Light-induced deformation and instability of a liquid interface. I. Statics

Optical Actuation of Nanoparticle-Loaded Liquid-Liquid Interfaces for Active Photonics

Liquid-liquid interfaces hold the potential to serve as versatile platforms for dynamic processes, due to their inherent fluidity and capacity to accommodate surface-active materials. This study explores laser-driven actuation of liquid-liquid interfaces with and without loading of gold nanoparticles and further exploits the laser-actuated interfaces with nanoparticles for tunable photonics. Upon laser exposure, gold nanoparticles were rearranged along the interface, enabling the reconfigurable, small-aperture modulation of light transmission and the tunable lensing effect. Adapting the principles of optical and optothermal tweezers, we interpreted the underlying mechanisms of actuation and modulation as a synergy of optomechanical and optothermal effects. Our findings provide an analytical framework for understanding microscopic interfacial behaviors, contributing to potential applications in tunable photonics and interfacial material engineering.

Numerical simulation of universal morphogenesis of fluid interface deformations driven by radiation pressure

We report on numerical simulation of fluid interface deformations induced by either acoustic or optical radiation pressure. This is done by solving simultaneously the scalar wave propagation equation and the two-phase flow equations using the boundary element method. Using dimensional analysis, we show that interface deformation morphogenesis is universal, i.e., depends on the same dimensionless parameters in acoustics and electromagnetics. We numerically investigate a few selected phenomena-in particular the shape of large deformations and the slenderness transition and its hysteresis-and compare with existing and novel experimental observations. Qualitative agreement between the numerical simulations and experiments is found when the mutual interaction between wave propagation and wave-induced deformations is taken into account. Our results demonstrate the leading role of the radiation pressure in morphogenesis of fluid interface deformations and the importance of the propagation-deformation interplay.

Optofluidic Resonance of a Transparent Liquid Jet Excited by a Continuous Wave Laser

We report a new optofluidic resonating phenomenon that naturally links the optical radiation pressure, total internal reflection, capillary wave, and Rayleigh-Plateau instability together. When a transparent liquid jet is radiated by a focused continuous wave laser beam, the highly ordered periodic jet breakup is unexpectedly triggered and maintained. The capillary wave enables the liquid-gas interface to serve as a rotating mirror reflecting the laser beam in a wide range of angles, including the critical angle for total internal reflection. The liquid jet acts as an optical waveguide to periodically transmit the laser beam to the upstream of the jet. The periodic optical beam transmittance inside the liquid jet exerts time-dependent optical pressure to the jet that triggers the Rayleigh-Plateau instability. The jet breakup process locks in at the frequency corresponding to the peak growth rate of the Rayleigh-Plateau instability of the liquid jet, which agrees with the prediction from the dispersion relation of a traveling liquid jet.

Contactless thin-film rheology unveiled by laser-induced nanoscale interface dynamics

One of the classical limitations for the investigation of the local rheology of small scale soft objects and/or confined fluids is related to the difficulty to control mechanical contact and its consequences. In order to overcome these issues, we implement a new local, active, fast and contactless optical strategy, called optorheology, which is based on both the optical radiation pressure of a laser wave to dynamically deform a fluid interface and interferometry to probe this deformation with nanometric resolution. This optical approach is first validated by measuring the surface tension and the viscosity of transparent Newtonian liquids. We also show how non-equilibrium situations, such as continuous evaporation, can be used to deduce the thickness dependence of the rheology of thin films and the concentration dependence of the viscosity of binary liquid mixtures and suspensions. We further extend the investigation to elasticity and viscoelasticity measurements of polymer solutions. Finally, since liquids may absorb light, we discuss the influence of a weak laser heating and the triggering of interface deformations by thermocapillary tangential stresses that could represent a complementary approach to probe the rheology at small scale.

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.

Non-conservative optical forces

Undoubtedly, laser tweezers are the most recognized application of optically induced mechanical action. Their operation is usually described in terms of conservative forces originating from intensity gradients. However, the fundamental optical action on matter is non-conservative. We will review different manifestations of non-conservative optical forces (NCF) and discuss their dependence on the specific spatial properties of optical fields that generate them. New developments relevant to the NCF such as tractor beams and transversal forces are also discussed.

Simulations of radiation pressure experiments narrow down the energy and momentum of light in matter

Consensus on a single electrodynamic theory has yet to be reached. Discord was seeded over a century ago when Abraham and Minkowski proposed different forms of electromagnetic momentum density and has since expanded in scope with the gradual introduction of other forms of momentum and force densities. Although degenerate sets of electrodynamic postulates can be fashioned to comply with global energy and momentum conservation, hope remains to isolate a single theory based on detailed comparison between force density predictions and radiation pressure experiments. This comparison is two-fold challenging because there are just a handful of quantitative radiation pressure measurements over the past century and the solutions developed from different postulates, which consist of approximate expressions and inferential deductions, are scattered throughout the literature. For these reasons, it is appropriate to conduct a consolidated and comprehensive re-analysis of past experiments under the assumption that the momentum and energy of light in matter are degenerate. We create a combined electrodynamic/fluid dynamic simulation testbed that uses five historically significant sets of electrodynamic postulates, including those by Abraham and Minkowski, to model radiation pressure under diverse configurations with minimal assumptions. This leads to new interpretations of landmark investigations of light momentum, including the Balazs thought experiment, the Jones-Richards and Jones-Leslie measurements of radiation pressure on submerged mirrors, observations of laser-deformed fluid surfaces, and experiments on optical trapping and tractor beaming of dielectric particles. We discuss the merits and demerits of each set of postulates when compared to available experimental evidence and fundamental conservation laws. Of the five sets of postulates, the Abraham and Einstein-Laub postulates provide the greatest consistency with observations and the most physically plausible descriptions of electrodynamic interactions. Force density predictions made by these two postulates are unique under many conditions and their experimental isolation is potentially within reach.

Phase-transition oscillations induced by a strongly focused laser beam

We report the observation of a surprising phenomenon consisting in a oscillating phase transition which appears in a binary mixture when this is enlightened by a strongly focused infrared laser beam. The mixture is poly-methyl-meth-acrylate (PMMA)-3-octanone, which has an upper critical solution temperature at T(c)=306.6K and volume fraction ϕ(c)=12.8% [Crauste et al., arXiv:1310.6720, 2013]. We describe the dynamical properties of the oscillations, which are produced by a competition between various effects: the local accumulation of PMMA produced by the laser beam, thermophoresis, and nonlinear diffusion. We show that the main properties of this kind of oscillations can be reproduced in the Landau theory for a binary mixture in which a local driving mechanism, simulating the laser beam, is introduced.

Optical trapping of interfaces at ultra-low interfacial tension

We achieve active control of interfacial phenomena by optically trapping the interface using the gradient forces of a strongly focussed laser beam parallel to the interface. We illustrate our technique in a phase separated colloid-polymer mixture by distorting the interface in a very controlled way. The static structure of the manipulated interface as well as its dynamic relaxation behaviour are analysed. Both the statics and dynamics can be related to the capillary wave height-height correlation functions using the fluctuation dissipation theorem up to surprisingly large deformations of the interface. To underline the novelty and potential of our approach we also show multiple interface distortions and the controlled snap-off of liquid droplets.

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.

Optohydrodynamics of soft fluid interfaces: optical and viscous nonlinear effects

Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state.

Nonlinear laser-induced deformations of liquid-liquid interfaces: an optical fiber model

Experimentally, it turns out that radiation forces from a cw laser on a liquid-liquid interface are able to produce giant deformations (up to about 100 microm), if the system is close to the critical point where the surface tension becomes small. We present a model for such a fingerlike deformation, implying that the system is described as an optical fiber. One reason for introducing such a model is that the refractive index difference in modern experiments, such as those of the Bordeaux group, is small, of the same order as in practical fibers in optics. It is natural therefore to adopt the hybrid HE11 mode, known from fiber theory as the fundamental mode for the liquid system. We show how the balance between hydrodynamical and radiation forces leads to a stable equilibrium point for the liquid column. Also, we calculate the narrowing of the column radius as the depth increases. Comparison with experimental results of the Bordeaux group yields quite satisfactory agreement as regards the column width.

Bridging dielectric fluids by light: a ray optics approach

Rayleigh-Plateau instability is known to impose a stability limit for the length of a liquid bridge in weightless conditions. This fundamental limit may be exceeded by using a light field to form and stabilize dielectric fluid bridges (A. Casner, J.P. Delville, Europhys. Lett. 65, 337 (2004)). Using both new experimental data as well as a new theoretical approach, we show that both the size and the stability of such light-sustained dielectric bridge can be qualitatively explained. We present a ray optics model that encompasses the competition between surface tension effects and optical radiation pressure arising from total internal reflection inside the bridge. A critical power below which a liquid bridge can no longer be sustained by light is predicted and confirmed experimentally. The observed power dependence of the bridge diameter also agrees with the proposed stabilization mechanism.

Stretching and squeezing of sessile dielectric drops by the optical radiation pressure

We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semiangle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these "optical cones" become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable "optical torus." These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated "optical domain" and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale.

Light-induced deformation and instability of a liquid interface. II. Dynamics

We study the dynamics of the deformation of a soft liquid-liquid interface by the optical radiation pressure of a focused cw Gaussian laser beam. We measured the temporal evolution of both the hump height and the hump curvature by direct observation and by detecting the focusing effect of the hump acting as a lens. Extending the results of Yoshitake [J. Appl. Phys. 97, 024901 (2005)] to the case of liquid-liquid interfaces and to the Bo approximately =1 regime [Bo=(omega0/lc)2, , where omega0 is the beam waist and lc the capillary length], we show that, in the Bo<<1 and Bo approximately =1 ranges, the small-amplitude deformations are correctly described by a linear hydrodynamic theory predicting an overdamped dynamics. We also study the dynamics of the large-amplitude interface deformations at the onset of optohydrodynamic instability [Phys. Rev. Lett. 90, 144503 (2003)]. Using a simple, phenomenological model for the nonlinear evolution of the hump height, we interpret the observed interface dynamics at the instability onset as the signature of an imperfect subcritical instability.