Large Shape Deformation Research Articles

Large-Deformation Curling Actuators Based on Carbon Nanotube Composite: Advanced-Structure Design and Biomimetic Application.

In recent years, electroactive polymers have been developed as actuator materials. As an important branch of electroactive polymers, electrothermal actuators (ETAs) demonstrate potential applications in the fields of artificial muscles, biomimetic devices, robotics, and so on. Large-shape deformation, low-voltage-driven actuation, and ultrafast fabrication are critical to the development of ETA. However, a simultaneous optimization of all of these advantages has not been realized yet. Practical biomimetic applications are also rare. In this work, we introduce an ultrafast approach to fabricate a curling actuator based on a newly designed carbon nanotube and polymer composite, which completely realizes all of the above required advantages. The actuator shows an ultralarge curling actuation with a curvature greater than 1.0 cm(-1) and bending angle larger than 360°, even curling into a tubular structure. The driving voltage is down to a low voltage of 5 V. The remarkable actuation is attributed not only to the mismatch in the coefficients of thermal expansion but also to the mechanical property changes of materials during temperature change. We also construct an S-shape actuator to show the possibility of building advanced-structure actuators. A weightlifting walking robot is further designed that exhibits a fast-moving motion while lifting a sample heavier than itself, demonstrating promising biomimetic applications.

Feature description using local neighborhoods

Feature description and matching is an essential part of many computer vision applications. Numerous feature description algorithms have been developed to achieve reliable performance in image matching, e.g. SIFT, SURF, ORB, and BRISK. However, their descriptors usually fail when the images have undergone large viewpoint changes or shape deformation. To remedy the problem, we propose a novel feature description and similarity measure based on local neighborhoods. The proposed descriptor and similarity is useful for a wide range of matching methods including nearest neighbor matching methods and popular graph matching algorithms. Experimental results show that the proposed method detects reliable matches for image matching, and performs robustly to viewpoint changes and shape deformation.

Numerical Simulation of the Solute‐Induced Marangoni Effect with the Semi‐Lagrangian Advection Scheme

AbstractThe level set method is combined with the concentration transformation method to solve the interphase mass transfer process. However, the artificial diffusion generated in the mass transfer convection term across the interface is inevitable, especially when large shape deformation is encountered at high Reynolds numbers. A semi‐Lagrangian advection scheme is introduced to overcome this disadvantage. The methyl isobutyl ketone (MIBK)‐acetic acid‐water system is adopted to study the unsteady mass transport process accompanied with the Marangoni effect of a single deformable drop ascending in the infinite continuous phase. The predicted overall mass transfer coefficients agree with experimental data very well. The configuration of Marangoni convection is revealed and its effect on the interphase mass transfer process is investigated.

On the self-propulsion of the euglenoid

We characterize the swimming speed of an euglenoid, which is propelled by large shape deformations traveling along its body.

An instant multi-responsive porous polymer actuator driven by solvent molecule sorption.

Fast actuation speed, large-shape deformation and robust responsiveness are critical to synthetic soft actuators. A simultaneous optimization of all these aspects without trade-offs remains unresolved. Here we describe porous polymer actuators that bend in response to acetone vapour (24 kPa, 20 °C) at a speed of an order of magnitude faster than the state-of-the-art, coupled with a large-scale locomotion. They are meanwhile multi-responsive towards a variety of organic vapours in both the dry and wet states, thus distinctive from the traditional gel actuation systems that become inactive when dried. The actuator is easy-to-make and survives even after hydrothermal processing (200 °C, 24 h) and pressing-pressure (100 MPa) treatments. In addition, the beneficial responsiveness is transferable, being able to turn 'inert' objects into actuators through surface coating. This advanced actuator arises from the unique combination of porous morphology, gradient structure and the interaction between solvent molecules and actuator materials.

Stick-slip transition of a water droplet vibrated on a superhydrophobic surface

We have studied the vibration behavior of a water droplet vibrated on a superhydrophobic surface via a high-speed camera. The resonance frequencies of the droplet satisfy the Rayleigh equation, suggesting that the droplet on a superhydrophobic surface can be regarded as a free droplet. Its real oscillation frequency is half of the driving frequency when it is vibrated at low frequencies(<200 Hz). It shows large shape deformation from a compressed puddle to a stretched spheroid. The three-phase contact line exhibits a stick-slip behavior. However, when the droplet is vibrated at frequencies greater than 200 Hz, the three-phase contact line is pinned to the substrate and the droplet is vibrated at the same frequencies as the external driving frequencies. It is found that the oscillation of the contact line and the large shape deformation of the droplet are responsible for the distinct behavior at low frequency.

Infrared-Actuated Recovery of Polyurethane Filled by Reduced Graphene Oxide/Carbon Nanotube Hybrids with High Energy Density

Optically actuated shape recovery materials receive much interest because of their great ability to control the creation of mechanical motion remotely and precisely. An infrared (IR) triggered actuator based on shape recovery was fabricated using polyurethane (TPU) incorporated by sulfonated reduced graphene oxide (SRGO)/sulfonated carbon nanotube (SCNT) hybrid nanofillers. Interconnected SRGO/SCNT hybrid nanofillers at a low weight loading of 1% dispersed in TPU showed good IR absorption and improved the crystallization of soft segments for a large shape deformation. The output force, energy density and recovery time of IR-triggered actuators were dependent on weight ratios of SRGO to SCNT (SRGO:SCNT). TPU nanocomposites filled by a hybrid nanofiller with SRGO:SCNT of 3:1 showed the maximum IR-actuated stress recovery of lifting a 107.6 g weight up 4.7 cm in 18 s. The stress recovery delivered a high energy density of 0.63 J/g and shape recovery force up to 1.2 MPa due to high thermal conductivity (1.473 W/mK) and Young's modulus of 23.4 MPa. Results indicate that a trade-off between the stiffness and efficient heat transfer controlled by synergistic effect between SRGO and SCNT is critical for high mechanical power output of IR-triggered actuators. IR-actuated shape recovery of SRGO/SCNT/TPU nanocomposites combining high energy density and output forces can be further developed for advanced optomechanical systems.

A Novel Mesh Morphing Technique for Large Shape Deformation and Its Application to Optimal Design Problems

In the process of finding the optimal solution by parameter sweeping analysis for optimal shape design problems, the silhouette of the device to be optimized usually needs to be changed many times. To avoid the tedious and time-consuming mesh regeneration process when the shape parameters change, a parameterized mesh technique is adopted in this paper. In the proposed method, the new coordinates of the nodes and their displacement vectors can be obtained instantly because all nodes are associated with the design parameters. To eliminate inverted elements in case of large shape deformation, a smart edge swapping technique is proposed. The edge swapping process is based on the nodal displacement vectors and the edge facing the displacement vector of the node is to be swapped. The parameterized mesh technique is fully described and examples are also given to showcase the effectiveness of the proposed method.

High pressure phase transitions in the fluid region surrounding the collapse point of large single bubbles in water

Observations from imaging experiments will be presented, which have shown persistent, long-lived spherical objects to form in the fluid region surrounding large, single bubbles in highly over-pressured water. Objects have been observed to form in a region of fluid where pressures are first predicted to exceed 0.8 GPa, and to extend radially inward to where fluid pressures are predicted to reach 6 GPa. These pressures bound those requisite for transitions in water to the crystalline phases of Ice-VI and Ice-VII, at 1.1 GPa and 2.1 GPa, respectively. The objects have been observed to behave in a fashion more consistent with a highly viscous fluid. They support and recover from large shape deformations, as well as support fluid flows within them. While water does have phases which are known to exhibit properties of highly viscous fluids, they have only been observed to form at or near cryogenic temperatures, typically via hyperquenching or quasi-static pressurization at low temperatures. Here, we present evidence for a high pressure liquid-liquid phase transition in water surrounding collapsing bubbles at room temperature. [Work supported by Impulse Devices, Inc.]

Smoothed particle hydrodynamics-based numerical investigation on sessile, oscillating droplets

Forced oscillations in sessile droplets can be exploited in electrowetting mixing of fluid fractions. The necessary complex flows and large shape deformations require a numerical investigation of fluid dynamics in the transient regime. We provide a means to characterize oscillations qualitatively and quantitatively with the goal to examine and to classify flow patterns occurring inside. A superposition of different harmonic excitation patterns gives the possibility to control the convective flow. In this investigation, we apply a generic and accurate multi-phase smoothed particle hydrodynamics model to a two-dimensional three-phase flow and consider an oscillating droplet sitting on a substrate and immersed in a fluidic phase. These vibrations are investigated in two ways: the analysis of a step response due to an abrupt change of the contact angle is applied to identify the resonance frequencies. Secondly, the time evolution of the shape of the droplet in terms of harmonic functions is determined. Their amplitudes are examined in the time and frequency domain. This gives the possibility to relate resonance frequencies to mode shapes and to detect a coupling between them. Our approach is successfully applied to different numerical case studies.

Tank-treading and tumbling frequencies of capsules and red blood cells

This study is motivated in part by the discrepancy that exists in the literature with regard to the dependence of the tank-treading frequency of red blood cells on the shear rate and suspending medium viscosity. Here we consider three-dimensional numerical simulations of deformable capsules of initially spherical and oblate spheroidal shapes and biconcave discoid representing the red blood cell resting shape. By considering a much broader range of the viscosity ratio (ratio of capsule or cell interior to suspending fluid viscosity), shear rate, and aspect ratio (ratio of minor to major axes) than that considered in the previous experiments, we find several new characteristics of the tank-treading and tumbling frequencies that have not been reported earlier. These new characteristics are the result of the large shape deformation and the coupling between shape and angular oscillations of the capsules or cells. For the spherical and oblate spheroidal capsules, the tank-treading frequency shows a nonmonotonic trend that is characterized by an initial decrease leading to a minimum followed by an increase with increasing viscosity ratio. For red blood cells, we find two regimes of the viscosity dependence of the tank-treading frequency: an exponential regime in which the tank-treading frequency decreases at a slower rate with increasing viscosity ratio, and a logarithmic range in which it decreases at a much faster rate. While this trend agrees well with different theoretical models of shape-preserving capsules, it was not evident in previous experimental results. When the shear rate dependence is considered, the tank-treading frequency of red blood cells and capsules of highly elongated initial shapes exhibits a nonmonotonic trend that is characterized by an initial increase leading to a maximum followed by a sharp decrease with decreasing shear rate. This anomalous behavior of the tank-treading frequency is shown to be due to a breathing-like dynamics of the capsule or cell that is characterized by a repeated emergence and absence of deep, crater-like dimples, and a large swinging motion. We further observe that the tumbling frequency exhibits a decreasing trend with increasing viscosity ratio that is in contrast to the theoretical result for the shape-preserving capsules and is due to the periodic deformation and preferential alignment of the capsules in the extensional quadrant of the flow.

Adhesion of Nanoparticles

We have developed a new model of nanoparticle adhesion which explicitly takes into account the change in the nanoparticle surface energy. Using combination of the molecular dynamics simulations and theoretical calculations we have showed that the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter beta proportional, variant gamma(p)(GR(p))(-2/3)W(-1/3), where G is the particle shear modulus, R(p) is the initial particle radius, gamma(p) is the polymer interfacial energy, and W is the particle work of adhesion. In the case of small values of the parameter beta < 0.1, which is usually the case for strongly cross-linked large nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. However, we observed a significant deviation from the classical JKR theory in the case of the weakly cross-linked nanoparticles that experience large shape deformations upon particle adhesion. In this case the interfacial energy of the nanoparticle plays an important role controlling nanoparticle deformation. Our model of the nanoparticle adhesion is in a very good agreement with the simulation results and provides a new universal scaling relationship for nanoparticle deformation as a function of the system parameters.

Behavior of a small single bubble rising in rotating flow field

A small single bubble was generated with a single-hole nozzle facing upward in a water bath contained in a rotating cylindrical vessel. The bubble size falls in the surface tension force dominant regime. The vertical, radial, and tangential migration velocities of the bubble were measured with two CCD cameras and a high-speed video camera. The tangential velocity component of water flow was measured with particle image velocimetry. A helical motion of the bubble was observed under every experimental condition. The direction of the helical motion was the same as that of the tangential velocity component. This helical motion is associated with the large initial shape deformation of the bubble near the nozzle exit and the subsequent regular shedding of vortices behind it. The period and amplitude of the helical motion were obtained by analyzing the trajectory of the bubble. These quantities were non-dimensionalized by the volume equivalent bubble diameter and the terminal bubble velocity in the vertical direction and correlated as functions of the Eotvos number. Empirical equations were proposed for the period and amplitude.

Direct Observation of Water Droplet Motion on a Hydrophobic Self-assembled Monolayer Surface under Airflow

A wind tunnel to produce a uniform air stream and a high-speed camera system assembled for this study enabled direct and simultaneous measurements of sliding droplets' shape and acceleration. Using this system, wind velocity dependence of deformation and the moving acceleration of a water droplet on an inclined octadecyltrimethoxysilane (ODS) coating were investigated. The motions of a water droplet in an air stream are classifiable into three : sliding, stopping, and climbing. In the sliding zone, the sliding acceleration decreases with increasing wind velocity. During stopping, the droplet shape changes from a forward-bent to backward-bent posture against the surface inclination. In the climbing zone under large wind velocity, water droplets have large climbing acceleration with large shape deformation.

Budding Dynamics of Multicomponent Tubular Vesicles

Real-time budding dynamics of multicomponent, tubular lipid vesicles was investigated. By using a fluorescence microscope, three typical growth modes of the buds were observed, corresponding, respectively, to bud growth through coalescence between flat patches, a bud and a patch, as well as that of two buds. The spatial and temporal scales measured in the observation were used to estimate the bending rigidity of the membrane. In the late stage, the continuing coalescence between the buds resulted in large shape deformation of the vesicles, from tubular to spherical vesicles, and the number of the buds decayed with time as N approximately t-2/3. This scaling relation was observed for the first time in experiment and confirmed early theoretical predictions. Our observation showed a difference between the diffusivity of the buds on the lipid membrane and that of the embedded membrane proteins.

Dynamics of acoustically levitated disk samples

The acoustic levitation force on disk samples and the dynamics of large water drops in a planar standing wave are studied by solving the acoustic scattering problem through incorporating the boundary element method. The dependence of levitation force amplitude on the equivalent radius R of disks deviates seriously from the R3 law predicted by King's theory, and a larger force can be obtained for thin disks. When the disk aspect ratio gamma is larger than a critical value gamma(*) ( approximately 1.9 ) and the disk radius a is smaller than the critical value a(*) (gamma) , the levitation force per unit volume of the sample will increase with the enlargement of the disk. The acoustic levitation force on thin-disk samples ( gamma</= gamma(*) ) can be formulated by the shape factor f(gamma,a) when a</= a(*) (gamma) . It is found experimentally that a necessary condition of the acoustic field for stable levitation of a large water drop is to adjust the reflector-emitter interval H slightly above the resonant interval H(n) . The simulation shows that the drop is flattened and the central parts of its top and bottom surface become concave with the increase of sound pressure level, which agrees with the experimental observation. The main frequencies of the shape oscillation under different sound pressures are slightly larger than the Rayleigh frequency because of the large shape deformation. The simulated translational frequencies of the vertical vibration under normal gravity condition agree with the theoretical analysis.

Behavior of a small single bubble rising in a rotating flow field

A small single bubble was generated with a single-hole nozzle facing upward in a water bath contained in a rotating cylindrical vessel. The bubble size falls in the surface tension force dominant regime. The vertical, radial, and tangential migration velocities of the bubble were measured with two CCD cameras and a high-speed video camera. The tangential velocity component of water flow was measured with particle image velocimetry. A helical motion of the bubble was observed under every experimental condition. The direction of the helical motion was the same as that of the tangential velocity component. This helical motion is associated with the large initial shape deformation of the bubble near the nozzle exit and the subsequent regular shedding of vortices behind it. The period and amplitude of the helical motion were obtained by analyzing the trajectory of the bubble. These quantities were non-dimensionalized by the volume equivalent bubble diameter and the terminal bubble velocity in the vertical direction and correlated as functions of the Eotvos number. Empirical equations were proposed for the period and amplitude.

Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel

The time-temperature-transformation (TTT) diagram within the medium temperature range of medium carbon steel has been determined. A single type of C-curve is found within the bainite temperature range for the studied steel. Distinct reaction C-curves were not observed for both types of microstructure, upper bainite and lower bainite in the TTT diagram. Experimental results on the kinetics of the isothermal formation of bainite at different temperature have demonstrated that both type of microstructure, upper and lower bainite, possesses similar overall transformation kinetics. Some applications of phase transformation theory towards the formation of bainitic microstructures are discussed, with particular emphasis on the bainite start temperature, transition temperature from upper to lower bainite, martensite start temperature and the thickness of bainitic plates. Bainite can be regarded as a non-lamellar mixture of ferrite and carbides, but within this broad description, it is possible to identify two classical morphologies, traditionally named upper and lower bainite. Both upper and lower bainite consist of plates of ferrite, known as sub-units, separated by cementite. The plates of ferrite grow in aggregates called sheaves of bainite. Within each sheaf, the plates of ferrite are parallel and share a common crystallographic orientation. The essential difference between upper and lower bainite is with respect to the carbide precipitation. In upper bainite, the bainitic ferrite is free of precipitation; carbides grow from the regions of carbon enriched austenite, which are trapped between the sub-units of ferrite. By contrast lower bainitic ferrite contains a fine dispersion of plate-like carbides within the bainitic ferrite plates. Both upper and lower bainite are formed by the propaga- tion of displacive ferrite sub-units followed by the partition of the excess carbon into the residual austenite. Cementite can then precipitate from the enriched austenite between the ferrite plates. The growth of the bainitic sub-units is accompanied by a change in the shape of the transformed region, which can be described as an invariant-plane strain with a large shear component. The parent austenite cannot accommodate the large shape deformation elastically and relaxes by plastic deformation in the region adjacent to the bainite. This plastic deformation stifles the growth of bainite sub-units before they hit any obstacle, and the transformation proceeds with the formation of a new sub-unit. 1) The mechanism of transformation continues until the carbon concentration of the residual austenite reach the To curve, at which ferrite and austenite with identical chemical compo-

Terminal velocity of single bubbles in surface tension force dominant regime

Terminal velocity V T of a single bubble rising through an infinite stagnant liquid in surface tension force dominant regime was investigated theoretically and experimentally. A theoretical V T model, which is applicable to a distorted spheroidal bubble with a high bubble Reynolds number, was deduced from a jump condition and a potential flow theory for a flow about an oblate spheroid. Experiments were conducted using air and water to measure bubble trajectories, shapes and velocities. As a result, it was confirmed that (1) the primal cause of widely scattered V T in this regime is not surfactant concentration but initial shape deformation, (2) small initial shape deformation results in a low V T and a high aspect ratio, whereas large initial shape deformation results in a high V T and a low aspect ratio, (3) the primal role of surfactants in this regime is to cause the damping of shape oscillation, by which a contaminated bubble behaves as if it were a clean bubble with low initial shape deformation, and (4) the proposed model gives good predictions of V T for single distorted bubbles.

AI-EigenSnake: an affine-invariant deformable contour model for object matching

Abstract An affine-invariant (AI) deformable contour model for object matching, called AI-EigenSnake (AI-ES), is proposed in the Bayesian framework. In AI-ES, the prior distribution of object shapes is estimated from the sample data. This distribution is then used to constrain the prototype contour, which is dynamically adjustable in the matching process. In this way, large shape deformations due to the variations of samples can be tolerated. Moreover, an AI internal energy term is introduced to describe the shape deformations between the prototype contour in the shape domain and the deformable contour in the image domain. Experiments based on real object matching demonstrate that the proposed model is more robust and insensitive to the positions, viewpoints, and large deformations of object shapes, as compared to the Active Shape Model and the AI-Snake Model.