Jean Claude Legros

Improved three-dimensional imaging with a digital holography microscope with a source of partial spatial coherence

A digital holographic technique is implemented in a microscope for three-dimensional imaging reconstruction. The setup is a Mach-Zehnder interferometer that uses an incoherent light source to remove the coherent noise that is inherent in the laser sources. A phase-stepping technique determines the optical phase in the image plane of the microscope. Out-of-focus planes are refocused by digital holographic computations, thus considerably enlarging the depth of investigation without the need to change the optical focus mechanically. The technique can be implemented in transmission for various magnification ratios and can cover a wide range of applications. Performances and limitations of the microscope are theoretically evaluated. Experimental results for a test target are given, and examples of two applications in particle localization and investigation of biological sample are provided.

Digital holographic microscopy for the three-dimensional dynamic analysis of in vitro cancer cell migration.

Cancer cell motility and invasion are critical targets for anticancer therapeutics. Whereas in vitro models could be designed for rapid screening with a view to investigate these targets, careful consideration must be given to the construction of appropriate model systems. Most investigations focus on two-dimensional (2-D) assays despite the fact that increasing evidence suggests that migration across rigid and planar substrates fails to recapitulate in vivo behavior. In contrast, few systems enable three-dimensional (3-D) cell migration to be quantitatively analyzed. We previously developed a digital holographic microscope (DHM) working in transmission with a partially spatial coherence source. This configuration avoids the noise artifacts of laser illumination and makes possible the direct recording of information on the 3-D structure of samples consisting of multiple objects embedded in scattering media, such as cell cultures in matrix gels. The software driving our DHM system is equipped with a time-lapse ability that enables the 3-D trajectories of living cells to be reconstituted and quantitatively analyzed.

Interfacial convection in multilayer systems

Introduction.- Types of convective instabilities in systems with an interface.- Benard problem in multilayer systems with undeformable interfaces.- Benard problem in multilayer systems with deformable interfaces.- Stability of flows.- Outlook.

Validity domain of the Benney equation including the Marangoni effect for closed and open flows

The Benney equation including thermocapillary effects is considered to study a liquid film flowing down a homogeneously heated inclined wall. The link between the finitetime blow-up of the Benney equation and the absence of the one-hump travelling-wave solution of the associated dynamical system is accurately demonstrated in the whole range of linearly unstable wavenumbers. Then the blow-up boundary is tracked in the whole space of parameters accounting for flow rate, surface tension, inclination and thermocapillarity. In particular, the latter two effects can strongly reduce the validity range of the Benney equation. It is also shown that the subcritical bifurcation found for falling films with the Benney equation is related to the blow-up of solutions and is unphysical in all cases, even with the thermocapillary effect though in contrast to horizontally heated films. The accuracy of bounded solutions of the Benney equation is determined by comparison with a reference weighted integral boundary layer model. A distinction is made between closed and open flow conditions, when calculating travelling-wave solutions; the former corresponds to the conservation of mass and the latter to the conservation of flow rate. The open flow condition matches experimental conditions more closely and is explored for the first time through the associated dynamical system. It yields bounded solutions for larger Reynolds numbers than the closed flow condition. Finally, solutions that are conditionally bounded are found to be unstable to disturbances of larger periodicity. In this case, coalescence is the pathway yielding finite-time blow-up.

Nonlinear evolution of nonuniformly heated falling liquid films

The present theoretical study focuses on the dynamics of a thin liquid film falling down a vertical plate with a nonuniform, sinusoidal temperature distribution. The results are compared to those obtained in the case of the uniform temperature distribution. The governing evolution equation for the film thickness profile based on long-wave theory accounts for two instability mechanisms related to thermocapillarity. The first mechanism is due to an inhomogeneity of the temperature at the liquid–gas interface induced by perturbations of the film thickness, when heat transfer to the gas phase is present, while the second one is due to the nonuniform heating imposed at the plate and leads to steady-state deformations of the liquid–gas interface. For a moderate nonuniform heating the traveling waves obtained in the case of a uniform heating are modulated by an envelope. When the temperature modulation along the plate increases the shape of the liquid–gas interface becomes “frozen” and the oscillatory traveling ...

Long-wave instabilities of non-uniformly heated falling films

We consider the problem of a thin liquid layer falling down an inclined plate that is subjected to non-uniform heating. The plate temperature is assumed to be linearly distributed and both directions of the temperature gradient with respect to the flow are investigated. The film flow is not only influenced by gravity and mean surface tension, but in addition by the thermocapillary force acting along the free surface. The coupling of thermocapillary instability and surface-wave instabilities is studied for two-dimensional disturbances. Applying the long-wave theory, a nonlinear evolution equation is derived. When the plate temperature is decreasing in the downstream direction, linear stability analysis exhibits a film stabilization, compared to a uniformly heated film. In contrast, for increasing temperature along the plate, the film becomes less stable. Numerical solution of the evolution equation indicates the existence of permanent finite-amplitude waves of different kinds. The shape of the waves depends mainly on the mean flow and the mean surface tension, but their amplitudes and phase speeds are influenced by thermocapillarity.

On the principle of corresponding states for the viscosity of simple liquids

Synopsis The aim of this paper is to discuss the validity of the principle of corresponding states as applied to the viscosity of simple liquids like Ar, Kr, Xe, 02, Nz, CO, CH4, CD4. New measurements of one ot us (J.C.L.) for CO and Nz are reported. The apparatus used by the authors in Brussels is also described. A. Introduction. The principle of corresponding states has been applied by the authorsl) 2) to the phenomenon of liquid viscosity, and they have investigated this field from a purely phenomenological point of view,**) on the basis of their experiments on liquified gases3) 4) 5) and their mixtures2) 6) 7). In this paper, we shall be concerned only with the pure liquids, our purpose being to find out whether or not simple liquids, according to their structure, satisfy a law of corresponding states for a transport property (namely the viscosity coefficient). Section B is devoted to a description of the experimental procedure. The results for the viscosity coefficients are given in section C and displayed in tables I to VIII as a function of the temperature: in our previous publications3) 4) relative data were given for Ax-, Kr, 02, CH4 and CD,; all the coefficients are reported in the present paper as absolute values. ***) Since the data found in the literature for Nz and CO seem to be partially inconsistent we have thought it to be of interest to re-investigate these two liquids and obtained results slightly different from the previously reported data. The coefficients for liquid Xe have been recalculated from our previous measurementss). Finally in section D the results are discussed in terms of the theorem of corresponding states.

Heat transfer and rivulet structures formation in a falling thin liquid film locally heated

An experimental investigation of the heat transfer from a local heat source to a liquid film falling down a vertical plate is performed. The thermocapillary counterflow, induced by non-uniform heating, causes a deformation of the film surface having a horizontal bump-shape. This shape becomes unstable above a critical value of the imposed heat flux and deforms into vertical downstream rivulets. This variation of patterns is expected to modify significantly the heat transfer through the film. Experiments are carried out at atmospheric pressure with three varying parameters: the streamwise heater length, the Reynolds number and the imposed heat flux density. Velocimetry, shadowgraphy and infrared thermography are used to study the behavior of the interface and the heat transfer. We put in evidence the presence of a thermocapillary counter flow producing a stagnation line at the upper edges of the horseshoe structures, beyond the instability threshold, and observe a decrease of the heat transfer with the Reynolds number.

Phase-shifting schlieren: high-resolution quantitative schlieren that uses the phase-shifting technique principle

A quantitative autocalibrated high-resolution schlieren technique for quantitative measurement of reflective surface shape is proposed. It combines the schlieren principle with the phase-shifting technique that is generally used in interferometry. With an appropriate schlieren filter and appropriately tailored setup, some schlieren fringes are generated. After application of the phase-shift technique, the schlieren phase is calculated and converted into beam deviation values. Theoretical and experimental demonstrations are given. The technique is validated on a reference target, and then its application in a fluid physics experiment is demonstrated. These two examples show the potential of the phase-shifting schlieren technique that in some situations can become competitive with interferometry but with a much better dynamic range and with variable sensitivity. The technique can also be used to measure refractive-index gradients in transparent media.

OSCILLATORY CONVECTIVE MOTION IN DEFORMED LIQUID BRIDGES

The transition from two-dimensional thermoconvective steady flow to a time-dependent flow is considered for a liquid with a high Prandtl number (Pr=105) in a liquid bridge with a curved free surface. Both thermocapillary and buoyancy mechanisms of convection are taken into account. The computer program developed for this simulation transforms the original nonrectangular physical domain into a rectangular computational domain. To solve the problem in body-fitted curvilinear coordinates, the time-dependent Navier–Stokes equations were approximated by central differences on a stretched mesh. For liquid bridges with a flat interface, the instability corresponding to an azimuthal wave number of m=0 is not found for the investigated range of Marangoni numbers. The instability corresponding to an m=0 is found for relatively low Marangoni numbers only in liquid bridges with a nonflat, free surface, and nonzero Rayleigh number. The steady state becomes unstable to axially running waves. It is shown that the onset ...