We investigate the onset of thermosolutal instabilities in a moderately dense nanoparticle suspension layer with a deformable interface. The suspension is deposited on a solid substrate subjected to a specified constant heat flux. The Soret effect and the action of gravity are taken into account. A mathematical model for the system considered with nanoparticle concentration-dependent density, viscosity, thermal conductivity and the Soret coefficient is presented in dimensional and non-dimensional forms. Linear stability analysis of the obtained base state is carried out using disturbances in the normal mode, and the corresponding eigenvalue problem is derived and numerically investigated. The onset of various instabilities is investigated for cases of both heating and cooling at the substrate. The monotonic solutocapillary instability is found in the case of cooling at the substrate, which exhibits two competing mechanisms that belong to two different disturbance wavelength domains. We identify the occurrence of both monotonic and oscillatory thermocapillary instabilities when the system is heated at the substrate. Furthermore, we show the emergence of the solutal buoyancy instability due to density variation which is promoted by the Soret effect adding nanoparticles heavier than the carrier fluid in the proximity of the layer interface. Transitions from the monotonic to oscillatory thermocapillary instability are found with variation in the gravity- and solutocapillarity-related parameters. Notably, we identify a previously unknown transition from monotonic to the oscillatory thermocapillary instability due to the variation in the strength of the thermal-conductivity stratification coupled with the Soret effect.

The thermocapillary flows generated by an inclined temperature gradient in and around a floating droplet are studied in the framework of the lubrication approximation. Numerical simulations of nonlinear flow regimes are fulfilled. It is shown that under the action of Marangoni stresses, a droplet typically moves as a whole. It is found that an inclined temperature gradient can lead to the excitation of periodic oscillations. With an increase of the inclination of the temperature gradient, temporally quasi-periodic oscillations have been obtained. In a definite region of parameters, an inclined temperature gradient can suppress oscillations, changing the droplet’s shape. The diagram of regimes in the plane of longitudinal and transverse Marangoni numbers has been constructed. Bistability has been found.

The action of temporal heating modulation on Marangoni flows in a droplet on a liquid substrate is investigated. The problem is studied numerically in the framework of long-wave amplitude equations and precursor model. It is shown that temporal modulation can lead to a change of the droplet shape. Specifically, rhombic droplets have been obtained. A modulated cooling from below can lead to periodic or quasiperiodic oscillations or the droplet's decomposition.

The dynamics of a droplet on an inhomogeneously cooled liquid substrate is investigated numerically. The longwave approximation is applied. It is shown that spatial temperature modulation leads to the droplet's motion towards the region of lower temperature, which is accompanied by the change of the droplet shape. An intensive cooling from below can lead to periodic or quasiperiodic oscillations or the droplet's decomposition. A spatial temperature modulation can suppress the oscillatory instability.

In the presence of temperature gradients along the gas–liquid interface, a liquid bridge is prone to hydrothermal instabilities. In the case of a coaxial gas stream, in addition to buoyancy and thermocapillary forces, the shear stresses and interfacial heat transfer affect the development of these instabilities. By combining experimental data with three-dimensional numerical simulations, we examine the evolution of hydrothermal waves in a liquid bridge with $Pr=14$ with a gas flow parallel to the interface. The gas moves from the cold to the hot side with a constant velocity of $0.5\ \textrm {m}\ \textrm {s}^{-1}$ and its temperature is the main control parameter of the study. When the thermal stress ${\rm \Delta} T$ exceeds a critical value ${\rm \Delta} T_{cr}$, a three-dimensional oscillatory flow occurs in the system. A stability window of steady flow has been found to exist in the map of dynamical states in terms of gas temperature and applied thermal stress ${\rm \Delta} T$. The study is carried out by tracking the evolution of hydrothermal waves with increasing gas temperature along three distinct paths with constant values of ${\rm \Delta} T$: path 1 is selected to be just above the threshold of instability while path 2 traverses the stability window and path 3 lies above it. We observe a variety of dynamics including standing and travelling waves, determine their dominant and secondary azimuthal wavenumbers, and suggest the mathematical equations describing hydrothermal waves. Multimodal standing waves, coexistence of travelling waves with several wavenumbers rotating in the same or opposite directions are among the most intriguing observations.

We use linear proportional control for the suppression of the Marangoni instability in a thin film heated from below. Our keen interest is focused on the recently revealed oscillatory mode caused by a coupling of two long-wave monotonic instabilities, the Pearson and deformational ones. Shklyaev et al. (Phys. Rev. E, vol. 85, 2012, 016328) showed that the oscillatory mode is critical in the case of a substrate of very low conductivity. To stabilize the no-motion state of the film, we apply two linear feedback control strategies based on the heat flux variation at the substrate. Strategy (I) uses the interfacial deflection from the mean position as the criterion of instability onset. Within strategy (II) the variable that describes the instability is the deviation of the measured temperatures from the desired, conductive values. We perform two types of calculations. The first one is the linear stability analysis of the nonlinear amplitude equations that are derived within the lubrication approximation. The second one is the linear stability analysis that is carried out within the Bénard–Marangoni problem for arbitrary wavelengths. Comparison of different control strategies reveals feedback control by the deviation of the free surface temperature as the most effective way to suppress the Marangoni instability.

The nonlinear dynamics of waves generated by the deformational oscillatory Marangoni instability in a two-layer film under the action of a two-dimensional temperature modulation on the solid substrate is considered. A system of long-wave equations governing the deformations of the upper surface and the interface between the liquids is presented. The long-wave approach is applied. The nonlinear simulations reveal the existence of different dynamic regimes, including stationary, time-periodic and quasi-periodic flows. The general diagrams of the flow regimes are constructed.

The nonlinear dynamics of waves generated by the deformational oscillatory Marangoni instability in a two-layer film under the action of a spatial temperature modulation on the solid substrate is considered. A system of long-wave equations governing the deformations of the upper surface and the interface between the liquids is derived. The nonlinear simulations reveal the existence of numerous dynamical regimes, including two-dimensional stationary flows and standing waves, three-dimensional standing waves with different spatial periods, and three-dimensional travelling waves. The general diagram of the flow regimes is constructed.

We have discovered a peculiar behaviour of the interface between two miscible liquids placed in a finite-size container under horizontal vibration. We provide evidence that periodic wave patterns created by the Kelvin–Helmholtz instability and Faraday waves simultaneously exist in the same system of miscible liquids. We show experimentally in reduced and normal gravity that large-scale frozen waves yield Faraday waves with a smaller wavelength on a diffusive interface. The emergence of the different scale patterns observed in the experiments is confirmed numerically and explained theoretically.

Longwave Marangoni convection in two-layer films under the action of heating modulation is considered. The analysis is carried out in the lubrication approximation. The capillary forces are assumed to be sufficiently strong, and they are taken into account. Periodic or symmetric boundary conditions are applied on the boundaries of the computational region. Numerical simulations are performed by means of a finite-difference method. Two regions of parametric instabilities have been found. In the first region, one observes the competition or coexistence of standing waves parallel to the boundaries of the computational region. The multistability of the flow regimes is revealed. In the second region, the regimes found in the case of periodic boundary conditions are more diverse than in the case of symmetric boundary conditions.

The influence of time-periodic vibrations on long Marangoni waves in two-layer films is investigated. The problem is governed by a system of nonlinear equations obtained in the framework of the lubrication approximation. Periodic boundary conditions are applied on the boundaries of the computational region. The development of instabilities is investigated by means of nonlinear simulations. Excitation of two-dimensional and three-dimensional subharmonic wavy regimes is studied. A new phenomenon, the excitation of nonlinear waves with a temporal period that is four times larger than that of the gravity modulation, is revealed.

We consider Marangoni convection in a heated layer of a binary liquid. The solute is a surfactant, which is present in both surface and bulk phases; the bulk gradient of the concentration is formed due to the Soret effect. Linear stability analysis demonstrates a well-pronounced stabilization of the layer due to the adsorption kinetics and advection of the surface phase. We derive nonlinear amplitude equations for longwave perturbations in the case of fast sorption kinetics (small Langmuir number) and demonstrate that with increase in the effect of the adsorption, subcritical excitation occurs. In the case of a finite Langmuir number, the weakly nonlinear problem is ill-posed. A physical mechanism of subcritical bifurcation is discussed.

We present the results of a numerical study of the thermocapillary (Marangoni) convection in a liquid bridge of $\mathit{Pr}= 12$ ( $n$ -decane) and $\mathit{Pr}= 68$ (5 cSt silicone oil) when the interface is subjected to an axial gas stream. The gas flow is co- or counter-directed with respect to the Marangoni flow. In the case when the gas stream comes from the cold side, it cools down the interface to a temperature lower than that of the liquid beneath and in a certain region of the parameter space that cooling causes an instability due to a temperature difference in the direction perpendicular to the interface. The disturbances are swept by the thermocapillary flow to the cold side, which leads to the appearance of axisymmetric waves propagating in the axial direction from the hot to cold side. The mechanism of this new two-dimensional oscillatory instability is similar to that of the Pearson’s instability of the rest state in a thin layer heated from below (Pearson, J. Fluid Mech., vol. 4, 1958, p. 489), and it appears at the value of the transverse Marangoni number ${ \mathit{Ma}}_{\perp }^{cr} \approx 39\text{{\ndash}} 44$ lower than that of the Pearson’s instability in a horizontal layer ( $48\lt { \mathit{Ma}}_{\perp }^{cr} \lt 80$ , depending on the Biot number). The generality of the instability mechanism indicates that it is not limited to cylindrical geometry and might be observed in a liquid layer with cold gas stream.

We study the influence of low-frequency vibration on Marangoni instability in a layer of a binary mixture with the Soret effect. A linear stability analysis is performed numerically for perturbations of a finite wavelength (short-wave perturbations). Competition between long-wave and short-wave modes is found: the former ones are critical at smaller absolute values of the Soret number $\chi $ , whereas the latter ones lead to instability at higher $\vert \chi \vert $ . In both cases the vibration destabilizes the layer. Two variants of calculations are performed: via Floquet theory (linear asymptotic stability) and taking noise into consideration (empirical criterion). It is found that fluctuations substantially reduce the domains of stability. Further, while studying a limiting case within the empirical criterion, we have found a short-wave instability mode overlooked in former investigations of coupled Rayleigh–Marangoni convection in a layer of pure liquid.

The influence of a global delayed feedback control which acts on a system governed by asubcritical complex Ginzburg-Landau equation is considered. The method based on avariational principle is applied for the derivation of low-dimensional evolution models.In the framework of those models, one-pulse and two-pulse solutions are found, and theirlinear stability analysis is carried out. The application of the finite-dimensional modelallows to reveal the existence of chaotic oscillatory regimes and regimes withdouble-period and quadruple-period oscillations. The diagram of regimes resembles thosefound in the damped-driven nonlinear Schrödinger equation. The obtained results arecompared with the results of direct numerical simulations of the original problem.

The effect of gravity on the dynamics of non-isothermic ultra-thin two-layer films is studied in this paper. The joint action of disjoining pressure and thermocapillary forces is taken into account. The problem is considered in a long-wave approximation. The linear stability of a quiescent state and thermocapillary flows is investigated. It has been found that the influence of the upper fluid density is significantly stronger than that of the difference of fluid densities. Nonlinear flow regimes are studied by means of numerical simulations. The gravity can lead to the formation of stripes or holes instead of droplets. The two-dimensional wavy patterns are replaced by one-dimensional waves with the fronts inclined or transverse to the direction of the horizontal temperature gradient.

A global feedback control of a system that exhibits a subcritical monotonic instabilityat a non-zero wavenumber (short-wave, or Turing instability) in the presence of a zeromode is investigated using a Ginzburg-Landau equation coupled to an equation for the zeromode. The method based on a variational principle is applied for the derivation of alow-dimensional evolution model. In the framework of this model the investigation of thesystem’s dynamics and the linear and nonlinear stability analysis are carried out. Theobtained results are compared with the results of direct numerical simulations of theoriginal problem.

The development of instabilities under the joint action of the van der Waals forces and Marangoni stresses in a two-layer film in the presence of an inclined temperature gradient is investigated. The problem is solved by means of a linear stability theory and nonlinear simulations. It has been found that for sufficiently large values of the ratio between the longitudinal and transverse Marangoni numbers, the real part of the linear growth rate does not depend on the direction of the wavenumber, except the case of nearly longitudinal disturbances. Numerous types of nonlinear evolution have been observed, among them are ordered systems of droplets, ‘splashes’, oblique waves, modulated transverse and longitudinal structures.