Layer Of Viscoelastic Liquid Research Articles

Purely elastic instabilities in the airways and oral area

The present study considers a shear-thinning viscoelastic liquid layer sheared by the air and flowing past a deformable-solid layer in the presence of a surfactant at the air–liquid interface to model the airflow in the oral area and airways. The stability analysis reveals the existence of purely elastic and unconditionally unstable ‘liquid elastic’ and ‘solid elastic’ modes. The mechanism responsible for the destabilisation of the solid elastic mode is the shear stresses exerted by the air on the liquid and by the liquid on the deformable solid while for the liquid elastic mode, the mechanism is the first normal stress difference across the air–liquid interface. The liquid and solid elastic modes undergo resonance, resulting in the ‘resonance mode’ of instability. The resonance mode exhibits a much higher growth rate than the liquid and solid elastic modes. The shear-thinning characteristic of the liquid and presence of the surfactant leads to enhancement in the growth rate of the resonance mode. An estimate shows a good correlation between the exhaled fluid particle (i.e. droplets and aerosols) diameters and the wavelength of the perturbations with maximum growth rate. In essence, the present analysis predicts that the airflow in the airways and oral area could lead to an elastic instability arising due to the elastic nature of the saliva, mucus and underlying muscle layers.

Effect of internal heat source modulations on the onset of triple diffusive convection in viscoelastic liquids

The paper aims to study the dynamic behavior of a triple diffusive system subjected to sinusoidal (trigonometric cosine) and non-sinusoidal wave forms (square, sawtooth and triangular) of internal heat source modulation. The configuration of the system is such that a layer of viscoelastic liquid is heated and salted with two solutes from below. An Oldroyd-B type model is made use for viscoelastic liquids. In order to regulate the convection onset, internal heat source modulation is applied. This investigation is modelled using a linear stability analysis where a stationary convection is preferred. Venezian approach facilitates a solution by finding the eigen values of the problem. The influence of pertinent parameters which are varied for a wide range of values have been reported. It is captured via graphs that for small values of frequency of modulation, square wave form is more stable while sawtooth wave form is more stable for an increment in the values of frequency of modulation. Further, liquids such as Newtonian, Maxwell and Rivlin-Ericksen are analysed as the limiting cases of the problem. It seems worthwhile to discuss the results of the present study as it is the first work on linear theory of different wave forms of internal heat source modulation and thus paves a way for new theoretical and experimental endeavors.

Azimuthal instability modes in a viscoelastic liquid layer flowing down a heated cylinder

In this paper the non axisymmetric longwave instability of a thin viscoelastic liquid film flowing down a vertical heated cylinder is investigated. The stability of the film coating a cylinder in the absence of gravity is also investigated. In a previous paper it is found that viscoelasticity stimulates the appearance of azimuthal modes but the axial mode is the most unstable one. Other calculations in a former paper show that for flow outside a heated cylinder azimuthal modes can be the more unstable when the Marangoni number is large and, in particular, when the Reynolds number and wavenumber are small. Therefore, the small wavenumber and large cylinder radius approximation is assumed with the simultaneous action of viscoelasticity and thermocapillarity on the stability of azimuthal modes. In the presence and in the absence of gravity, it is found that, in comparison with the Newtonian case, it is easier to excite the azimuthal modes when viscoelasticity and thermocapillarity destabilize at the same time. Moreover, it is shown that, despite the axial mode is the most unstable one, there are wide wavenumber ranges where higher modes are the more unstable and they can show up by means of a periodic time dependent perturbation.

Biomimetic Wet Adhesion of Viscoelastic Liquid Films Anchored on Micropatterned Elastic Substrates

Inspired by the natural adhesives in the toe pads of arthropods and some other animals, we explore the effectiveness and peel failure of a thin viscoelastic liquid film anchored on a micropatterned elastic surface. In particular, we focus on the role of the substrate pattern in adhesion energy of the liquid layer and in allowing its clean separation without cohesive failure. Peel tests on the microfabricated wet adhesives showed two distinct modes of adhesive (interfacial) and cohesive (liquid bulk) failures depending on the pattern dimensions. The adhesion energy of a viscoelastic liquid layer on an optimized micropatterned elastic substrate is ~3.5 times higher than that of a control flat bilayer and ~26 times higher than that of a viscoelastic film on a rigid substrate. Adhesive liquid layers anchored by narrow microchannels undergo clean, reversible adhesive failure rather than the cohesive failure seen on flat substrates. An increase in the channel width engenders cohesive failure in which droplets of the wet adhesive remain on the peeled surface.

Onset of electroconvective instability of Oldroydian viscoelastic liquid layer in Brinkman porous medium

The problem of the onset of electrohydrodynamic instability in a horizontal layer of Oldroydian viscoelastic dielectric liquid through Brinkman porous medium under the simultaneous action of a certical ac electric field and a vertical temperature gradient is analyzed. Applying linear stability theory, we derive an equation of eight order. Under somewhat suitable boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. Both the cases of stationary and oscillatory instabilities are discussed if the liquid layer is heated from below or above. The effects of the porosity of porous medium, the medium permeability, the Prandtl number, the ratio of retardation time to relaxation time, the elastic number, in the presence or absence of Rayleigh number are shown graphically for both cases. Some of the known results are derived as special cases. The electrical force has been shown to be the sole agency causing instability of the considered system since it is much more important than the buoyancy force even if the medium is porous.

Shear horizontal acoustic waves at the boundary of two piezoelectric crystals separated by a liquid layer

Transverse acoustic waves that occur at the boundary of two piezoelectric halfspaces separated by a viscoelastic liquid layer are studied theoretically. Three variants of layered structures with different numbers of metallized interfaces are considered. It is demonstrated that two types of waves exist in each of these structures. The waves are localized near the boundaries and differ in both their structure and the dependences of the complex wave numbers on the frequency or the gap width. The properties of this family of shear horizontal waves are described.

Autooscillation instability of the free surface of a viscoelastic medium

Oscillation instability of a layer of viscoelastic liquid is revealed on the basis of the numerical analysis of the dispersion equation. The instability is observed in the case of time-independent uniform external force action on the free surface of the liquid and may lead to the formation of a regular wavelike relief of a finite amplitude.

Growth rates of the Marangoni instability in a layer of elastic liquid

The problem of thermocapillary (Marangoni) convection in a layer of viscoelastic liquid is considered. The stability boundary for this problem has been previously calculated in various cases by a number of authors. Here attention is fixed on the magnitude of the growth rate in the parameter regime corresponding to instability. Two noteworthy features are pointed out. First, there are anomalously large values of the growth rate at or near the limiting special case of a Maxwell fluid. Second, the complex values of the growth rate (corresponding to overstability, or the onset of instability via oscillatory motion) coalesce into real (positive) values at moderately supercritical values of the Marangoni number, suggesting that overstability might be elusive to observation.

The stability of natural convection in a vertical layer of viscoelastic liquid

Linear stability theory is applied to the problem of stability of natural convection that occurs in a layer of viscoelastic liquid (Oldroyd liquid B) between two parallel vertical plates maintained at constant and different temperatures. The modified Orr-Sommerfeld equation and the thermal disturbance equation are derived without making any approximation other than the Boussinesq one. The Chebyshev collocation method is used to obtain the eigenvalue equation, which is then solved numerically with the aid of the Muller method, and the stability conditions are obtained for wide ranges of various parameters. It is found that depending upon the values of various parameters, instability sets in as either stationary or travelling-wave mode, and that elasticity has both stabilizing and destabilizing effects on the flow.

Overstability of a viscoelastic liquid layer with internal heat generation

Overstability of a viscoelastic liquid layer with internal heat generation

Thermoconvective Waves in a Viscoelastic Liquid Layer Situated in a Magnetic Field

The effect of the presence of a magnetic field on the propagation of thermo-convective waves (TCW) in a horizontal layer of electrically conducting, viscoelastic liquid (Oldroyd's liquid B) heated from below is investigated on the assumption that both layer surfaces are free. The presence of a magnetic field decreases (increases) the logarithmic decrement (the damping length) of TCW for supercritical Rayleigh numbers and increases the frequency above which the thermal waves propagate almost undamped, although the qualitative properties of TCW are hardly altered by the presence of a magnetic field.

Thermoconvective Waves in a Viscoelastic Liquid Layer

The propagation of thermoconvective waves in a horizontal layer of viscoelastic liquid (Oldroyd's liquid B) heated from below is investigated. It is shown that the wave propagates without damping at the critical condition for the onset of overstability and that as the elastic parameter increases the damping length (the logarithmic decrement) becomes large (small) near the critical Rayleigh number for the onset of stationary instability. When the ratio of retardation time to relaxation time is very small, the weakly damped wave can exist for high frequencies even if the liquid layer is isothermal.

Electrohydrodynamic Instability in a Viscoelastic Liquid Layer

The problem of the onset of instability in a horizontal layer of viscoelastic dielectric liquid under the simultaneous action of a vertical ac electric field and a vertical temperature gradient is analyzed. Applying linear stability theory, an equation of eighth order is derived. Under somewhat artificial boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. It is shown that oscillatory modes of instability exist only when the thickness of the liquid layer is smaller than about 0.5 mm and for such a thin layer the force of electrical origin is much more important than buoyancy force.

Thermal instability of a horizontal layer of viscoelastic liquid

The convective instability of a horizontal layer of viscoelastic liquid during heating from below has been examined. Critical values of the Rayleigh numbers for two forms of limiting conditions have been determined.

Instability of a horizontal layer of viscoelastic liquid on an oscillating plane

Secular instability of unsteady flow of a horizontal layer of viscoelastic liquid set in motion by simple harmonic motion of the lower boundary in its own plane is investigated. Using an extension of Floquet's theory for ordinary differential equations, the stability of long waves is studied by a regular perturbation method. The elastic parameter of the fluid is found to be destabilizing and stabilizing in different ranges of frequency.