103 research outputs found
Monostable Super Antiwettability
Super-antiwettability is an extreme situation of wetting where liquids stay
at the tops of rough surfaces, in the so-called Cassie state1. Owing to the
dramatic reduction of solid/liquid contact, it has many applications, such as
antifouling2,3, droplet manipulation4,5, and self-cleaning6-9. However,
super-antiwettability is often destroyed by impalement transitions caused by
environmental disturbances10-16 while inverse transitions without energy input
have never been observed12,17-21. Here we show through controlled experiments
that there is a "monostable" region in the phase space of the receding contact
angle and roughness parameters where transitions between (impaled) Wenzel and
Cassie states can be reversible. We describe the transition mechanism and
establish a simple criterion that predicts the experimentally observed
Wenzel-to-Cassie transitions for different liquids placed on
micropost-patterned substrates. These results can guide for designing and
engineering robust super-antiwetting surfaces.Comment: 12 pages, 4 figure
Control of Surface Wettability via Strain Engineering
Reversible control of surface wettability has wide applications in
lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a
graphene sheet as a sample material and molecular dynamic (MD) simulations, we
demonstrate that strain engineering can serve as an effective way to control
the surface wettability. The contact angles of water droplets on a
graphene vary from 72.5 to 106 under biaxial strains ranging
from -10% to 10% that are applied on the graphene layer. For an intrinsic
hydrophilic surface (at zero strain), the variation of upon the
applied strains is more sensitive, i.e., from 0 to 74.8.
Overall the cosines of the contact angles exhibit a linear relation with
respect to the strains. In light of the inherent dependence of the contact
angle on liquid-solid interfacial energy, we develop an analytic model to show
the as a linear function of the adsorption energy of a
single water molecule over the substrate surface. This model agrees with our MD
results very well. Together with the linear dependence of on biaxial
strains, we can thus understand the effect of strains on the surface
wettability. Thanks to the ease of reversibly applying mechanical strains in
micro/nano-electromechanical systems (MEMS/NEMS), we believe that strain
engineering can be a promising means to achieve the reversibly control of
surface wettability.Comment: Submitted to Physical Review E on September 17, 2012, manuscript ID:
EW1084
An Irreducible Function Basis of Isotropic Invariants of A Third Order Three-Dimensional Symmetric Tensor
In this paper, we present an eleven invariant isotropic irreducible function
basis of a third order three-dimensional symmetric tensor. This irreducible
function basis is a proper subset of the Olive-Auffray minimal isotropic
integrity basis of that tensor. The octic invariant and a sextic invariant in
the Olive-Auffray integrity basis are dropped out. This result is of
significance to the further research of irreducible function bases of higher
order tensors
Superlubric Nanogenerators with Superb Performances
Nanogenerators promise self-powered sensors and devices for extensive
applications in internet of things, sensor networks, big data, personal
healthcare systems, artificial intelligence, et al. However, low electric
current densities and short product lifespans have blocked nanogenerators'
applications. Here we show that structural superlubricity, a state of nearly
zero friction and wear between two contacted solid surfaces, provides a
revolutionary solution to the above challenge. We investigate three types of
superlubric nanogenerators (SLNGs), namely the capacitor-based, triboelectric,
and electret-based SLNGs, and systematically analyze the influences of material
and structural parameters to these SLNGs' performances. We demonstrate that
SLNGs can achieve not only enduring lifespans, but also superb performances -
three orders of magnitude in current densities and output powers higher than
those of conventional nanogenerators. Furthermore, SLNGs can be driven by very
weak external loads (down to ~1 N) in very low frequencies (down to ~1
Hz), and are thus capable to harvest electric energies from an extremely
board spectrum of environments and biosystems. Among the three types of SLNGs,
the capacitor-based is synthetically most competitive in the senses of
performance, fabrication and maintaining. These results can guide designs and
accelerate fabrications of SLNGs toward real applications.Comment: 14 pages, 4 figure
Curvature Gradient Driving Droplets in Fast Motion
Earlier works found out spontaneous directional motion of liquid droplets on
hydrophilic conical surfaces, however, not hydrophobic case. Here we show that
droplets on any surface may take place spontaneous directional motion without
considering contact angle property. The driving force is found to be
proportional to the curvature gradient of the surface. Fast motion can be lead
at surfaces with small curvature radii. The above discovery can help to create
more effective transportation technology of droplets, and better understand
some observed natural phenomena.Comment: 18 pages, 7 figure
Rotational instability in superlubric joints
Surface and interfacial energies play important roles in a number of
instability phenomena in liquids and soft matters, but are rare to play a
similar role in solids. Here we report a new type of mechanical instabilities
that are controlled by surface and interfacial energies and are valid for a
large class of materials, in particular two-dimensional layered materials. When
sliding a top flake cleaved from a square microscale graphite mesa by using a
probe acted on the flake through a point contact, we observed that the flake
moved unrotationally for a certain distance before it suddenly transferred to a
rotating-moving state. The theoretical analysis that agrees well with the
experimental observation reveals that this mechanical instability is an
interesting effect of the structural superlubricity (a state of nearly zero
friction). Our further analysis shows that this type of instability holds
generally for various sliding joints on different scales, as long as the
friction is ultralow. Thus, the uncovered mechanism provides useful knowledge
for manipulating and controlling all these sliding joints, and can guide design
of future structural superlubricity based devices.Comment: 9 pages, 4 figure
Drop Impact on Two-Tier Monostable Superrepellent Surfaces
Superrepellency is a favorable non-wetting situation featured by a
dramatically reduced solid/liquid contact region with extremely low adhesion.
However, drop impact often brings out a notable extension of the contact region
associated with rather enhanced water affinity, such renders irreversible
breakdowns of superhydrophobicity. Here, we report an alternative outcome, a
repeated Cassie-Wenzel-Cassie (CWC) wetting state transition in the microscale
occurs when a drop impacts a two-tier superhydrophobic surface, which exhibits
a striking contrast to the conventional perspective. Influences of material
parameters on the impact dynamics are quantified. We demonstrate that
self-cleaning and dropwise condensation significantly benefit from this outcome
- dirt particles or small droplets in deep textures can be taken away through
the transition. The results reported in this study allows us to promote the
strategy to design functional superrepellency materials.Comment: 20 pages, 5 figure
Frictional scattering and frictional waveguides: achieving persistent superlubricity at high velocity on the nanoscale
Nanomechanical devices can operate at much higher speeds than their
macroscopic analogues, due to low inertia. For example, peak speeds >100m/s
have been predicted for carbon nanotube devices. This stimulates our interest
in the atomic-scale physics of friction at high velocity. Here we study a model
nanosystem consisting of a graphene flake moving freely on a graphite substrate
at >100m/s. Using molecular dynamics we discover that ultra-low friction, or
superlubricity, is punctuated by high-friction transients as the flake rotates
through successive crystallographic alignments with the substrate. We term this
phenomenon frictional scattering and show that it is mathematically analogous
to Bragg scattering. We also show that frictional scattering can be eliminated
by using graphitic nanoribbons as frictional waveguides to constrain the flake
rotation, thus achieving persistent superlubricity. Finally, we propose an
experimental method to study nanoscale high-velocity friction. These results
may guide the design of efficient high-frequency nanomechanical devices
Accurate Measurement of the Cleavage Energy of Graphite
The basal plane cleavage energy (CE) of graphite is a key material parameter
for understanding many of the unusual properties of graphite, graphene, and
carbon nanotubes. The CE is equal to twice the surface energy and is closely
related to the interlayer binding energy and exfoliation energy of graphite.
Nonetheless, a wide range of values for these properties have been reported and
no consensus has yet emerged as to their magnitude. Here, we report the first
direct, accurate experimental measurement of the CE of graphite using a novel
method based on the recently discovered self-retraction phenomenon in graphite.
The measured value, 0.37 +/- 0.01 J/m2 for the incommensurate state of
bicrystal graphite, is nearly invariant with respect to temperature (from
22{\deg}C to 198{\deg}C) and bicrystal twist angle, and insensitive to
impurities (from the atmosphere). The cleavage energy for the ideal ABAB
graphite stacking, 0.39 +/- 0.02 J/m2, is calculated based upon a combination
of the measured CE and a theoretical calculation. These experimental
measurements are ideal for use in evaluating the efficacy of competing
theoretical approaches
Interlayer shear strength of single crystalline graphite
Reported values (0.2 MPa ~ 7.0 GPa) of the interlayer shear strength (ISS) of
graphite are very dispersed. The main challenge to obtain a reliable value of
ISS is the lack of precise experimental methods. Here we present a novel
experimental approach to measure the ISS, and obtain the value as 0.14 GPa. Our
result can serve as an important basis for understanding mechanical behavior of
graphite or graphene-based materials.Comment: 11 pages, 5 figure
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