Retraction Behavior Research Articles

Splitting behavior and breakup mechanism of bubbles in the split‐and‐recombine microchannel

AbstractSplit‐and‐recombine (SAR) microreactor is an advanced reactor for chemical process intensification. Bubble flow in the SAR microchannel is an important phenomenon that affects reaction efficiency, however drew little attention before. This study aims to explore the underlying mechanisms of bubble splitting, retraction, and breakup behaviors in a compact SAR microchannel. Two breakup flow patterns, unilateral flow and unilateral alternate flow were identified with symmetric or asymmetric splitting, respectively. Mechanism analysis indicates that the splitting symmetry issue is related to liquid slug size, viscous effect and novel retraction behavior of a splitting filament. The retraction is induced by the interconnection and unequal pressures between the splitting filaments. The correlation between normalized breakup time and Ca number confirms the Capillary‐pressure breakup mechanism for the splitting gas filaments. Two empirical correlations for the two breakup flow patterns were proposed, which illustrate the significant contribution of bubble retraction to the breakup degree.

Effects of confinement geometry on shape transition and interfacial behavior of nanodroplets in externally applied electric field

The electric field-induced shape transition of a nanodroplet confined in a silicon nanogroove with variable structure was investigated through molecular dynamics simulations. Our work provides insight into the relationship between the shape transition and the molecular interaction. We demonstrated that the geometric structure of the groove significantly influences the responsive behaviors of the droplet to the electric field. The simulations elucidate increasing the tilt angle of the groove reduces the critical field strength for the shape transition of the droplet under parallel electric field. Above the critical field, the droplet detaches from the groove and forms a liquid bridge across the groove, leading to weakened interfacial interactions. The detachment process of the droplet from the rectangular groove, which originates from the spreading of water molecules on perpendicular side walls, is different from the droplet initially confined in the groove with tilt angle higher than 90°. Under the perpendicular electric field, the droplet undergoes various shape transitions depending on the groove structure. In particular, the droplet in the groove with a moderate opening forms two asymmetrical liquid columns. The retraction behavior of the droplet in the groove with the small opening sheds light on the competition between interfacial interactions. The shape transition of droplets in the electric field due to the change of confinement nanostructures, such as liquid bridge, asymmetrical extension, branching and merging, is helpful to enrich the understanding of electrowetting and confinement dynamics of the droplets.

Effects of concentration of hydrophobic component and swelling in saline solutions on mechanical properties of a stretchable hydrogel.

An elastic biopolymer, resilin possesses exceptional qualities such as high stretchability and resilience. Such attributes are utilized in nature by many species for mechanical energy storage to facilitate movement. The properties of resilin are attributed to the balanced combination of hydrophilic and hydrophobic segments. To mimic the properties of resilin, we developed a hydrogel system composed of hydrophilic acrylic acid (AAc) and methacrylamide (MAM) chains and hydrophobic poly(propylene glycol diacrylate) (PPGDA) chains. The gel was produced through free-radical polymerization in 0.8 M NaCl solutions using KPS as an initiator. In these gels, AAc and MAM can form hydrogen bonds, whereas the association between PPGDA chains can lead to hydrophobic domains. The PPGDA concentration affects the level of hydrogen bonding and gel mechanical properties. Tensile experiments revealed that the elastic modulus increased with a higher PPGDA concentration. Retraction experiments demonstrated increased velocity and acceleration when released from a stretched state with increasing PPGDA concentration. Swelling and deswelling of gels in saline solutions led to a change in mechanical properties and retraction behavior. This study shows that the stretchability and resilience of these hydrogels can be adjusted by changing the concentration of hydrophobic components.

Local subsidence behavior of linear guideway assembly resting on a rough elastic foundation

The accurate stiffness estimation of linear guideway assembly is determined by the structural deformation of carriage and supporting rail-pedestal-bolt joint. This paper proposes an analytical model to investigate the stiffness and local subsidence behaviors of linear guideway assembly. The different preload states between the upper and lower raceways are revealed, and the retraction behavior of carriage wing is found. Considering the local subsidence of the rail below a carriage, the proposed model reports the non-uniform load distribution along the finite-length rail, which is placed on the rough foundation and assembled by bolts. Loading tests are performed to verify the local sinking effect and the model accuracy. The developed model is promising to serve to the deformation error compensation.

Spread and retraction dynamics of droplet coalescence on a rectangular pixel for organic light-emitting diode inkjet printing

In organic light-emitting diodes (OLEDs) manufacturing, inkjet printing has rapidly become popular due to its high resolution and low cost. However, the mechanism of droplet impacting on a rectangular pixel in OLEDs inkjet printing is not yet clear. In this paper, coalescence dynamics of droplet impacting on a rectangular pixel including the spread and retraction stages is simulated by the software of fluent. In addition, a dynamic model is proposed to analyze the spread and retraction behavior of droplet impacting on a rectangular pixel. In the spread stage, the dynamic equation of spread length with respect to time is derived based on energy and mass conservation. In the retraction stage, the variational principle instead of solving the complete Navier–Stokes equation is used to simplify the calculation. On this basis, the dynamic equation of retraction length with respect to time is achieved, and the theoretical solution is in agreement with the numerical result. Finally, the influences of droplet offset, size, and initial velocity on coalescence dynamics are analyzed. Furthermore, the dynamic model proposed in this paper provides a new possibility for the coalescence of a droplet impacting on a heteromorphic microcavity instead of a flat substrate.

Network topology enables efficient response to environment in Physarum polycephalum

The network-shaped body plan distinguishes the unicellular slime mould Physarum polycephalum in body architecture from other unicellular organisms. Yet, network-shaped body plans dominate branches of multi-cellular life such as in fungi. What survival advantage does a network structure provide when facing a dynamic environment with adverse conditions? Here, we probe how network topology impacts P. polycephalum’s avoidance response to an adverse blue light. We stimulate either an elongated, I-shaped amoeboid or a Y-shaped networked specimen and subsequently quantify the evacuation process of the light-exposed body part. The result shows that Y-shaped specimen complete the avoidance retraction in a comparable time frame, even slightly faster than I-shaped organisms, yet, at a lower almost negligible increase in migration velocity. Contraction amplitude driving mass motion is further only locally increased in Y-shaped specimen compared to I-shaped—providing further evidence that Y-shaped’s avoidance reaction is energetically more efficient than in I-shaped amoeboid organisms. The difference in the retraction behaviour suggests that the complexity of network topology provides a key advantage when encountering adverse environments. Our findings could lead to a better understanding of the transition from unicellular to multicellularity.

Magnetic Control of Water Droplet Impact onto Ferrofluid Lubricated Surfaces

Controlling the impact process of a droplet impacting a liquid film has remained a wide-open challenge. The existing passive techniques lack precise on-demand control of the impact dynamics of droplets. The present study introduces a magnet-assisted approach to control water droplets' impact dynamics. We show that by incorporating a thin, magnetically active ferrofluid film, the overall droplet impact phenomena of the water droplets could be controlled. It is found that by modifying the distribution of the magnetic nanoparticles (MNPs) present inside the ferrofluid using a permanent magnet, the spreading and retraction behavior of the droplet could be significantly controlled. In addition to that, we also show that by altering the impact Weber number (Wei), and the magnetic Bond number (Bom), the outcomes of droplet impact could be precisely controlled. We reveal the role of the various forces on the consequential effects of droplet impact with the help of phase maps. Without the magnetic field, we discovered that the droplet impact on ferrofluid film results in no-splitting, jetting, and splashing regimes. On the other hand, the presence of magnetic field results in the no-splitting and jetting regime. However, beyond a critical magnetic field, the ferrofluid film gets transformed into an assembly of spikes. In such scenarios, the droplet impact only results in no-splitting and splashing regimes, while the jetting regime remains absent. The outcome of our study may find potential applications in chemical engineering, material synthesis, and three-dimensional (3D) printing where the control and optimization of the droplet impact process are desirable.

Magnetoresistance retraction behaviour of Ag/p‐Ge:Ga/Ag device under pulsed high magnetic field

In non-magnetic semiconductor materials, unsaturated magnetoresistance (MR) effect has attracted lots of attention due to its physical interests and potential applications in electronic devices. Under the extremely high magnetic field, the stability and reliability of MR effects based on the non-ohmic transport has been rarely researched. In this paper, the transport properties of non-magnetic Ag/p-Ge:Ga/Ag devices under 45 T pulsed high magnetic field at 300 K are investigated. It is found that in ohmic conduction region ( I <5 mA) where the single dominant carrier is hole, the MR values increase with increasing the applied magnetic field, presenting a conventional unsaturated behavior. In the two non-ohmic regions (5 mA≤ I ≤ 100 mA) where the transport is dominated by bipolar (electrons and holes), a MR retraction has been obviously observed under pulsed high magnetic field. Combining the Hall measurement results and calculation of Hall effect with bipolar-driven transport model, the mechanism of the MR retraction is analysed, in which the MR retraction may be related to the strong regulation of electron-to-hole density ratio by pulsed high magnetic field. This work provides a reference for evaluating the stability and reliability of the properties of non-magnetic semiconductor based MR devices under the interference of strong magnetic pulses.

Drop Impact on a Superhydrophilic Spot Surrounded by a Superhydrophobic Surface.

The spatial variation in the wettability of a surface can have a significant effect on the spreading and retraction behavior of an impacting droplet and hence the overall impact dynamics. Although composite surfaces have proven applications, there is a lack of understanding of droplet impact on surfaces with a sudden jump in wettability. Here, we study the behavior of a liquid drop impacting a composite surface having a superhydrophilic (SHL) spot surrounded by a superhydrophobic (SHB) region. We find that the droplet exhibits different regimes: no-splitting, jetting, and splashing, depending upon the spot size (βs) and the Weber number (We). At a smaller βs, the behavior shifts from the stable to jetting regime and then to the splashing regime, with increasing We. We find that by increasing the value of βs, one can avoid the undesirable splashing and jetting regimes and attain a stable regime even at a higher We. Our study reveals that βs has a significant influence on the maximum spreading diameter βmax at a smaller We but a negligible effect at a higher We. We show that the dominance of capillary energy at a smaller We and viscous energy at a higher We underpins the phenomena. We employ an energy conservation approach to develop an analytical model to predict βmax on a composite SHL-SHB surface by considering the total energy of the system before the impact and at the maximum spread position. We find K = (Re1/2/We) emerges as a key parameter in the model that accurately predicts the experimentally measured βmax. Our study reveals the existence of an inertia-viscous dominated regime at a smaller K and an inertia-capillary dominated regime at a larger K. The outcome of our study may find applications in stable and precise positioning of impacting droplets.

Experimental insights on the water entry of hydrophobic sphere

This article reports experimental insights into the physics of water entry of hydrophobic spheres. In the set of experiments, parameters such as sphere density, diameter, and impact velocity are varied. The trajectory of the sphere after impact, the dynamics of trapped air-cavity, including the cavity formation, and the retraction analysis are given. Furthermore, analysis of the Worthington-jet, the cavity ripple, and early bubble shedding after the air-cavity detachment is carried out. At the location of cavity closure, radial expansion and contraction behavior are reported for the case of the shallow seal (near the air–water interface), while for the deep seal, only one such behavior is observed. Further, five cavity shapes are recorded based on the cavity retraction behavior (i.e., shallow, deep seal), namely, conical shape, slender-cone shape, telescopic shape, spearhead shape, and the thick spearhead shape. The radial dynamics and radial surface energy analysis are reported at various locations on these cavity shapes to find that the thick spearhead cavities hold the most cross-sectional surface energy. The slender-cone shaped cavity generates the fastest Worthington-jet, followed by the telescopic shaped cavities. The thick spearhead shaped cavities are reported to have the longest intact Worthington-jets, followed by the spearhead shaped cavities. Finally, a new regime map is presented for single ripple and multiple ripple behaviors at the time of retraction in the wake of descending spheres. A bubble shedding behavior has also been characterized as the most frequent bubble shedding for shallow seal and associated longer bubble length compared to the other cases.

Modeling and Experimental Validation of the Time Delay in a Pilot Operated Proportional Directional Valve

The pilot-operated proportional directional valve is a crucial component in the airplane landing gear retraction system. The time delay of the proportional valve directly affects the retraction behavior even the flight safety and needs to be analyzed. Therefore, the modeling and analysis of a typical pilot-operated proportional directional valve are presented in this paper. Substantially, the nonlinear valve system is composed of four subsystems: mechanical, electronic, electromagnetic, and fluid dynamic subsystems. The subsystems are modeled based on a lumped parameter approach. The coupling among the subsystems is analyzed. The nonlinearity is implemented using a specific set of equations in the model. Especially, the nonlinearity observed in practical application is considered in modeling the effect of the electronic anti-unloading power drive circuit on the pulse-width modulation signal. The analytical results of the model show good agreement with the experimental ones. The investigation shows that the delay of the valve results from multiple subsystems’ dynamic performances which act sequentially on the main spool. The effect of the electronic part and the structural part is analyzed, and the proportion of each part is given based on the simulation results. The analysis shows that the model can provide a direct suggestion in choosing the valve part to optimize and the optimization effect limit.

Modeling and simulation of viscoelastic film retraction

In this paper, we investigate the retraction of a circular viscoelastic liquid film with a hole initially present in its center by means of finite element numerical simulations. We study the whole retraction process, aiming at understanding the hole opening dynamics both when the hole does not feel any confinement and when it interacts with the solid wall bounding the film. The retraction behavior is also interpreted through a simple toy model, that highlights the physical mechanism underlying the process.We consider three different viscoelastic constitutive equations, namely, Oldroyd-B, Giesekus (Gsk), and Phan Thien–Tanner (PTT) models, and several system geometries, in terms of the film initial radius and thickness. For each given geometry, we investigate the effects of liquid inertia, elasticity, and flow-dependent viscosity on the dynamics of the hole opening. Depending on the relative strength of such parameters, qualitatively different features can appear in the retracting film shape and dynamics.When inertia is relevant, as far as the opening hole does not interact with the wall bounding the film, the influence of liquid elasticity is very moderate, and the retraction dynamics tends to the one of Newtonian sheets; when the hole starts to interact with the solid wall, hole radius/opening velocity oscillations are detected. Such oscillations are enhanced at increasing elasticity. From the morphological point of view, the formation of a rim at the edge of the retracting film is observed. If inertial forces become less relevant with respect to viscous forces, R-oscillations disappear, the hole opening velocity goes through a maximum and then monotonically decays to zero, and no rim forms during the film retraction. Geometrical changes have the effect of enlarging or reducing the portion of the retraction dynamics not influenced by the presence of the solid wall with respect to the one governed by the hole-wall interactions.

Retraction of Complaints Among Female Victims of Intimate Partner Violence Living in Poverty in Nicaragua.

Retraction among female victims of intimate partner violence (IPV) who report their abuser is a major problem in all societies. This article describes a study of 136 female victims of physical IPV living in poverty in Nicaragua, one of the countries with the lowest levels of development in Central America. This article analyses the aspects that differentiate women living in poverty who retracted after reporting IPV from those who did not. The results show that retraction is widespread among female victims of IPV living in poverty in León (Nicaragua). Although it is difficult to predict the retraction behaviour of the respondents, some differences between the women who retracted their complaint and those who did not were observed. A combination of five variables (including personal circumstances and beliefs about the intimate partner relationship and family) was the best alternative for discriminating between women who had retracted and those who had not.

Eye retraction in the giant guitarfish, Rhynchobatus djiddensis (Elasmobranchii: Batoidea): a novel mechanism for eye protection in batoid fishes

Eye retraction behavior has evolved independently in some vertebrate linages such as mudskippers (fish), frogs and salamanders (amphibians), and cetaceans (mammals). In this paper, we report the eye retraction behavior of the giant guitarfish (Rhynchobatus djiddensis) for the first time, and discuss its mechanism and function. The eye retraction distance was nearly the same as the diameter of the eyeball itself, indicating that eye retraction in the giant guitarfish is probably one of the largest among vertebrates. Eye retraction is achieved by unique arrangement of the eye muscle: one of the anterior eye muscles (the obliquus inferior) is directed ventrally from the eyeball and attaches to the ventral surface of the neurocranium. Due to such muscle arrangement, the obliquus inferior can pull the eyeball ventrally. This mechanism was also confirmed by electrical stimulation of the obliquus inferior. The eye retraction ability of the giant guitarfish likely represents a novel eye protection behavior of elasmobranch fishes.

Type-IV Pilus deformation can explain retraction behavior.

Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of 100 pN during their retraction before stalling, powering surface translocation(twitching). Single TFP level experiments have shown remarkable nonlinearity in the retraction behavior influenced by the external load as well as levels of PilT molecular motor protein. This includes reversal of motion near stall forces when the concentration of the PilT protein is loweblack significantly. In order to explain this behavior, we analyze the coupling of TFP elasticity and interfacial behavior with PilT kinetics. We model retraction as reaction controlled and elongation as transport controlled process. The reaction rates vary with TFP deformation which is modeled as a compound elastic body consisting of multiple helical strands under axial load. Elongation is controlled by monomer transport which suffer entrapment due to excess PilT in the cell periplasm. Our analysis shows excellent agreement with a host of experimental observations and we present a possible biophysical relevance of model parameters through a mechano-chemical stall force map.

Influence of Substrate Elasticity on Droplet Impact Dynamics

Droplet impact dynamics is vital to the understanding of several phase-change and heat-transfer phenomena. This work examines the role of substrate elasticity on the spreading and retraction behavior of water droplets impacting flat and textured superhydrophobic substrates. Experiments reveal that droplet retraction on flat surfaces decreases with decreasing substrate elasticity. This trend is confirmed through a careful measurement of droplet impact dynamics on multiple PDMS surfaces with varying elastic moduli and comparison with impact dynamics on hard silicon surfaces. These findings reveal that surfaces tend to become more wettable upon droplet impact as the elastic modulus is decreased. First-order analyses are developed to explain this reduced retraction in terms of increased viscoelastic dissipation on soft substrates. Interestingly, superhydrophobic surfaces display substrate-elasticity-invariant impact dynamics. These findings are critical when designing polymeric surfaces for fluid-surface interaction applications.

Numerical study of the parameters governing the impact dynamics of yield-stress fluid droplets on a solid surface

The impact dynamics of a droplet onto a solid surface are important in a variety of applications, such as inkjet printing and spray coating. Many fluids encountered in practical industrial applications exhibit non-Newtonian behavior, and therefore more research associated with non-Newtonian fluids is necessary. This paper reports on a numerical study of the impact dynamics of yield-stress fluid droplets. The numerical simulation is performed using a computational fluid dynamics package, Fluent 6.3, with a volume of fluid model. The numerical simulation models the presence of yield-stress and shear-rate dependent viscosity using the Herschel–Bulkley rheological model. The numerical results are found to be in qualitative agreement with experimental data in the literature. By performing extensive numerical simulations varying the impact velocity, rheological parameters, and surface tension, the influence of these parameters on the impact dynamics are evaluated, and the dominant effects that govern the spreading and relaxation phases are determined. The results show that while the spreading behavior is determined by the power-law index n, the non-Newtonian Reynolds number Ren, and the Weber number We, the retraction behavior is determined by the non-Newtonian capillary Can and the Bingham-capillary number B^. In addition, the scaling law that predicts the maximum spreading diameter is presented.

Superhydrophobic Properties of Nonaligned Boron Nitride Nanotube Films

Superhydrophobicity is highly desirable for numerous applications. Here, we report that a semierect but nonaligned boron nitride nanotube (BNNT) film showed superhydrophobicity with contact angle above 170 degrees and a small contact angle hysteresis. This superhydrophobicity was stable over a large range of drop sizes, and the measured critical transition pressure was about 10 kPa. However, the prostrate BNNT films only showed hydrophobicity. The drop retraction behavior during evaporation, the pressure effect on contact angle, the critical transition pressure, the drop impact behavior, and the self-cleaning efficiency between these two kinds of films were systematically investigated and compared.

Pom-pom model predictions on nonlinear stress relaxation in single-step strain flow

A phenomenological pom-pom model, which utilizes linear stress relaxation data as an input, is proposed for simulating the detailed retraction behavior of a full pom-pom chain as well as the associated nonlinear stress relaxation in single-step shear flows. Specifically, the initial, Rouse-like arm and/or backbone retraction as well as the long-time, renormalized (dumbbell-like) backbone retraction is simulated at one time, and a possible coupling between linear orientation relaxation and nonlinear stretch relaxation due to diffusive or convective constraint release is self-consistently accounted for. The model’s predictions are systematically tested against nonlinear stress relaxation data on three nearly monodisperse pom-pom melts, including two six-arm pom-pom polybutadienes (PPBDs) and an H-shaped polyisoprene (PPI). The model can describe the present data reasonably well by incorporating a previously proposed effect of drag–strain coupling. Accounting further for the effect of constraint release, due primarily to arm diffusive motion and its coupling with a known polydispersity in arm molecular weight distribution, appreciably broadens the predicted nonlinear stress relaxation and substantially improves the results of theory/data comparisons. The significance of the current findings is discussed.

A Langevin-elasticity-theory-based constitutive equation for rubberlike networks and its comparison with biaxial stress–strain data. Part I

A structure-based constitutive equation for rubberlike networks is proposed. It is obtained by combining the Langevin-statistics-based theory of Arruda and Boyce (AB) with a term based on the first invariant of the generalized deformation tensor which follows from some theoretical treatments of the constraint effect. The combined (ABGI) four-parameter strain–energy function has been found to give (with the exception of the very-low strain region) a very good description (deviations<5–8%) of a representative selection of published biaxial stress–strain data obtained on networks of isoprene and natural rubbers at low and medium strains. For the description of biaxial extension data up to high strains, the concept of a strain-induced increase in the network mesh size (strain-dependent finite extensibility parameter) used previously for tensile strains, is shown to apply generally to all deformation modes. It also enables a prediction of the retraction behavior. Reasonable values were obtained for the network parameters; the exponent in the strain invariant assumes values at the higher limit of the Kaliske and Heinrich extended tube theory prediction or even somewhat outside the limits.