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.

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.

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.

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.

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.

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.

The Constructions and Pyrolysis of 3D Kerogen Macromolecular Models: Experiments and Simulations.

Kerogens are extracted from deep shales to study pyrolysis of deep shale samples. The 2D molecular models of kerogens are obtained by a series of physical and chemical experiments by which the macromolecular models of kerogens are constructed. Then, the reasonable 3D macromolecular models are established by molecular mechanics and global energy minimization. The effects of temperature and heating rate on the chemical kinetics of kerogen pyrolysis are studied using reactive force field (ReaxFF). The hybrid molecular dynamics/force-biased Monte Carlo (MD/fbMC) approach is used to simulate the pyrolytic process at the experimental temperature, which is lower than the conventional one. The gaseous products and residues obtained by the simulations agree with the experimental results, which means a reliable simulation method for pyrolysis at experimental temperature is provided. This study constructs the rational macromolecular models of kerogen by experiments, and proposes the mechanisms of typical reactions of kerogen pyrolysis, which may help in understanding the formation of shale oil and gas.

The effect of sharp solid edges on the droplet wettability.

Understanding the role of pinning force in droplet dynamic wetting is of critical importance for surface science studies. Generally, the pinning force is only related to the surface tension and the change of contact angle. However, there is an obvious correlation between the pinning force and the surface geometry. In this paper, the relation between the surface geometry and contact line pinning was studied with systematical experiments and theoretical analysis. We compared the samples with different edge angles and carried out plenty experiments with different liquids. Meanwhile, the theoretical analysis and molecular simulation were carried out. The results show that the sharp edge has a strong pinning effect on the contact line and can significantly change the contact angle and wetting state of droplets. The maximum contact angle of droplet has a linear relation with the edge angle of substrate. The formula of pinning force was revised to consider the impact of surface topography. According to the relationship between surface defect and contact line, we proposed a model to classify the cases of contact line pinning for the first time. Our research will deepen the understanding of contact line pinning and provide help for potentially industrial production designs.

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.

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.

Statics and dynamics of electrowetting on pillar-arrayed surfaces at the nanoscale.

The statics and dynamics of electrowetting on pillar-arrayed surfaces at the nanoscale are studied using molecular dynamics simulations. Under a gradually increased electric field, a droplet is pushed by the electromechanical force to spread, and goes through the Cassie state, the Cassie-to-Wenzel wetting transition and the Wenzel state, which can be characterized by the electrowetting number at the microscale ηm. The expansion of the liquid is direction-dependent and influenced by the surface topology. A positive voltage is induced in the bulk droplet, while a negative one is induced in the liquid confined among the pillars, which makes the liquid hard to spread and further polarize. Based on the molecular kinetic theory and the wetting states, theoretical models have been proposed to comprehend the physical mechanisms in the statics and dynamics of electrowetting, and are validated by our simulations. Our findings may help to understand the electrowetting on microtextured surfaces and assist the future design of engineered surfaces in practical applications.

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.

Phase transitions of a water overlayer on charged graphene: from electromelting to electrofreezing.

We show by using molecular dynamics simulations that a water overlayer on charged graphene experiences first-order ice-to-liquid (electromelting), and then liquid-to-ice (electrofreezing) phase transitions with the increase of the charge value. Corresponding to the ice-liquid-ice transition, the variations of the order parameters indicate an order-disorder-order transition. The key to this novel phenomenon is the surface charge induced change of the orientations of water dipoles, which leads to the change of the water-water interactions from being attractive to repulsive at a critical charge value qc. To further uncover how the orientations of water dipoles influence the interaction strength between water molecules, a theoretical model considering both the Coulomb and van der Waals interactions is established. The results show that with the increase of the charge value, the interaction strength between water molecules decreases below qc, then increases above qc. These two inverse processes lead to electromelting and electrofreezing, respectively. Combining this model with the Eyring equation, the diffusion coefficient is obtained, the variation of which is in qualitative agreement with the simulation results. Our findings not only expand our knowledge of the graphene-water interface, but related analyses could also help recognize the controversial role of the surface charge or electric field in promoting phase transitions of water.

Kinetic behaviour of the cells touching substrate: the interfacial stiffness guides cell spreading.

To describe detailed behaviour of cell spreading under the influence of substrate stiffness, A549 cells cultured on the surfaces of polydimethylsiloxane (PDMS) and polyacrylamide (PAAm) with bulk rigidities ranging from 0.1 kPa to 40 kPa were in situ observed. The spreading behaviour of cells on PAAm presented a positive correlation between spreading speed and substrate stiffness. After computing the deformations of PAAm gels and collagen, the bulk stiffness of PAAm, rather than matrix tethering, determined the cell behaviour. On the other hand, spreading behaviour of the cells was unaffected by varying the bulk stiffness of PDMS. Based on simulation analyses, the elasticity of silica-like layer induced by UV radiation on PDMS surface dominated cell-substrate interaction, rather than the bulk stiffness of the material, indicating that it is the interfacial stiffness that mainly guided the cell spreading. And then the kinetics of cell spreading was for the first time modeled based on absolute rate theory.

Wetting on flexible hydrophilic pillar-arrays.

Dynamic wetting on the flexible hydrophilic pillar-arrays is studied using large scale molecular dynamics simulations. For the first time, the combined effect of the surface topology, the intrinsic wettability and the elasticity of a solid on the wetting process is taken into consideration. The direction-dependent dynamics of both liquid and pillars, especially at the moving contact line (MCL), is revealed at atomic level. The flexible pillars accelerate the liquid when the liquid approaches, and pin the liquid when the liquid passes. The liquid deforms the pillars, resulting energy dissipation at the MCL. Scaling analysis is performed based on molecular kinetic theory and validated by our simulations. Our results may expand our knowledge of wetting on pillars and assisting the future design of active control of wetting in practical applications.