Researchers solve mystery of attractive surfaces

August 2, 2006
Researchers solve mystery of attractive surfaces
Panel A shows the tip/substrate position just prior to cavitation, which is shown ~33 msec later in Panel B. Panel C shows the cavity meniscus, during tip retraction, one frame prior to its unstable collapse leaving a cavity “bubble” behind on both the tip and substrate. These bubbles, attributed to air supplied from water and the porous superhydrophobic (SH) surface, are unstable and readsorbed in approximately six seconds. In all frames the circular image at the bottom is the reflection of the spherical 150-µm diameter SH tip from the flat SH surface.

When smooth surfaces that hate water approach each other underwater, scientists have observed that they snap into contact. This is apparently due to attractive forces that extend for tens to hundreds of nanometers.

But action over these distances — though small to us — are unexplainable by conventional theories, which find no standard force sufficiently long-ranged to accomplish this task.

The action is of considerable possible importance. Long-range attractions between hydrophobic surfaces might help guide the complex folding of proteins, for example, from their initial passive clothesline-like shape into the active fist-like formations upon which life depends.

In a paper published this Thursday in the journal Nature, Sandia National Laboratories researchers were able to increase the long-ranged attraction from nanometers to microns by inserting rough hydrophobic surfaces in place of smooth ones. They also were able to slow the reaction down, enabling them to measure the attraction and visually observe its origin — a cavitation called a vapor bubble that bridges the gap between the submerged surfaces.

These experiments offer new insight into the long-range attractions that encourage hydrophobic surfaces to snap together under water. The improved observation led the group to conclude that cavitation may be responsible in general for the hydrophobic interactions that exceed the known range of van der Waals or electrostatic forces.

The new test conditions were effected, first, by using rough, so-called superhydrophobic surfaces rather than conventional smooth hydrophobic surfaces.

Superhydrophobic surfaces, on which water droplets roll like marbles, can be formed simply by evaporating liquid from a silica solution in an assembly process developed by Sandia Fellow Jeff Brinker.

The interactions of superhydrophobic materials underwater have not been studied.

“Previous experimentalists had always used smooth materials — but the common materials of nature are rough, and roughness greatly influences the interaction with water,” says Brinker.

In addition, a microscope that resists the ‘snap-together’ effect enabled the Sandia team to measure the forces involved as the surfaces closed upon each other.

The microscope, called an Interfacial Force Microscope, is similar to an Atomic Force Microscope, but a teeter-totter end piece allows the tip to maintain its distance and measure the forces acting on it rather than succumbing to them. The IFM was developed and patented under the direction of Sandia researcher Jack Houston and is now available at some universities.

Through IFM resistance, the group slowed the ‘snap’ into a longer time frame that allowed step-by-step observation of what exactly was happening in the formerly indecipherable moment.

“When force becomes overwhelming for an AFM, surfaces snap together uncontrollably,” says Houston. “The IFM just measures the force without caving in to it. We can move in as slowly as we want until we reach the point of contact.”

“There’s no other instrument that can do that,” says first author Seema Singh, who did the experimental work under direction of Brinker and Houston.

The group observed that two superhydrophobic surfaces approaching each other force the water between them to change state to a vapor, creating a cavity. This cavitation has less internal pressure, so external water pressure forces the two hydrophobic surfaces at each end of the cavity closer.

This very long-range attractive interaction may be a longer scale version of the unexplained interactions seen to-date for smooth surfaces.

The superhydrophobic material was self-assembled by simply drying a slurry of hydrophobically modified silica in a technique originally developed to create super low-density silica aerogels. During drying, the silica gel shrinks and re-expands to create a rough, rather than smooth, surface. The roughness creates a spike-like effect, causing a water drop to adopt an almost spherical shape.

“This greater hydrophobicity apparently increased the distance over which cavitation could occur, allowing it to be visually imaged for the first time,” says Sandia researcher Frank Van Swol, who calculated the theoretical cavitation distance and the energy and forces associated with cavitation.

Asked whether the observed reaction might offer some insight into the mechanisms by which proteins fold, Brinker said, “The only evidence so far for things snapping together comes from the measurements of interactions between flat smooth hydrophobic surfaces underwater. The longer-range interactions for rough surfaces may more closely represent how proteins fold, since proteins are certainly not flat surfaces.”

Rough superhydrophobic surfaces have been of much recent interest for their self-cleaning properties — the so-called Lotus effect, where rolling drops of water cleanse such surfaces of particles and parasites.

The work was sponsored by Sandia’s Laboratory-Directed Research and Development (LDRD) program, and then by DOE’s Office of Science and by the Air Force.

Source: Sandia National Laboratories

Explore further: Study reveals how nanochannels select potassium ions

Related Stories

Study reveals how nanochannels select potassium ions

August 25, 2015

(Phys.org)—One of the mysteries in biology is how cells can selectively diffuse potassium across a membrane. Biological systems rely on a delicate balance between these potassium and sodium ion concentrations in the surrounding ...

Scientists grow high-quality graphene from tea tree extract

August 21, 2015

(Phys.org)—Graphene has been grown from materials as diverse as plastic, cockroaches, Girl Scout cookies, and dog feces, and can theoretically be grown from any carbon source. However, scientists are still looking for a ...

Is graphene hydrophobic or hydrophilic?

August 18, 2015

The National Physical Laboratory's (NPL) Quantum Detection Group has just published research investigating the hydrophobicity of epitaxial graphene, which could be used in the future to better tailor graphene coatings to ...

Research clarifies the physics of water repelling surfaces

July 3, 2015

Researchers have gained valuable insights into the behaviour of water on strongly hydrophobic (water-repelling) surfaces. Understanding this behaviour should help scientists develop new types of surfaces with applications ...

Recommended for you

New nanomaterial maintains conductivity in 3-D

September 4, 2015

An international team of scientists has developed what may be the first one-step process for making seamless carbon-based nanomaterials that possess superior thermal, electrical and mechanical properties in three dimensions.

Graphene made superconductive by doping with lithium atoms

September 2, 2015

(Phys.org)—A team of researchers from Germany and Canada has found a way to make graphene superconductive—by doping it with lithium atoms. In their paper they have uploaded to the preprint server arXiv, the team describes ...

Making nanowires from protein and DNA

September 3, 2015

The ability to custom design biological materials such as protein and DNA opens up technological possibilities that were unimaginable just a few decades ago. For example, synthetic structures made of DNA could one day be ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.