The amplitude of optical forces on flowing dielectric microparticles can be actuated by coating them partially with metallic nanospheres and exposing them to laser light within the surface plasmon resonance. Here, optical forces on both pure silica particles and silica-gold raspberries are characterized within an optical chromatography setup by measuring the Stokes drag versus laser beam power. Results are compared to Mie theory predictions for both core dielectric particles and core-shell ones where the shell is described by a continuous dielectricmetal composite of dielectric constant determined from the Maxwell Garnett approach. The nice observed quantitative agreement demonstrates that radiation pressure forces are directly related to the metal concentration present at the microparticle surface and that nano-metallic objects increase the magnitude of optical forces compared to pure dielectric particles of the same overall size, even at very low metal concentration. Behaving as “micro-sized nanoparticles", the benefit of microparticles coated with metallic nanospheres is thus twofold: (i) to enhance optofluidic manipulation and transport at the microscale and (ii) to increase sensing capabilities at the nanoscale, compared to separated pure dielectric particles and single metallic nanosystems.
We show how optocapillary stresses and optical radiation pressure effects in two-phase liquids open the way for investigating the difficult problem of liquid thread breakup at small scales when surfactants are present at the interface or when the roughness of the interface becomes significant. Using thermocapillary stresses driven by light to pinch a surfactant-laden microjet, we observe deviations from the expected visco-capillary law governed by a balance between viscosity and interfacial tension. We suggest that these deviations are due to time varying interfacial tension resulting from the surfactant depletion at the neck pinching location, and we experimentally confirm this mechanism. The second case is representative of the physics of nanojets. Considering a near critical liquid-liquid interface, where the roughness of the interfaces may be tuned, we use the radiation pressure of a laser wave to produce stable fluctuating liquid columns and study their breakup. We show how pinching crosses over from the visco-capillary to a fluctuation dominated regime and describe this new regime. These experiments exemplify how optofluidics can reveal new physics of fluids.
Laser induced heating at the interface between two immiscible fluids is used to produce thermocapillary stresses along this interface. When the interface is heated locally, the surface tension is reduced at the hot spot and the fluids are drawn in the direction opposite to the temperature gradient. This effect, known as the Marangoni effect, is amplified in miniaturized systems since the temperature and surface tension gradients are increased as the typical distances are reduced. When implemented in an adequate microchannel geometry, the Marangoni effect allows us to devise fundamental building blocks for microfluidic systems. In particular, the motion along the surface of a fluid immediately causes movement in the bulk for low Reynolds numbers, another feature of miniaturized systems. This allows us to apply the laser heating technique to make a pump in a microchannel, by focusing a laser beam on an oil-water interface. More surprisingly, localized heating can also be applied to create a microfluidic valve when implemented in an adequate geometry. A mixer can also be produced by using a fluid-fluid interface which changes in time. In this talk we present some of our realizations of these actuators in microfluidic channels and discuss some of the physical background underlying their behavior. Once these devices are created they can be combined in a complex circuit, yielding a contactless, scalable solution for the practical limitations that plague microfluidics.
The bending of fluid interfaces by the optical radiation pressure is now recognized as an appealing contactless tool to probe microscopic surface properties of soft materials. However, as the radiation pressure is intrinsically weak (typically of the order of a few Pascal), investigations are often limited to the regime of weak deformations. Non-linear behaviors can nevertheless be investigated using very soft fluid interfaces. Either a large stable tether is formed, or else a break-up of the interface occurs above a well-defined beam power threshold, depending on the direction of the beam propagation. This asymmetry originates from the occurrence of total reflection condition of light at deformed interface. Interface instability results in the formation of a stationary beam-centered liquid micro-jet that emits droplets. Radiation-induced jetting can also lead to giant tunable liquid columns with aspect ratio up to 100, i.e. well beyond the fundamental Rayleigh-Plateau limitation. Consequently, the applications range of the opto-hydrodynamic interface instability is wide, going from adaptative micro-optics (lensing and light guiding by the induced columns) to micro-fluidics and microspraying, as fluid transfer is optically monitored and directed in three dimensions.