Spontaneous Motion and Rotation of Acid Droplets on the Surface of a Liquid Metal.

Self-propulsion of droplets is of great significance in many fields. The spontaneous horizontal motion and self-jumping of droplets have been well realized in various ways. However, there is still a lack of an effective method to enable a droplet to rotate spontaneously and steadily. In this paper, by employing an acid droplet and a liquid metal, the spontaneous and steady rotation of droplets is achieved. For an acid droplet, it may spontaneously move when it is deposited on the surface of the liquid metal. By adjusting experimental parameters to the proper range, the self-rotation of droplet happens. This phenomenon originates from the fluctuation of the droplet boundary and the collective movement of bubbles that are generated by the chemical reactions between the acid droplet and liquid metal. This rotation has a simpler implementation method and more steady rotation state. Its angular velocity is much higher than that driven by other mechanisms. Moreover, the movements of acid droplets on the liquid metal are classified according to experimental conditions. The internal flow fields, the movements and distribution of bubbles, and the fluctuation of the droplet boundary are also explored and discussed. The theoretical model describing the rotational droplet is given. Our work may deepen the understanding of the physical system transition affected by chemical reactions and provide a new way for the design of potential applications, e.g., micro- and nanodevices.

Probing Micro-Newton Forces on Solid/Liquid/Gas Interfaces Using Transmission Phase Shift.

Many of the nature and life systems are driven by capillary interactions on solid/liquid/gas interfaces. Here, we present a profilometry technique called transmission phase shift for visualizing the liquid/gas interfaces in three dimensions with high resolution. Using this approach, we probe the change in tiny forces with particle radius at a solid/liquid/gas interface. We provide the first direct evidence that in the issues of floating versus sinking at small-scale, Archimedes' principle should be generalized to include the crucial role of surface tension and reveal the dominant regimes of floating particles based on the Bond number. Remarkably, the measured forces are in the range of micro-Newtons, suggesting that this terse methodology may guide the future design of a liquid microbalance and will be a universal tool for investigating capillarity and interface issues.

Topography-induced symmetry transition of droplets on quasi-periodically patterned surfaces.

Quasi-periodic structures of quasicrystals yield novel effects in diverse systems. However, there is little investigation on employing quasi-periodic structures in morphology control. Here, we show the use of quasi-periodic surface structures in controlling the transition of liquid droplets. Although surface structures seem random-like, we find that on these surfaces, droplets spread to well-defined 5-fold symmetric shapes and the symmetry of droplet shapes spontaneously restores during spreading, hitherto unreported in the morphology control of droplets. To obtain physical insights into these symmetry transitions, we conduct energy analysis and perform systematic experiments by varying the properties of both liquid droplets and patterned surfaces. The results show the dominant factors in determining droplet shapes to be surface topography and the self-similarity of the surface structure. Quantified results of the droplet spreading process show distinct dynamics from the spreading experiments on periodically micropatterned surfaces. Our findings significantly advance the control capability of the droplet morphology. Such a quasi-periodic patterning strategy can offer a new method to achieve complex patterns, and may be used to model patterns in the study of rough surfaces.

Dynamics of Dissolutive Wetting: A Molecular Dynamics Study.

Dissolutive wetting, i.e., dynamic wetting of a liquid droplet on dissolvable substrates, has been studied by molecular dynamics simulations. In dissolutive wetting, the geometry and properties of the solid-liquid interface evolve with the solid dissolving into the droplet; meanwhile, the droplet spreads on the receding solid surfaces. The droplets advance on the dissolvable substrate following different dynamic laws, compared with spreading on nondissolutive substrate. On the basis of molecular kinetic theory, we develop a theoretical model to reveal physical mechanisms behind the dissolutive wetting phenomena. We also find that solid particles are pulled by their hydration shells to dissolve into liquid, changing the flow field, the atomic structure, and the hydrogen bond network in the droplet. Our findings may help to comprehend the dynamics of dissolutive wetting and assist future design in practical applications.

The effect of a capillary bridge on the crack opening of a penny crack.

Young's relation is based on the equilibrium of horizontal components of surface tensions for a liquid droplet on a "rigid" substrate without addressing the substrate deformation induced by the net vertical component of surface tensions. Upon realizing the importance of wetting in controlling the integrity of flexible structures and electronics, the effect of a capillary bridge or a liquid droplet on the crack opening of a penny crack under the action of a far-field tensile stress is analyzed. Closed-form solutions are derived for both the crack opening and the stress intensity factor, which are functions of the size of the capillary bridge or the droplet, surface tension, and the contact angle. Both the capillary bridge and the droplet can introduce the crack closure. The minimum far-field tensile stresses needed for complete crack opening, i.e. no crack closure, are obtained analytically. The net vertical component of the surface tensions introduces the formation of a surface ridge on the crack face at the edge of the droplet for an open crack. The amplitude of the surface ridge increases with the increase of the net vertical component of the surface tensions and the decrease of the breadth width.

A phase field model coupling lithium diffusion and stress evolution with crack propagation and application in lithium ion batteries.

Cracking and fracture of electrodes under diffusion during lithiation and delithiation is one of the main factors responsible for short life span of lithium based batteries employing high capacity electrodes. Coupling effects among lithium diffusion, stress evolution and crack propagation have a significant effect on dynamic processes of electrodes during cycling. In this paper, a phase field model coupling lithium diffusion and stress evolution with crack propagation is established. Then the model is applied to a silicon thin film electrode to explore the coupling effects on diffusion and crack propagation paths. During lithiation, simulation results show that lithium accumulates at crack tips and the lithium accumulation further reduces the local hydrostatic stress. Single and multiple crack geometries are considered to elucidate some of the crack patterns in thin film electrodes as a consequence of coupling effects and crack interactions.

Molecular dynamics simulations of the enhanced recovery of confined methane with carbon dioxide.

For the first time, the enhanced recovery of confined methane (CH4) with carbon dioxide (CO2) is investigated through molecular dynamics simulations. The adsorption energy and configuration of CH4 and CO2 on the carbon surface were compared, which shows that CO2 is a good candidate in displacing confined CH4. The energy barrier required for displacing CH4 by CO2 injection was found to depend on the displacement angle. When CO2 approached vertically to the carbon surface, the displacement of CH4 occurred most easily. The curvature and size effects of the carbon nanopores on CH4 recovery were revealed and indicated that there exists an optimum pore size making the displacement occur most efficiently. The underlying mechanisms of these phenomena were uncovered. Our findings and related analyses may help to understand CO2 enhanced gas recovery from the atomic level and assist the future design in engineering.

Mass and Force Sensing of an Adsorbate on a Beam Resonator Sensor.

The mass sensing superiority of a micro-/nano-mechanical resonator sensor over conventional mass spectrometry has been, or at least is being firmly established. Because the sensing mechanism of a mechanical resonator sensor is the shifts of resonant frequencies, how to link the shifts of resonant frequencies with the material properties of an analyte formulates an inverse problem. Besides the analyte/adsorbate mass, many other factors, such as position and axial force, can also cause the shifts of resonant frequencies. The in situ measurement of the adsorbate position and axial force is extremely difficult if not impossible, especially when an adsorbate is as small as a molecule or an atom. Extra instruments are also required. In this study, an inverse problem of using three resonant frequencies to determine the mass, position and axial force is formulated and solved. The accuracy of the inverse problem solving method is demonstrated, and how the method can be used in the real application of a nanomechanical resonator is also discussed. Solving the inverse problem is helpful to the development and application of a mechanical resonator sensor for two reasons: reducing extra experimental equipment and achieving better mass sensing by considering more factors.

Tuning structural and mechanical properties of two-dimensional molecular crystals: the roles of carbon side chains.

A key requirement for the future applicability of molecular electronics devices is a resilience of their properties to mechanical deformation. At present, however, there is no fundamental understanding of the origins of mechanical properties of molecular films. Here we use quinacridone, which possesses flexible carbon side chains, as a model molecular system to address this issue. Eight molecular configurations with different molecular coverage are identified by scanning tunneling microscopy. Theoretical calculations reveal quantitatively the roles of different molecule-molecule and molecule-substrate interactions and predict the observed sequence of configurations. Remarkably, we find that a single Young's modulus applies for all configurations, the magnitude of which is controlled by side chain length, suggesting a versatile avenue for tuning not only the physical and chemical properties of molecular films but also their elastic properties.

Electro-capillary peeling of thin films.

Thin films are widely-used functional materials that have attracted much interest in academic and industrial applications. With thin films becoming micro/nanoscale, developing a simple and nondestructive peeling method for transferring and reusing the films remains a major challenge. Here, we develop an electro-capillary peeling strategy that achieves thin film detachment by driving liquid to percolate and spread into the bonding layer under electric fields, immensely reducing the deformation and strain of the film compared with traditional methods (reaching 86%). Our approach is evaluated via various applied voltages and films, showing active control characterizations and being appropriate for a broad range of films. Theoretically, electro-capillary peeling is achieved by utilizing the Maxwell stress to compete with the film's adhesion stress and tension stress. This work shows the great potential of the electro-capillary peeling method to provide a simple way to transfer films and facilitates valid avenues for reusing soft materials.

Hierarchical self-assembly of achiral amino acid derivatives into dendritic chiral nanotwists.

The organogel formation and self-assembly of a glycine-based achiral molecule were investigated. It has been found that the compound could gel organic solvents either at a lower temperature with lower concentration or at room temperature with higher concentration, which showed different self-assembled nanostructures. At a low temperature of -15 °C, the compound self-assembled into fibrous structures, whereas it formed distinctive flat microbelts at room temperature. When the organogel with nanofibers formed at -15 °C was brought into an ambient condition, chiral twist nanostructures were immediately evolved, which subsequently transferred to a giant microbelt through a hierarchical dendritic twist with the time. Although the compound is achiral, it formed chiral twist with both left- and right-handed twist structures simultaneously. When a trace analogical chiral trigger, L-alanine or D-alanine derivative, was added, a complete homochiral dendritic twist was obtained. Interestingly, a reverse process, i.e. the transformation of the microbelts into twists, could occur upon dilution of the organogel with microbelt structure. During the dilution, both left- and right-handed chiral twists could be formed again. Interestingly, the same branch from the microbelt formed the twist with the same handedness. A combination of the density functional theory (DFT), molecular mechanics (MM), and molecular dynamics (MD) simulations demonstrates that the temperature-induced twisting of the bilayer is responsible for the morphological transformation and evolution of the dendrite twist. This research sheds new light on the hierarchical transformation of the chiral structures from achiral molecules via controlled self-assembly.

Active control of electro-visco-fingering in Hele-Shaw cells using Maxwell stress.

Viscous fingering is an extensively observed phenomenon in porous media or Hele-Shaw cells. In general, this instability is particularly difficult to control for given fluids and geometries. Therefore, investigating a control method of viscous fingering is quite attractive. Here, we present that electro-visco-fingering (EVF) in fluids with different relative permittivity shows a controllable performance under electric fields. The theoretical model is established from the perspective of force analysis to indicate that active control of EVF is achieved by the competition between the Maxwell stress jump and hydrostatic pressure gradient. In addition, an Electric Control number (EC) is adopted to characterize the electric effect on EVF and experimentally confirmed for broad ranges of flow rates and voltages. Unlike the electro-osmotic flow, this method shows a considerable achievement in energy efficiency. Our work provides a new way to actively control viscous fingering and opens new routes for applications of interfacial instabilities.

Mode Localization and Eigenfrequency Curve Veerings of Two Overhanged Beams.

Overhang provides a simple but effective way of coupling (sub)structures, which has been widely adopted in the applications of optomechanics, electromechanics, mass sensing resonators, etc. Despite its simplicity, an overhanging structure demonstrates rich and complex dynamics such as mode splitting, localization and eigenfrequency veering. When an eigenfrequency veering occurs, two eigenfrequencies are very close to each other, and the error associated with the numerical discretization procedure can lead to wrong and unphysical computational results. A method of computing the eigenfrequency of two overhanging beams, which involves no numerical discretization procedure, is analytically derived. Based on the method, the mode localization and eigenfrequency veering of the overhanging beams are systematically studied and their variation patterns are summarized. The effects of the overhang geometry and beam mechanical properties on the eigenfrequency veering are also identified.

The Influence of Background Ultrasonic Field on the Strength of Adhesive Zones under Dynamic Impact Loads.

The influence of background ultrasonic field on the ultimate dynamic strength of adhesive joints is studied using fracture mechanics analysis. Winkler foundation-type models are applied to describe the cohesion zone, and the incubation time fracture criterion is used. The challenging task is to study whether relatively weak ultrasound is able to decrease the threshold values of the external impact load depending on a joint model, such as an "elastic membrane" or "beam" approximation, and various boundary conditions at the ends. The specific task was to investigate the case of short pulse loading through application of time-dependent fracture criterion instead of the conventional principle of critical stress. Three different load cases, namely, step constant force, dynamic pulse, and their combination with ultrasonic vibrations, were also studied. The analytical solution to the problem demonstrates that background vibrations at certain frequencies can significantly decrease threshold values of fracture impact load. Specific calculations indicate that even a weak background sonic field is enough to cause a significant reduction in the threshold amplitude of a dynamic short pulse load. Additionally, non-monotonic dependency of threshold amplitude on pulse duration for weak background field was observed, which demonstrates the existence of optimal regimes of impact energy input. Moreover, this phenomenon does not depend on the way in which the beam edges mount, whether they are clamped or hinged, and it could be applied for micro-electro-mechanical switch design processes as an additional tool to control operational regimes.

Precursor film in dynamic wetting, electrowetting, and electro-elasto-capillarity.

Dynamic wetting and electrowetting are explored using molecular dynamics simulations. The propagation of the precursor film (PF) is fast and obeys the power law with respect to time. Against the former studies, we find the PF is no slip and solidlike. As an important application of the PF, the electro-elasto-capillarity, which is a good candidate for drug delivery at the micro- or nanoscale, is simulated and realized for the first time. Our findings may be one of the answers to the Huh-Scriven paradox and expand our knowledge of dynamic wetting and electrowetting.

Simulated pathogenic conformational switch regions matched well with the biochemical findings.

Pathogenic conformational conversion is a general causation of many disease, such as transmissible spongiform encephalopathy (TSE) caused by misfolding of prion, sickle cell anemia, and etc. In such structural changes, misfolding occurs in regions important for the stability of native structure firstly. This destabilizes the normal conformation and leads to subsequent errors in folding pathway. Sites involved in the first stage can be deemed switch regions of the protein, and are vital for conformational conversion. Namely it could be a switch of disease at residue level. Here we report an algorithm that can identify such sites computationally with an accuracy of 93%, by calculating the probability of the native structure of a short segment jumping to a mistake one. Knowledge of such switch sites could be used to target clinical therapy, study physiological and pathologic mechanism of protein, and etc.

Geomaterials Evaluation: A New Application of Ashby Plots.

The development of fossil energy resources and the occurrence of geological hazards call for a quick and effective identification of geological situations. In this study, we present rapid evaluations of geological structures from the pure point of view of material properties. For the first time, Ashby plots are applied to the evaluation of geomaterials such as rocks and coals. A series of case studies are presented and related Ashby plots are drawn. The stability of rocks facing natural hazards is analyzed and compared; the stability of coals formed in different periods in China is studied; and a new brittleness index for reservoir rocks is proposed. The Ashby plots show a strong vitality and a wide application prospect in geomaterial evaluation and geological engineering.

Hydroelectric voltage generation based on water-filled single-walled carbon nanotubes.

A DFT/MD mutual iterative method was employed to give insights into the mechanism of voltage generation based on water-filled single-walled carbon nanotubes (SWCNTs). Our calculations showed that a constant voltage difference of several mV would generate between the two ends of a carbon nanotube, due to interactions between the water dipole chains and charge carriers in the tube. Our work validates this structure of a water-filled SWCNT as a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device at the atomic level.

Donut-shaped fingerprint in homologous polypeptide relationships--a topological feature related to pathogenic structural changes in conformational disease.

Features of homologous relationship of proteins can provide us a general picture of protein universe, assist protein design and analysis, and further our comprehension of the evolution of organisms. Here we carried out a study of the evolution of protein molecules by investigating homologous relationships among residue segments. The motive was to identify detailed topological features of homologous relationships for short residue segments in the whole protein universe. Based on the data of a large number of non-redundant proteins, the universe of non-membrane polypeptide was analyzed by considering both residue mutations and structural conservation. By connecting homologous segments with edges, we obtained a homologous relationship network of the whole universe of short residue segments, which we named the graph of polypeptide relationships (GPR). Since the network is extremely complicated for topological transitions, to obtain an in-depth understanding, only subgraphs composed of vital nodes of the GPR were analyzed. Such analysis of vital subgraphs of the GPR revealed a donut-shaped fingerprint. Utilization of this topological feature revealed the switch sites (where the beginning of exposure of previously hidden "hot spots" of fibril-forming happens, in consequence a further opportunity for protein aggregation is provided; 188-202) of the conformational conversion of the normal alpha-helix-rich prion protein PrP(C) to the beta-sheet-rich PrP(Sc) that is thought to be responsible for a group of fatal neurodegenerative diseases, transmissible spongiform encephalopathies. Efforts in analyzing other proteins related to various conformational diseases are also introduced.

A scheme for multiple sequence alignment optimization--an improvement based on family representative mechanics features.

As a basic tool of modern biology, sequence alignment can provide us useful information in fold, function, and active site of protein. For many cases, the increased quality of sequence alignment means a better performance. The motivation of present work is to increase ability of the existing scoring scheme/algorithm by considering residue-residue correlations better. Based on a coarse-grained approach, the hydrophobic force between each pair of residues is written out from protein sequence. It results in the construction of an intramolecular hydrophobic force network that describes the whole residue-residue interactions of each protein molecule, and characterizes protein's biological properties in the hydrophobic aspect. A former work has suggested that such network can characterize the top weighted feature regarding hydrophobicity. Moreover, for each homologous protein of a family, the corresponding network shares some common and representative family characters that eventually govern the conservation of biological properties during protein evolution. In present work, we score such family representative characters of a protein by the deviation of its intramolecular hydrophobic force network from that of background. Such score can assist the existing scoring schemes/algorithms, and boost up the ability of multiple sequences alignment, e.g. achieving a prominent increase (approximately 50%) in searching the structurally alike residue segments at a low identity level. As the theoretical basis is different, the present scheme can assist most existing algorithms, and improve their efficiency remarkably.