Jean-Pierre Delville

Thermocapillary valve for droplet production and sorting

Droplets are natural candidates for use as microfluidic reactors, if active control of their formation and transport can be achieved. We show here that localized heating from a laser can block the motion of a water-oil interface, acting as a microfluidic valve for two-phase flows. A theoretical model is developed to explain the forces acting on a drop due to thermocapillary flow, predicting a scaling law that favors miniaturization. Finally, we show how the laser forcing can be applied to sorting drops, thus demonstrating how it may be integrated in complex droplet microfluidic systems.

Laser switching and sorting for high speed digital microfluidics

We used thermocapillary stresses locally induced by laser on flowing drops to build high throughput drop switchers and sorters for digital microfluidics. Since the laser is disconnected to the chip, the method does not require dedicated micropatterning. We show switching efficiencies of 100% for drop velocities up to 1.3cm∕s, demonstrate the involved mechanism, and apply laser switching for sorting droplets of different natures for lab-on-a-chip applications.

Laser-induced hydrodynamic instability of fluid interfaces.

We report on a new class of electromagnetically driven fluid interface instability. Using the optical radiation pressure of a cw laser to bend a very soft near-critical liquid-liquid interface, we show that it becomes unstable for sufficiently large beam power P, leading to the formation of a stationary beam-centered liquid microjet. We explore the behavior of the instability onset by tuning the interface softness with temperature and varying the size of the exciting beam. The instability mechanism is experimentally demonstrated. It simply relies on total reflection of light at the deformed interface whose condition provides the universal scaling relation for the onset P(S) of the instability.

Liquid Transport Due to Light Scattering

Using experiments and theory, we show that light scattering by inhomogeneities in the index of refraction of a fluid can drive a large-scale flow. The experiment uses a near-critical, phase-separated liquid, which experiences large fluctuations in its index of refraction. A laser beam traversing the liquid produces a interface deformation on the scale of the experimental setup and can cause a liquid jet to form. We demonstrate that the deformation is produced by a scattering-induced flow by obtaining good agreements between the measured deformations and those calculated assuming this mechanism.

Deformation of acoustically transparent fluid interfaces by the acoustic radiation pressure

We experimentally study the deformations of liquid-liquid interfaces induced by a high-intensity focused ultrasonic beam. We quantitatively verify that small-amplitude deformations of a transparent chloroform-water interface are well described by the theory of Langevin acoustic radiation pressure, in both static and dynamic regimes. The large-amplitude deformations depend on the direction of propagation of the beam and are qualitatively similar to those induced by electromagnetic radiation pressure.

Adaptative lensing driven by the radiation pressure of a continuous-wave laser wave upon a near-critical liquid–liquid interface

The bending of a liquid interface by the radiation pressure of a cw laser wave is generally weak. To strongly enhance the coupling, we investigate lensing in a near-critical phase-separated liquid mixture. By continuously tuning the softness of the meniscus by varying the temperature, we observed huge stationary interface deformations at low beam power that led to an important variation of the resultant soft lensing. We also illustrate the crucial role played by temperature in this process to demonstrate its potential use in lens adaptation.

Fluid flows driven by light scattering

We report on the direct experimental observation of laser-induced flows in isotropic liquids that scatter light. We use a droplet microemulsion in the two-phase regime, which behaves like a binary mixture. Close to its critical consolute line, the microemulsion undergoes large refractive index fluctuations that scatter light. The radiation pressure of a laser beam is focused onto the soft interface between the two phases of the microemulsion and induces a cylindrical liquid jet that continuously emits droplets. We demonstrate that this dripping phenomenon takes place as a consequence of a steady flow induced by the transfer of linear momentum from the optical field to the liquid due to light scattering. We first show that the cylindrical jet guides light as a step-index liquid optical fiber whose core diameter is self-adapted to the light itself. Then, by modelling the light-induced flow as a low-Reynoldsnumber, parallel flow, we predict the dependence of the dripping flow rate on the thermophysical properties of the microemulsion and the laser beam power. Satisfying agreement is found between the model and experiments.

Gas spreading on a heated wall wetted by liquid

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.

Thermocapillary flows and interface deformations produced by localized laser heating in confined environment

The deformation of a fluid-fluid interface due to the thermocapillary stress induced by a continuous Gaussian laser wave is investigated analytically. We show that the direction of deformation of the liquid interface strongly depends on the viscosities and the thicknesses of the involved liquid layers. We first investigate the case of an interface separating two different liquid layers while a second part is dedicated to a thin film squeezed by two external layers of same liquid. These results are predictive for applications fields where localized thermocapillary stresses are used to produce flows or to deform interfaces in presence of confinement, such as optofluidics.

Break-up dynamics of fluctuating liquid threads

The thinning dynamics of a liquid neck before break-up, as may happen when a drop detaches from a faucet or a capillary, follows different rules and dynamic scaling laws depending on the importance of inertia, viscous stresses, or capillary forces. If now the thinning neck reaches dimensions comparable to the thermally excited interfacial fluctuations, as for nanojet break-up or the fragmentation of thermally annealed nanowires, these fluctuations should play a dominant role according to recent theory and observations. Using near-critical interfaces, we here fully characterize the universal dynamics of this thermal fluctuation-dominated regime and demonstrate that the cross-over from the classical two-fluid pinch-off scenario of a liquid thread to the fluctuation-dominated regime occurs at a well-defined neck radius proportional to the thermal length scale. Investigating satellite drop formation, we also show that at the level of the cross-over between these two regimes it is more probable to produce monodisperse droplets because fluctuation-dominated pinch-off may allow the unique situation where satellite drop formation can be inhibited. Nonetheless, the interplay between the evolution of the neck profiles from the classical to the fluctuation-dominated regime and the satellites’ production remains to be clarified.