ATOMS & MOLECULES

MOLECULAR ENGINEERING & SIMULATION

Background

Most of the matter around us is made up of individual elements called atoms. When atoms join together, they make larger assemblies named molecules. Molecular engineering allows modifying the structure of these assemblies, making possible creating new materials. Molecular simulation, on the other hand, is the computerized study of atoms and molecules, helping to better understand the material world around us and to systematically design new materials.

Molecular Engineering of the Surface

One of the applications of molecular engineering is modifying the surface of solid materials. Through surface engineering, various intriguing characteristics can be given to the surface of solid objects. For instance, car windshields can be made water repellent while doorknobs can be made antibacterial. Nanotubes are one group of materials that can significantly benefit from molecular engineering. The one-dimensional hollow geometry of nanotubes, like carbon nanotubes and boron nitride nanotubes, create curved crystalline sheets of matter that can respond to various surface engineering methods, such as grafting-to and grafting-from approaches.

Smart Surfaces

A smart surface – or a stimuli-responsive surface – is a surface that can change its properties in the presence of an external stimulus, such as temperature, humidity, or light. Using high precision molecular engineering techniques like atom transfer radical polymerization, large molecules with dual features can be created. Amphiphilic diblock copolymers, such as polystyrene-block-polyacrylic acid, is one of these molecules. Acting like an on-off switch, diblock copolymers can help creating smart surfaces that can change their affinity towards water when the humidity of the environment is varied.

NANOMATERIALS

NANOFABRICATION

Background

When the size of matter is within the range of 1 to 100 nm, matter can obtain various intriguing properties. For instance, gold can become green, while carbon can become harder than steel. Nanomaterials can have various shapes, ranging from one-dimensional nanopillars to two-dimensional nanofilms to three-dimensional nanoparticles.

Hierarchical Teflon AF Nanopillars

Within nanoscales, the adhesive properties of matter change. A non-sticky fluoropolymer, Teflon AF, was employed to fabricate hierarchical nanopillars that can generate large adhesion forces in both dry conditions and under water. Terminated with a fluffy top layer, both the nanopillars and the terminating layer were fabricated through a one-step molding process using aluminum oxide as the mold. Concurrent fabrication of the terminating nanostructure helped the fabrication of extremely tall nanopillars which up to the aspect-ratio of 185, neither collapsed at the tip nor bundled.

MICROMATERIALS

MICROFABRICATION

Background

Materials with the size of approximately 0.1 to 100 μm are called micromaterials.

μ.dusters

Polymeric microfibrils of controlled interfacial and geometrical properties can effectively remove micrometric and submicrometric contaminant particles from a solid surface without damaging the underlying substrate. These polymeric microfibrils are called μ.dusters. Once these microfibrils are brought into contact with a contaminated surface, because of their soft and flexible structure, they develop intimate contact with both the surface contaminants and the substrate. While these intrinsically nonsticky micropillars have minimal interfacial interactions with the substrate, they produce strong interfacial interactions with the contaminant particles, granting the detachment of the particles from the surface upon retraction of the cleaning material.

MATTER & MATTER INTERACTIONS

ADHESION & SURFACE FORCES

Background

When matter reaches matter, electromagnetic forces create an adhesive or repulsive force between them. Electromagnetic forces that act between atoms and molecules can be of different origins, including chemical connections, like covalent bonds, and physical interactions, such as van der Waals and electrostatic forces.

Gecko Adhesion & Electrostatic Forces

Geckos are the world’s supreme climbers, capable of walking on walls and ceilings. A gecko’s exceptional locomotion capabilities stem from dry adhesion. The micro/nanoscale hierarchical fibrils (setae) on the toe pads of the gecko allows them to develop an intimate contact with the substrate the animal is walking on or clinging to. Given the closeness of contact, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification phenomenon. Even so, the possibility of the occurrence of contact electrification and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos was overlooked for several decades.

LIGHT & MATTER INTERACTIONS

ANTIREFLECTION & POLARITONICS

Background

When light – electromagnetic radiation – reaches matter, it can be reflected or absorbed by it. Reflection and absorption of electromagnetic radiations is dependent on both the type of the matter and its size. Changing the structure of the surface through surface patterning is an effective approach to create antireflection and controlled reflection; i.e., coloration. Light-absorbing materials, on the other hand, can be employed to generate electromagnetic shielding surfaces, solar energy harvesting devices, and light sensors.

Antireflection

During the course of evolution, many animals and insects have developed small scale features on their body parts to regulate their interactions with electromagnetic radiations. The structural coloration of a peacock tail feathers and the antireflective eyes of a moth are some well-known examples of such structural adaptations in the nature. A rare example, however, is Grta oto, a glass wing butterfly. Unlike many butterflies that benefit from structural coloration on their wings, a Greta oto possesses transparent wings with special nanostructures covering them, giving them antireflective characteristics.

Phonon Polaritons

Nanotubes are one-dimensional, hollow crystalline nanomaterials that can carry thermal and acoustic energies at very high speeds by vibrating their atom-thick walls. Some nanotubes, like boron nitride nanotubes, can also absorb electromagnetic radiations of specific wavelength to vibrate their walls. When light particles couple with the surface vibrations of a nanotube, they create phonon polaritons. Boron nitride nanotubes can create phonon polaritons by absorbing electromagnetic radiations within the infrared region of the electromagnetic spectrum. The capacity of boron nitride nanotubes to create phonon polaritons make them ideal for the fabrication of a plethora of advanced functional materials and devices, such as light sensors and emitters.

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