Instability Phenomena Research Articles

Determination of the Dominating Coalescence Pathways in Double Emulsion Formulations by Use of Microfluidic Emulsions

In water-in-oil-in-water (W1/O/W2) double emulsions several irreversible instability phenomena lead to changes. Besides diffusive processes, coalescence of droplets is the main cause of structural changes. In double emulsions, inner droplets can coalesce with each other (W1–W1 coalescence), inner droplets can be released via coalescence (W1–W2 coalescence) and oil droplets can coalesce with each other (O–O coalescence). Which of the coalescence pathways contributes most to the failure of the double emulsion structure cannot be determined by common measurement techniques. With monodisperse double emulsions produced with microfluidic techniques, each coalescence path can be observed and quantified simultaneously. By comparing the occurrence of all possible coalescence events, different hydrophilic surfactants in combination with PGPR are evaluated and discussed with regard to their applicability in double emulsion formulations. When variating the hydrophilic surfactant, the stability against all three coalescence mechanisms changes. This shows that measuring only one of the coalescence mechanisms is not sufficient to describe the stability of a double emulsion. While some surfactants are able to stabilize against all three possible coalescence mechanisms, some display mainly one of the coalescence mechanisms or in some cases all three mechanisms are observed simultaneously.

Effects of saturable function in three-core PIM-NIM-PIM coupler through modulation instability

We consider a model of three-core PIM-NIM-PIM coupler with Kerr-type saturable nonlinearity to study both analytically and numerically saturation effects on the modulational instability (MI) phenomenon. The analytical results show us that, in presence of saturable parameters in normal and anomalous dispersion regime the instability gain presents significant changes compared to Shafeeque et al. (Phys Lett A 11:223–229, 2015) in absence of saturable parameter. It clearly also shows that Power and nonlinear parameters can be used to control MI. Through numerical simulation, the generations of periodic soliton are obtained in presence of nonlinear saturation. However in absence of saturable parameter, the train of soliton obtained turns into a turbulent state after a certain propagations distance, and in that case, the system is prone to present MI.

Self-Sustained Euler Buckling of an Optically Responsive Rod with Different Boundary Constraints

Self-sustained oscillations can directly absorb energy from the constant environment to maintain its periodic motion by self-regulating. As a classical mechanical instability phenomenon, the Euler compression rod can rapidly release elastic strain energy and undergo large displacement during buckling. In addition, its boundary configuration is usually easy to be modulated. In this paper, we develop a self-sustained Euler buckling system based on optically responsive liquid crystal elastomer (LCE) rod with different boundary constraints. The buckling of LCE rod results from the light-induced expansion and compressive force, and the self-buckling is maintained by the energy competition between the damping dissipation and the net work done by the effective elastic force. Based on the dynamic LCE model, the governing equations for dynamic Euler buckling of the LCE rod is formulated, and the approximate admissible trigonometric functions and Runge-Kutta method are used to solve the dynamic Euler buckling. Under different illumination parameters, there exists two motion modes of the Euler rod: the static mode and the self-buckling mode, including alternating and unilateral self-buckling modes. The triggering conditions, frequency, and amplitude of the self-sustained Euler buckling can be modulated by several system parameters and boundary constraints. Results indicate that strengthening the boundary constraint can increase the frequency and reduce the amplitude. It is anticipated that this system may open new avenues for energy harvesters, signal sensors, mechano-logistic devices, and autonomous robots.

Case study of flow instabilities in subchannels via multidimensional CFD approach

Multidimensional simulations on two-phase flow instabilities based on a two-fluid CFD approach with water as working fluid were conducted for upward flow in both single subchannel and 2 × 1 subchannels at 4.5 MPa. During the simulations, the inlet subcooling was set to 10 K, 15 K, and 20 K respectively. The subchannel is 1060 mm in length with a rod wall pitch by rod diameter (P/D) of 12.6 mm/9.5 mm. Simulations were performed by fixing the pressure inlet and outlet boundary conditions and increasing the heat flux to reach flow instability conditions. The results showed that the CFD approach successfully simulated the flow instability phenomena. In the case of flow instability, the flow and heat transfer characteristics changed greatly, which resulted in rising temperature and further early CHF. This paper reveals the open connectivity structure in the subchannel system enhanced the stability of the two-phase flow system compared with one-dimensional simulation due to the multidimensional mixing effects between channels captured by CFD approach.

Experimental research of dynamic vibration damping for rigid busbar structures

Introduction: in the case when flexible structures interact with the wind flow, various phenomena of aerodynamic instability may arise. Typical representatives of such phenomena are vortex wind excitation of cylindrical structures, galloping of poorly streamlined structures with a square, rectangular or rhomboid cross-section, etc. The article highlights some basic ways of damping vibrations of rigid busbar structures. Materials and methods: the method of dynamic vibration damping consists in attaching additional devices to the vibration protection object to change its vibration state. The work of dynamic dampers is based on the formation of force effects transmitted to the object. It differs from another method of vibration reduction, characterized by the imposition of additional kinematic connections on the object such as fixing its particular points. Results: a mathematical model of the plate dynamic damper operation with a point load is presented. To determine the optimal parameters of dynamic vibration dampers, their calculation was performed, taking into account the joint action of the rigid busbar and the damper. Experimental research of the conjoint work of a rigid busbar with a plate dynamic damper is carried out. Conclusions: the effective application of plate dynamic dampers with a point load has been confirmed both outside and inside the tube busbar. Is proposed the special plate vibration damper. This allows to increase the logarithmic decrement of oscillations by 3-3.5 times and reduce the amplitude of oscillations of rigid busbar structures in the resonant mode by 12 time.

FRACTAL PULL-IN MOTION OF ELECTROSTATIC MEMS RESONATORS BY THE VARIATIONAL ITERATION METHOD

The dynamic pull-in instability of a microstructure is a vast research field and its analysis is of great significance for ensuring the effective operation and reliability of micro-electromechanical systems (MEMS). A fractal modification for the traditional MEMS system is suggested to be closer to the real state as a practical application in the air with impurities or humidity. In this paper, we establish a fractal model for a class of electrostatically driven microstructure resonant sensors and find the phenomenon of pull-in instability caused by DC bias voltage and AC excitation voltage. The variational iteration method has been extended to obtain approximate analytical solutions and the pull-in threshold value for the fractal MEMS system. The result obtained from this method shows good agreement with the numerical solution. The simple and efficient operability is demonstrated through theoretical analysis and results comparisons.

Strong effect of fluid rheology on electrokinetic instability and subsequent mixing phenomena in a microfluidic T-junction

When two fluids of different electrical conductivities are transported under the influence of an electric field, the electrokinetic instability (EKI) phenomenon often triggers in a microfluidic device once the electric field strength and conductivity gradient exceed some critical values. This study presents a detailed numerical investigation of how the rheological behavior of a fluid obeyed by the non-Newtonian power-law constitutive relation could influence this EKI phenomenon in a microfluidic T-junction. We find that as the fluid rheological behavior changes from shear-thickening (n >1) to shear-thinning (n <1), the EKI phenomenon is significantly influenced under the same conditions. In particular, the intensity of this EKI phenomenon is found to be significantly higher in shear-thinning fluids than in Newtonian and shear-thickening fluids. Also, the critical value of the applied electric field strength for the inception of this EKI phenomenon gradually increases as the fluid rheological behavior progressively moves from shear-thinning to shear-thickening. The corresponding mixing phenomenon, often achieved using this EKI phenomenon, is also notably higher in shear-thinning fluids compared to Newtonian and shear-thickening fluids. A detailed analysis of both the flow dynamics and mixing phenomena inside the microdevice is presented and discussed in this study. To perform so, we also employ the data-driven dynamic mode decomposition technique, considered one of the widely used reduced-order models to analyze a dynamical system. This analysis facilitates a better understanding of the EKI-induced chaotic convection and mixing phenomena inside the microdevice. We observe that the spatial expanse and intensity of the coherent flow structures differ significantly as the power-law index changes, thereby providing valuable insight into certain aspects of the underlying flow dynamics that, otherwise, are not apparent from other analyses.

Toppling of a rock block resting on a rough surface

The main aim of this study is trying to contribute to a better understanding of the role of surface roughness basal planes on toppling-related instability phenomena. In this way, the authors focus on the stability against toppling of a single block resting on a regular rough surface. To do that they have first artificially created sample rock blocks with a regular rough base and tested them against toppling in a tilt-test machine. The authors have also developed analytical formulations to theoretically estimate toppling instability under these circumstances and have carried out simple numerical DEM models to reproduce the corresponding tests. The comparison between obtained analytical and numerical results and the physical model response indicated good representativeness of both numerical and analytical approaches. Geometry characteristics of the saw cut artificially created blocks did affect results, so the numerical and analytical models were adapted by including an equivalent curvature radius in the corner of the cut block around which the overturning phenomenon takes place to account for this geometrical effect. Further research will extend these results to the case of blocks with natural irregular rough surfaces.

Computational evaluation of the compressive properties of different lattice geometries to be used as temporary implants

Additive manufactured iron lattice structures present unprecedented capabilities for fulfilling the role of bone temporary implants. Iron structures are biodegradable, eliminating the need for removal surgeries, and can help reduce stress shielding significantly by mimicking the properties of bone locally. Although several types of lattice structures have been proposed in the literature, there is the need to evaluate the effect of the relative density on their mechanical properties, in order to resemble bone properties.In the present work, the effects of relative density on the mechanical behavior and in particular, in elastic mechanical properties, were evaluated with finite element simulations, for several types of cells.Lattices, i.e, arrays of unit cells, were designed for 5% increments of relative density, from 5% up to the maximum allowed for each specific cell, for 5 different cell types, creating a total of 63 lattice structures. Finite element models were created for each geometry simulating a compression test, obtaining stress-strain curves and thus the Young's modulus. The material data introduced in the finite element analysis was obtained experimentally from a compression test on bulk iron.The results suggest that all lattice structures evaluated were stable for high density. Cubic (C), truncated cubic (TC), and rhombicuboctahedron (RCO) cells presented instability at low densities while truncated octahedron (TO) and rhombitruncated cuboctahedron (RTCO) cells remained stable at low densities, reaching the value of the Young's modulus of very low-density trabecular bone.Two cell types, RTCO and TO, showed the ability to reproduce the whole range of mechanical properties of bone in a stable manner. The other cell types experienced instability phenomena, like buckling, at low densities.

The impact of soil-vegetation-atmosphere interaction on a landslide initiation under torrential storms: a case study

Vegetation has long been used in geotechnical engineering to protect slopes from surface instability phenomena. However, its effects remain difficult to quantify, given the diversity of plants in situ, the variability of their morpho-mechanical characteristics and their impact on soil hydraulic states. For this purpose, the case of a landslide that occurred following torrential rains in Costa Rica was analysed. After the geotechnical characterisation of the soils affected by the movement, the plant species and their main morpho-mechanical characteristics were identified. These characteristics were used to evaluate the changes in the hydromechanical parameters of the soil due to the presence of the roots, and the transpiration rates generated by the plants themselves. In the FE model, a “continuum material” approach was adopted to simulate the vegetated soil numerically, jointly with a failure criterion for partially saturated soils. Using available meteorological data, the evolution of the hydraulic state of the slope in the weeks before and during the storm that caused the landslide was simulated, comparing the cases of vegetated and hypothetically not vegetated slopes. Results validated the observations carried out on the site and confirmed the hydromechanical effects of plants in delaying in time the triggering of the landslide.

Explosive growth for a constrained Hastings–Levitov aggregation model

We consider a constrained version of the HL(0) Hastings-Levitov model of aggregation in the complex plane, in which particles can only attach to the part of the cluster that has already been grown. Although one might expect that this gives rise to a non-trivial limiting shape, we prove that the cluster grows explosively: in the upper half plane, the aggregate accumulates infinite diameter as soon as it reaches positive capacity. More precisely, we show that after $nt$ particles of (half-plane) capacity $1/(2n)$ have attached, the diameter of the shape is highly concentrated around $\sqrt{t\log n}$, uniformly in $t\in [0,T]$. This illustrates a new instability phenomenon for the growth of single trees/fjords in unconstrained HL(0).

Self-Sustained Snapping Drives Autonomous Dancing and Motion in Free-Standing Wavy Rings.

Harnessing snapping, an instability phenomenon observed in nature (e.g., Venus flytraps), for autonomy has attracted growing interest in autonomous soft robots. However, achieving self-sustained snapping and snapping-driven autonomous motions in soft robots remains largely unexplored. Here, harnessing bistable, ribbon ring-like structures for realizing self-sustained snapping in a library of soft liquid-crystal elastomer wavy rings under constant thermal and photothermal actuation are reported. The self-sustained snapping induces continuous ring flipping that drives autonomous dancing or crawling motions on the ground and underwater. The 3D, free-standing wavy rings employ either a highly symmetric or symmetry-broken twisted shape with tunable geometric asymmetries. It is found that the former favors periodic self-dancing motion in place due to isotropic friction, while the latter shows a directional crawling motion along the predefined axis of symmetry during fabrication due to asymmetric friction. It shows that the crawling speed can be tuned by the geometric asymmetries with a peak speed achieved at the highest geometric asymmetry. Lastly, it is shown that the autonomous crawling ring can also adapt its body shape to pass through a confined space that is over 30% narrower than its body size.

MANAGERIAL PECULIARITIES OF STRATEGIC DEVELOPMENT OF THE TERRITORIAL COMMUNITY

The topicality of the topic is due to the fact that the current state of the economy in Ukraine is characterized by crisis phenomena. After all, an incorrectly chosen strategy or its absence leads to a decrease in the effectiveness of community activity. In this connection, there is a need to manage the strategic development of territorial communities, which will enable communities to improve production and economic activity and get out of the crisis state. We all know that a territorial community is a voluntary association of residents of several villages, towns, and cities that have a single administrative center. Modern managers have to carefully analyze the external environment in order to make timely changes to the community's chosen course in the modern conditions of European integration. In the conditions of martial law, the sharp increase in the importance of management in relation to strategic development is also explained by the acceleration of changes in the socio-economic environment, in the conditions of the growing dynamism of the entrepreneurial environment, the internationalization and globalization of business, the emergence of new unexpected challenges and opportunities for business in communities, the achievement of science and technology, the development information networks and digitization processes, which make it possible to spread and receive information quickly, wide access to modern technologies, changing the role of human resources, as well as a number of other reasons. The purpose of the article is to clarify the essence of the strategic development of the territorial community in the period of instability and crisis phenomena and to substantiate recommendations for the development of appropriate development strategies. In the process of the research, the concepts of "development" and "strategy" were substantiated and clarified, namely, it was stated that the development of the territorial community includes the organizing influences of the environment, dynamic balance with it through self-organization, progressive and regressive processes. In order to create the proper conditions for the development of territorial communities, the factors influencing the formation of their development programs were determined, that is, under such conditions, the community will have the opportunity to develop progressively, if it meets the requirements for openness, dynamism, consistency of processes and imbalance. Recommendations for strategic needs and the need to take into account certain specific factors influencing the formation of strategic development were also substantiated: location, dynamics of its development, its potential; infrastructure development and diversification of economic activity in the community; behavior of investors and partners; characteristics of products or services produced by the TC economy; state of the local economy, cultural environment; health care and the level of quality and consideration of the interests of community residents. The formation of proposals for the creation of basic mechanisms for the work of territorial communities in difficult conditions requires further study.

Aeroelastic tailoring of composite rudder skin considering variable angle tow laminates by a hybrid backtracking search-JAYA-Sine Cosine Algorithm

Flutter is a typical dynamic instability phenomenon of the wing or rudder structure. Besides, the phenomenon of aerodynamic heating will be quite significant and temperature can rise rapidly with the increase of the flight speed, thermal flutter characteristics should be taken into consideration for the rudder structure of supersonic vehicle. To improve the thermal flutter characteristics of the rudder structure and meet the design requirements of a lightweight structure, this article optimizes the variable stiffness laminates of the skin on the premise of keeping the thickness of the skin unchanged, so as to improve the critical flutter speed of the rudder. A novel intelligent optimization algorithm named BJASCA, which is a hybrid algorithm coupling of Backtracking Search Algorithm, Sine Cosine Algorithm, and JAYA Algorithms is proposed in this article to improve the efficiency of structural optimization and this algorithm shows obvious advantages in searching global optima compared with several other algorithms. The finite element model and aerodynamic model of the rudder are established to calculate the critical flutter speed. The results represent that the rudder structure optimized by the BJASCA algorithm shows better flutter characteristics and the critical flutter speed can be increased by 76.4% compared with the un-optimized structure.

Dynamic Instability Characteristics of Spherical Panels Subjected to linearly varying In-plane Loading

The present paper reports the parametric instability phenomenon of spherical shell panels under linearly varying harmonic load employing finite element technique. An isoparametric curved element with eight nodes is considered in the formulation.. The sanders’ theory is used to model the spherical panel incorporating the properties of transverse shear and rotary inertia. A computer programme is created based on the formulation to carry out all essential calculations.The correctness of the formulation is established by comparing with results available in the literature.The impact of different parameters such as load factors, shallowness ratio on the instability regions of spherical panels with three support conditions such as SSSS (all edges simply supported), CCCC (all sides clamped) and CFCF (two opposite edges clamped and other two edges free) are studied. The study demonstrates that the parametric instability behavior of the spherical shell panels is significantly influenced by its shallowness parameter, boundary conditions and type of loads. These results will help the designers for deciding the dimensions of the spherical panels exposed to linearly varying periodic load at the time of the preliminary design.

Mechanical instability tuning of a magnetorheological elastomer composite laminate

Magnetorheological elastomers (MRE) can change their shape and properties when activated by an external magnetic field. The attractive tunable properties of this stimuli-responsive composite can be further amplified by utilizing the magneto-mechanical instability phenomenon, frequently leading to dramatic microstructure transformation. In this work, we investigate the buckling behavior of the composite system consisting of a magnetoactive layer embedded into an inactive elastomeric matrix. The composite is subjected to compressive strains in the presence of a high magnetic field. Our experimental results show that the critical buckling strain is highly tunable by the applied magnetic field. In particular, the composite buckles significantly earlier when the field is applied, leading to well-developed controllable wavy patterns in the post-buckling regime. To elucidate the mechanisms associated with the experimentally observed magneto-mechanical instability, we investigated the stability using numerical analysis. The developed computational model utilizes our new experimental data on the mechanical and magnetic properties of the composite (including magnetization and the magnetic susceptibility of the MRE composite as a function of the volume fraction of magnetic particles). The numerical model is verified against the experimental results, showing its capability to predict the onset of the magneto-mechanical instability and the post-buckling behavior of the MRE. Collectively, the study demonstrates MRE instability tuning in a laminate form factor and outlines the strategy and benefits of harnessing two field physics for controlling bifurcations.

An efficient computational model for adhesive contact mechanics based on a lumped parameter formulation

Adhesive force due to van der Waals interactions plays a significant role in contact interactions at small scale. In this work, we propose a novel simulation module for performing adhesive contact analysis using a finite element (FE) software, based on a lumped parameter formulation. The numerical results are validated by comparing with analytical results. The proposed model significantly reduces computational resource requirements compared to direct implementation of body force in FE software. Adhesion-induced instability (jump-to-contact) phenomenon is studied with the help of this computational model. Finally, we demonstrate the applicability of the proposed formulation by studying the effect of adhesion applied to cantilever dynamics on an atomic force microscope.

Load Power Oriented Large-Signal Stability Analysis of Dual-Stage Cascaded dc Systems Based on Lyapunov-Type Mixed Potential Theory

Dual-stage cascaded dc systems are some of the most widely applied power interfaces in dc distributed power systems. However, in some practical situations, these systems might be unstable, especially if they incorporate tightly regulated load converters that operate as constant power loads (CPLs), whose power fluctuations could exert a cascading impact on the operation of the systems. Existing studies tend to describe the instability phenomena using bifurcation diagram analysis and the loci of eigenvalue analysis. However, it is usually difficult to derive the explicit expressions of the stability criterion. This paper addresses the large-signal stability issue of the dual-stage cascaded dc systems from a standpoint of load power and obtains the explicit form large-signal stability boundary in terms of load power by using Lyapunov-type mixed potential theory. Moreover, the prototype dual-stage cascaded dc system, in which the control strategies for the feeder converter and the load converter are different, is used as an example in this study. According to the results, the system remains stable when the load power is in [5.8, 23.2] W. When load power is less than 5.8 W or increased to [23.2, 32.8] W, the system is in a period-2 subharmonic oscillation state. Moreover, when the load power exceeds 32.8 W, the system falls into a chaotic state. The deduced boundary is highly consistent with the analysis results of both a bifurcation diagram and Jacobian matrix based analysis. Finally, both circuit-level simulation and experimental results validate the effectiveness of the load power stability boundary.

Influence of hydraulic properties to residual soil

Tropical country such as Malaysia experienced high intensity and long period of rainfall, which is the main factor of slope failure. The increasing of seepage subjected by rainfall not only distress suction distribution, but also affects hydraulic properties of soil lead to the decreasing of soil strength. This also leads to soil slope failure. The main hydraulic properties for soils are Soil-water characteristic curve (SWCC) and hydraulic conductivity, k. The relationship of these parameters is highly influenced by tropical soil, thus contributes to soil instability phenomenon. The main objective of this paper is to study the relationship between soils' hydraulic properties and particle size distribution (PSD). Four types of soils investigated in this research include one sample taken from Universiti Kebangsaan Malaysia (UKM)'s reserved forest, and three samples were commercially obtained. Other than soil basic properties, pressure plate extractor and permeability tests were conducted for SWCC and permeability, respectively. Tests show that different types of soils demonstrate different characteristic of SWCC and hydraulic conductivity. Result also shows that PSD has the influence and affect the result of hydraulic properties of soils. This study is expected to enhance the understanding of complex phenomenon of residual soil instability.

Origin of clay in granite intrusive contact zone and its influence on tunnel deformation and instability

The special properties of the clay derived alteration developed in the granite intrusive contact zone under the condition of humidity variation have extremely complex effects on engineering. In view of the deformation and destabilization of Yuxi Tunnel in China, detailed geological investigation has found that the deformation and destabilization section is located in the intrusive contact zone of granite and sandstone, in which the clay and marble are developed. The lithofacies and mineral composition test results of the clay show that it is derived from the alteration of hydrothermal fluids and belongs to the altered rock, rather than product of weathering or faulting. Results of laboratory tests show that the clay derived alteration has the properties of strong water absorption and expansion. In addition, its shear strength decreases significantly with the increase of water content. The characteristics of special topography, occurrence conditions, mineral composition and structure of the clay derived alteration are the inherent causes of the deformation and instability of the tunnel. Furthermore, numerical simulation of the response characteristics of the support structure under humidification conditions is realized. The calculation results show that when considering negative effects such as expansion and strength degradation of the surrounding rock, the disturbed range of surrounding rock and the deformation of tunnel vault are greatly increased, and significant stress concentration occurs at tunnel spandrel and arch foot. At the same time, the deformation and stress evolution of the supporting structure show certain accumulation and delay characteristics during the humidification process. The research in this paper not only clarifies the evolution of the displacement and stress of the tunnel support structure with the water content, but also reveals the origin of instability phenomena such as ground fissures, surface collapse, large deformation and collapse in the tunnel. This research methods and results can be used as reference for similar projects.