there [...] Read more.

The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there are still some unanswered questions. In the last two decades, it was experimentally proven that the advancing and receding contact angles, and the contact angle hysteresis of rough and chemically heterogeneous surfaces, are determined by interactions of the liquid and the solid at the three-phase contact line alone, and the interfacial area within the contact perimeter is irrelevant. However, confusion and misunderstanding still exist in this field regarding the relationship between contact angle and surface roughness and chemical heterogeneity. An extensive review was published on the debate for the dependence of apparent contact angles on drop contact area or the three-phase contact line in 2014. Following this old review, several new articles were published on the same subject. This article presents a review of the novel articles (mostly published after 2014 to present) on the dependency of contact angles on the three-phase contact line, after a short summary is given for this long-lasting debate. Recently, some improvements have been made; for example, a relationship of the apparent contact angle with the properties of the three-phase line was obtained by replacing the solid–vapor interfacial tension term, γ_SV, with a string tension term containing the edge energy, γ_SLV, and curvature of the triple contact line, k_m, terms. In addition, a novel Gibbsian thermodynamics composite system was developed for a liquid drop resting on a heterogeneous multiphase and also on a homogeneous rough solid substrate at equilibrium conditions, and this approach led to the same conclusions given above. Moreover, some publications on the line energy concept along the three-phase contact line, and on the “modified” Cassie equations were also examined in this review. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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16 pages, 2637 KiB

Open AccessReview

Mammalian Cell Behavior on Hydrophobic Substrates: Influence of Surface Properties

by Michele Ferrari, Francesca Cirisano and M. Carmen Morán

Colloids Interfaces 2019, 3(2), 48; https://0-doi-org.brum.beds.ac.uk/10.3390/colloids3020048 - 07 May 2019

Cited by 141 | Viewed by 10097

Abstract

The influence of different surface properties holding to a modification of the substrate towards hydrophobic or superhydrophobic behavior was reviewed in this paper. Cell adhesion, their communication, and proliferation can be strongly manipulated, acting on interfacial relationship involving stiffness, surface charge, surface chemistry, [...] Read more.

The influence of different surface properties holding to a modification of the substrate towards hydrophobic or superhydrophobic behavior was reviewed in this paper. Cell adhesion, their communication, and proliferation can be strongly manipulated, acting on interfacial relationship involving stiffness, surface charge, surface chemistry, roughness, or wettability. All these features can play mutual roles in determining the final properties of biomedical applications ranging from fabrics to cell biology devices. The focus of this work is the mammalian cell viability in contact with moderate to highly water repellent coatings or materials and also in combination with hydrophilic areas for more specific application. Few case studies illustrate a range of examples in which these surface properties and design can be fruitfully matched to the specific aim. Full article

(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)

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