Nonvolatile Film Research Articles

Stability of thin liquid films and sessile droplets under confinement

The stability of nonvolatile thin liquid films and of sessile droplets is strongly affected by finite size effects. We analyze their stability within the framework of density functional theory using the sharp kink approximation, i.e., on the basis of an effective interface Hamiltonian. We show that finite size effects suppress spinodal dewetting of films because it is driven by a long-wavelength instability. Therefore nonvolatile films are stable if the substrate area is too small. Similarly, nonvolatile droplets connected to a wetting film become unstable if the substrate area is too large. This instability of a nonvolatile sessile droplet turns out to be equivalent to the instability of a volatile drop which can attain chemical equilibrium with its vapor.

Bifurcation in a thin liquid film flowing over a locally heated surface

AbstractWe investigate the nonlinear dynamics of a two-dimensional film flowing down a finite heater, for a non-volatile and a volatile liquid. An oscillatory instability is predicted beyond a critical value of the Marangoni number using linear stability theory. Continuation along the Marangoni number using a nonlinear evolution equation is employed to trace the bifurcation diagram associated with the oscillatory instability. Hysteresis, a characteristic attribute of a subcritical Hopf bifurcation, is observed in a critical parametric region. The bifurcation is universally observed for both a non-volatile film and a volatile film.

Linear stability of a volatile liquid film flowing over a locally heated surface

The dynamics and linear stability of a volatile liquid film flowing over a locally heated surface are investigated. The temperature gradient at the leading edge of the heater induces a gradient in surface tension that leads to the formation of a pronounced capillary ridge. Lubrication theory is used to develop a model for the film evolution that contains three key dimensionless groups: a Marangoni parameter (M), an evaporation number (E), and a measure of the vapor pressure driving force for evaporation (K), which behaves as an inverse Biot number. The two-dimensional, steady solutions for the local film thickness are computed as functions of these parameters. A linear stability analysis of these steady profiles with respect to perturbations in the spanwise direction reveals that the operator of the linearized system can have both a discrete and a continuous spectrum. The continuous spectrum exists for all values of the spanwise wave number and is always stable. The discrete spectrum, which corresponds to eigenfunctions localized around the ridge, appears for values of M larger than a critical value for a finite band of wave numbers separated from zero. Above a second, larger critical value of M, a portion of the discrete spectrum becomes unstable, corresponding to rivulet formation at the forward portion of the capillary ridge. For sufficiently large heat transfer at the free surface, due either to phase change or to convection, a second band of unstable discrete modes appears, which is associated with an oscillatory, thermocapillary instability above the heater. The critical Marangoni parameter above which instability develops, Mcrit(K,E), has a nonmonotonic dependence on the steepness of the temperature increase at the heater, in contrast to the monotonic decrease for a nonvolatile film at vanishing Biot number. An energy analysis reveals that the dominant instability mechanism resulting from perturbations to the film thickness is either streamwise capillary flow or gravity for weakly volatile fluids and thermocapillary flow due to spanwise temperature gradients for more volatile fluids. The stability results are rather sensitive to the steepness of the temperature increase and heater width due to the nonlinear coupling of gravity, capillary pressure gradients, thermocapillary flow, and evaporation through the base states.

Three-dimensional nonlinear dynamics of thin liquid films

Three-dimensional dynamics of thin Newtonian liquid films subjected to long-range van der Waals forces on a horizontal coated solid surface is numerically studied in the framework of the long-wave theory. The dynamics of nonvolatile films results in the emergence of an isolated steady drop standing on a practically flat film, while volatile films uniformly disappear on the macroscale. In both cases the evolution of the initial small-amplitude noise spans over the stages of self-organization, fast thinning of the depressions, formation and expansion of the "holes," emergence of the polygonal network of liquid ridges, and its breakup, as seen in the experiments.

Ultrastructure of the thoracic dorso-medial field (TDM) in the elytra-to-body arresting mechanism in Tenebrionid Beetles (Coleoptera: Tenebrionidae).

Beetles with flying ability lock their elytra (forewings) to the thorax or/and abdomen using complex locking devices. These structures are often supplemented with microtrichia fields of the inner surface of the elytra and adjacent parts of the pterothorax. The present study provides information about the ultrastructure of microtrichia of the dorso-medial fields of the thorax (TDM) in tenebrionid beetles (Tribolium castaneum, Zophobas rugipes). Epidermal cells located under the TDM field contain large electron-lucent vesicles connected to rough endoplasmic reticulum. Microtrichia and underlying cuticle of the TDM have a high density of pore channels, which are responsible for transport of an epidermal secretion onto the TDM surface. In order to show the presence of the secretion on the cuticle surface, TDM fields of air-dried specimens were compared with those in specimens after two treatments, such as (1) dehydration in ethanol and acetone, and (2) dental-wax-cast technique applied to living beetles. This revealed the presence of the nonvolatile film on the intact microtrichial surface. Possible functions of this film are suggested to be (1) the increase of adhesive forces in the contact area and (2) providing soft coupling and release of two corresponding parts of the elytra-locking device. J. Morphol. 240:101-113, 1999. © 1999 Wiley-Liss, Inc.