Thermal Marangoni trapping driven by laser absorption in evaporating droplets for particle deposition.

Controlling the deposition of particles is of great importance in many applications. In this work, we study particle deposition driven by Marangoni flows, triggered by laser absorption inside an evaporating droplet. When the laser is turned on, thermal gradients are generated and produce a toroidal Marangoni flow that concentrates the particles around the laser beam and ultimately controls the final deposition. We experimentally characterize the radius of the Marangoni flows as a function of the laser parameters. Counter-intuitively, the radius of the Marangoni region appears to remain constant and is not proportional to the thickness of the drop which decreases due to evaporation. We develop a model to predict the size of the Marangoni region that combines evaporative flows and laser-induced Marangoni flows. The experimental data are in good agreement with the predictions, allowing us to estimate the particle overconcentration factor resulting from the laser heating effects. The addition of surfactants to the solution allows the coupling of solutal Marangoni flows with thermal ones to achieve a final micron-scale deposit located at the laser spot. These results pave the way for new methods with high tunability provided by spatio-temporal light control for surface patterning applications.

Conical Interfaces between Two Immiscible Fluids Induced by an Optical Laser Beam.

We demonstrate the existence of conical interface deformations induced by a laser beam that are similar to Taylor cones in the electrical regime. We show that the cone morphology can be manipulated by fluid and laser parameters. A theory is proposed to quantitatively describe these dependences in good agreement with experimental data obtained for different fluid systems with low interfacial tensions. Counterintuitively, the cone angle is proved to be independent of the refractive index contrast at leading order. These results open a new optofluidic route towards optical spraying technology-an analogue of electrospraying-and more generally for the optical shaping of interfaces.

Experimental study of hybrid nematic wetting films.

Liquid crystal layers, with thickness less than 1 µm, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as "hybrid nematics". The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.

Coalescence driven by line tension in thin nematic films.

Thin nematic films deposited on liquid substrates provide a unique situation to investigate coalescence: the whole process can be followed under microscope over a wide range of times, and temperature allows us to monitor the surface viscosity of the surrounding fluid. For the first time, the complete scenario of 2D coalescence has been recorded for a given system in both inviscid limit and viscous environment, enabling us to identify the successive routes of dissipation. In particular, 2D "viscous bubbles" of the surrounding viscous fluid with a bulbous shape formed in the gap between coalescing films are observed. Available models are adapted to our specific case and account satisfactorily for the whole process.

Thin nematic films on liquid substrates.

Thin films of cyanobiphenyl liquid crystals (nCB) deposited on water or glycerol have been studied in the nematic temperature range. A common property of the systems is the hybrid anchoring conditions at the film interfaces. The preferred orientation of the nematic director is planar at the liquid interface, and it is homeotropic and somewhat weaker at the air interface. The resulting structure of the film depends on its thickness. Films thicker than 0.5 microm show the usual defects of nematics. Between 0.5 microm and 20-30 nm, complex instability patterns such as stripes, "chevrons", or squares are observed in extended films. Then there is a forbidden range of thickness below in which much thinner structures (usually monolayers and trilayers) are present. The present paper investigates this common behavior in various systems and gives arguments for its analysis.

Some specificities of wetting by cyanobiphenyl liquid crystals.

The present paper provides an up to date restatement of the wetting behaviour of the series of cyanobiphenyl liquid crystals (LCs) on usual substrates, i.e. oxidized silicon wafers, water and glycerol, at both the macroscopic and microscopic scale, in the nematic range of temperature. We show that on water the systems are close to a wetting transition, especially 5CB and 7CB. In that case, the wetting behaviour is controlled by the presence of impurities. On a mesoscopic scale, we observe for all our (thin LC film-substrate) systems an identical, complex, but well defined general scenario, not accounted for by the available models. In the last part, we present a study on line tension which results from the specific organization of LCs at the edge of the nematic film. We report preliminary results on two-dimensional film coalescence where this line tension plays a major role.