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Written by Grant England AP225, Fall 2011


Wetting refers to how well or poorly a liquid contacts a surface. Usually the term applies to water, where if a surface is hydrophobic it will not wet well while if it is hydrophilic it will wet well. The relative hydrophobicity or hydrophilicity of a substrate can be determined by measuring the contact angle of water with the surface. A liquid wets a surface better if it has a low contact angle with that surface. In general, if the contact angle is lower than 90 degrees, the liquid is considered to be wetting for that surface; while, if the contact angle is greater than 90 degrees, the liquid is non-wetting for the surface.

Diagram showing states for a liquid on a surface with (A) little wetting, (B) moderate wetting, and (C) high wetting. (Taken fromWikipedia Article )

Cassie and Wenzel States

Surfaces can be made to be superhydrophobic or superhydrophilic by modification to have high aspect-ratio structures (micro-posts) on their surfaces, and chemical modification of these surfaces with hydrophobic or hydrophilic functional groups. Depending on the energetics of the surface, a drop of liquid on such a surface can be in either of two states--sitting on top of the micro-posts or sitting with the micro-posts embedded within it. The former is the Cassie state and the latter is the Wenzel state.

Ways to Change Wetting

Surfactants can change the wetting properties of a liquid, since they change the energetics of the surface of the liquid. Also, changing the properties of the surface by chemical functionalization or modification of the surface (etching or other methods of attaining high aspect-ratio structures on the surface which increase the microscopic surface area) can change a liquid-solid interaction from wetting to non-wetting or vice versa.

Applications of Wetting

Wetting can be used for chemical sensing as different chemicals and concentrations result in different surface properties. The sensing result can be extracted by a photonic-crystal based colourimetric method (WICK). The chemical selectivity can be greatly enhanced by an array of different surface functionalized WICKs, which is capable of identifying liquids as an optofluidic nose.

By changing the wetting properties of a material, one can change its contact angle hysteresis. If the contact angle hysteresis of a material is extremely small, droplets will slide off at a small incline. This has large implications for anti-ice coatings.

Stamou et al. showed that nonuniform wetting can explain the long-range attraction between colloidal particles at an interface. It is responsible for favored orientations for pairs of particles and larger aggregates, and explain how certain frustrated geometries do not appear in experimental results.

See also

Wetting from Lectures for AP225.



Aizenberg Lab @Harvard: Wettability

Wikipedia Article

Keyword in references:

Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures

Critical Casimir effect in three-dimensional Ising systems: Measurements on binary wetting films

Dewetting-Induced Membrane Formation by Adhesion of Amphiphile-Laden Interface

Encoding complex wettability patterns in chemically functionalized 3D photonic crystals

Pitcher plant inspired non-stick surface

Structural Transformation by Electrodeposition on Patterned Substrates (STEPS) - A New Versatile Nanofabrication Method

Enriching libraries of high-aspect-ratio micro- or nanostructures by rapid, low-cost, benchtop nanofabrication

Steering nanofibers: An integrative approach to bio-inspired fiber fabrication and assembly

Fine-Tuning the Degree of Stem Cell Polarization and Alignment on Ordered Arrays of High-Aspect-Ratio Nanopillars

Screening Conditions for Rationally Engineered Electrodeposition of Nanostructures (SCREEN): Electrodeposition and Applications of Polypyrrole Nanofibers using Microfluidic Gradients

Combinatorial Wetting in Colour: An Optofluidic Nose

Functionalized glass coating for PDMS microfluidic devices

Elastic Instability in Growing Yeast Colonies

Liquid-infused structured surfaces with exceptional anti-biofouling performance

Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance

Wetting in Color: Colorimetric Differentiation of Organic Liquids with High Selectivity

Electric-field-induced capillary attraction between like-charged particles at liquid interfaces

Dynamics of foam drainage

Dynamic mechanisms for apparent slip on hydrophobic surfaces

Linear stability and transient growth in driven contact lines

Spreading of Droplets on a Solid Surface

The single-cell chemostat: an agarose-based, microfluidic device for high-throughput, single-cell studies of bacteria and bacterial communities

DNA molecules and configurations in a solid- state nanopore microscope

Graphene as a subnanometre trans-electrode membrane

Non-stick water

How aphids lose their marbles

Elasticity of an interfacial particle raft

Nested self-similar wrinkling patterns in skins

The ‘‘Cheerios effect’’

Soft lubrication: The elastohydrodynamics of nonconforming and conforming contacts

Capillary rise between elastic sheets

Dynamics of Surfactant-Driven Fracture of Particle Rafts

Mechanics of Interfacial Composite Materials

The Universal Dynamics of Cell Spreading

Localized and extended deformations of elastic shells

Self-Organization of a Mesoscale Bristle into Ordered, Hierarchical Helical Assemblies

Hygromorphs: from pine cones to biomimetic bilayers

Infochemistry: Encoding Information as Optical Pulses Using Droplets in a Microfluidic Device

Control of Shape and Size of Nanopillar Assembly by Adhesion-Mediated Elastocapillary Interaction

How wet paper curls

Shock-driven jamming and periodic fracture of particulate rafts

Hydrodynamics of Writing with Ink

Skating on a Film of Air: Drops Impacting on a Surface

Cationic liposome–microtubule complexes: Pathways to the formation of two-state lipid–protein nanotubes with open or closed ends

Single molecule statistics and the polynucleotide unzipping transition

Stable island arrays by height-constrained Stranski–Krastanov growth

Nanoscale Domain Stability in Organic Monolayers on Metals

Electromigration lifetime and critical void volume

Statistics of Electromigration Lifetime Analyzed Using a Deterministic Transient Model

Saturated voids in interconnect lines due to thermal strains and electromigration

Drying-induced bifurcation in a hydrogel-actuated nanostructure

Foldable Printed Circuit Boards on Paper Substrates

One-step formation of multiple emulsions in microfluidics

A new device for the generation of microbubbles

Microfluidic synthesis of advanced microparticles for encapsulation and controlled release

Monodisperse Gas-Filled Microparticles from Reactions in Double Emulsions

Patterned Colloidal Coating Using Adhesive Emulsions

Surface-Tension-Induced Synthesis of Complex Particles Using Confined Polymeric Fluids

Glass coating for PDMS microfluidic channels by sol–gel methods

Microfluidic fabrication of complex-shaped microfibers by liquid template-aided multiphase microflow

Controllable Monodisperse Multiple Emulsions

Arrested fluid-fluid phase separation in depletion systems: Implications of the characteristic length on gel formation and rheology

Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature

Surface acoustic wave (SAW) directed droplet flow in microfluidics for PDMS devices

Self-Assembled Polymer Membrane Capsules Inflated by Osmotic Pressure

Dewetting Instability during the Formation of Polymersomes from Block-Copolymer-Stabilized Double Emulsions

Suppression of instabilities in multiphase flow by geometric confinement

Double-emulsion drops with ultra-thin shells for capsule templates

Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids

Multiple Polymersomes for Programmed Release of Multiple Components

Uniform Nonspherical Colloidal Particles with Tunable Shapes

Controlled production of emulsion drops using an electric field in a flow-focusing microfluidic device

Drop-based microfluidic devices for encapsulation of single cells

Highly monodisperse conjugated polymer particles synthesized with drop-based microfluidics

Nonspherical Colloidosomes with Multiple Compartments from Double Emulsions

Electric Control of Droplets in Microfluidic Devices

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Protein Expression, Aggregation, and Triggered Release from Polymersomes as Artificial Cell-like Structures

Electric-field-induced capillary attraction between like-charged particles at liquid interfaces

Short-time self-diffusion of nearly hard spheres at an oil–water interface

High throughput production of single core double emulsions in a parallelized microfluidic device

Functional patterning of PDMS microfluidic devices using integrated chemo-masks

Drop formation in non-planar microfluidic devices

Skating on a Film of Air: Drops Impacting on a Surface

Novel surface acoustic wave (SAW)-driven closed PDMS flow chamber

Osmotic spreading of Bacillus subtilis biofilms driven by an extracellular matrix

Polymers fit for function Making emulsions drop by drop

Dewetting-Induced Membrane Formation by Adhesion of Amphiphile-Laden Interfaces

Multicompartment Polymersomes from Double Emulsions

Hierarchical Porous Materials Made by Drying Complex Suspensions

Corrugated interfaces in multiphase core-annular flow

Microfluidic Fabrication of Monodisperse Biocompatible and Biodegradable Polymersomes with Controlled Permeability

Double Emulsion Templated Monodisperse Phospholipid Vesicles

Fabrication of Polymersomes using Double-Emulsion Templates in Glass-Coated Stamped Microfluidic Devices

Dripping, Jetting, Drops, and Wetting: The Magic of Microfluidics

Monodisperse Double Emulsions Generated from a Microcapillary Device

Mechanism for clogging of microchannels

Dynamics of Drying in 3D Porous Media

The pressure drop along rectangular microchannels containing bubbles

Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature

Interfacial instabilities in a microfluidic Hele-Shaw cell

Integrated Fabrication and Magnetic Positioning of Metallic and Polymeric Nanowires Embedded in Thin Epoxy Slabs

Basic Microfluidic and Soft Lithographic Techniques

Continuously tunable microdroplet-laser in a microfluidic channel

Fabrication of Micrometer-Scale, Patterned Polyhedra by Self-Assembly

Fabrication and Wetting Properties of Metallic Half-Shells with Submicron Diameters

Chemical Force Spectroscopy in Heterogeneous Systems: Intermolecular Interactions Involving Epoxy Polymer, Mixed Monolayers, and Polar Solvents

Surface Tension-Powered Self-Assembly of Microstructures—The State-of-the-Art

Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology

Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up