Photothermal Tuning and Size Locking of Salt-Water Microdroplets on a Superhydrophobic Surface (original) (raw)
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A self-stabilization mechanism locking the size of single inorganic salt (NaCl)–water microdroplets that are standing on a superhydrophobic surface and kept in a humidity-controlled chamber is demonstrated. The effect is based on the hysteretic behavior of a photothermal tuning cycle caused by the whispering gallery mode (WGM) absorption resonances that are observed when scanning the power of an infrared laser focused at the rim of a microdroplet. When locked, the microdroplet size and WGM spectrum are resilient to environmental perturbations and can be maintained for hours as the mechanism does not rely on a photobleachable dye. The bistable nature of the system is also demonstrated, enabling reversible switching between two sizes. A rate equation-based thermodynamical model of the hysteretic behavior is provided, giving good agreement with the experimental results. Our results may be used to establish stable experimental conditions for ultrahigh resolution spectroscopy of microdroplets. Other optical and biological applications that require exactly size-matched microdroplets can also benefit from the demonstrated self-stabilization mechanism.
2007
Abstract. Glycerol/water microdroplets take almost spherical shapes when standing on a superhydrophobic surface. Hence they are suitable to function as optical microcavities. Using Rhodamine B doped water microdroplets, large spectral tunability of the whispering gallery modes (WGMs)(> 5 nm) was observed. Tunability was achieved by evaporation/condensation in a current controlled mini humidity chamber. Experiments revealed a mechanism stabilizing the volume of these microdroplets with femtoliter resolution.
Applied Physics …, 2007
We demonstrate large spectral tuning of glycerol/water microdroplets standing on a superhydrophobic surface using the optical scattering force exerted by a 1064 nm Nd 3+ :YVO 4 solid-state laser. Spectral tuning up to 30 nm is presented in the whispering gallery modes as a result of the deformation of the microdroplets toward a truncated prolate spheroid geometry. Observed large spectral tuning is also reported to be highly reversible. This demonstration can inspire novel, largely tunable optical switches or filters based on liquid microdroplets kept in a sealed chamber.
Controlled Deformation and Raman Lasing in Microdroplets Standing on a Superhydrophobic Surface
2009
Liquid microdroplets are ideally suited for applications that benefit from the properties of the whispering gallery modes (WGMs) because of their spherical shapes, smooth surfaces, biocompatibility, and flexible nature. In this thesis, we present nondegeneracy in WGMs as a result of deformation in the glycerol-water microdroplets using uniform electric field. Raman lasing from individual pure water microdroplets standing on a superhydrophobic surface is also presented in this thesis.
Reversible photothermal tuning of a salty water microdroplet
2009
A fully reversible photothermal tuning of an inorganic salt (NaCl)–water microdroplet standing on a superhydrophobic surface is demonstrated. The size change of the microdroplet is caused by a focused infrared laser beam in a humidity-controlled chamber and a fully reversible large spectral tuning up to B40 nm is achieved.
Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting
Applied Physics Letters, 2008
Using electrowetting, we demonstrate reversible spectral tuning of the whispering gallery modes of glycerol/water microdroplets standing on a superhydrophobic surface by up to 4.7 nm at 400 V. Our results can inspire electrically tunable optical switches and filters based on microdroplets on a superhydrophobic surface. The sensitivity of the observed spectral drift to the contact angle can also be used to measure the contact angles of microdroplets on a superhydrophobic surface.
Journal of The Optical Society of America, 1993
The spectroscopy of the morphology-dependent resonances of a microdroplet has been studied at high precision. The line positions are shown to reveal optical dispersion and permit the refractive index to be determined with sufficient accuracy to provide an estimate of the droplet cooling that is due to evaporation. Comparison of the remaining discrepancies in mode positions for different radial modes indicates a small temperature gradient near the surface. Both the cooling and the temperature gradient are compatible with thermodynamic estimates. The mode quantum numbers are identified with high confidence, and the systematics of the line intensities permit an estimate of the extra radiative loss 1/QL over and above that predicted by Lorenz-Mie theory for a perfect homogeneous microsphere, for example, that which is due to internal scattering, with QL 2 X 108 for first-order modes.
Journal of the Optical Society of America B, 2016
Optical whispering gallery modes (WGMs) were observed in elastic scattering spectra recorded from oil-in-water emulsion droplets in a microfluidic channel. Droplets with diameters ranging between 15 and 50 μm were trapped by optical tweezers near the tip of a single mode fiber that enabled the excitation of the WGMs using a tunable laser. Quality factors of the WGMs were observed to increase with droplet size. WGMs with quality factors of more than 10 4 were observed for droplets with diameters around 45 μm. In some cases, recorded WGMs drifted monotonically to the blue end of the spectrum due to droplet dissolution in the host liquid. Fluctuating spectral shifts to both blue and red ends of the spectrum were also observed. These were attributed to the presence of randomly diffusing particulate contaminants in the droplet liquid, indicating the potential of optically trapped droplet resonators for optical sensing applications.
Optofluidic FRET microlasers based on surface-supported liquid microdroplets
Laser Physics Letters, 2014
We demonstrate optofluidic microlasers using highly efficient non-radiative Förster resonance energy transfer (FRET) for pumping of gain medium placed within liquid microdroplets situated on a superhydrophobic surface. Microdroplets generated from a mixture of ethylene glycol, glycerol, and water and stained with the FRET donor-acceptor dye pair Rhodamine 6G-Rhodamine 700 serve as active optical resonant cavities hosting high-quality whispering gallery modes. Upon direct optical pumping of the donor with a pulsed laser, lasing is observed in the emission band of the acceptor as a result of efficient FRET coupling between the acceptor and donor molecules. FRET lasing is characterized for different acceptor and donor concentrations, and threshold pump fluences of acceptor lasing as low as 6.3 mJ cm −2 are demonstrated. We also verify the dominance of the non-radiative FRET over cavity-assisted radiative energy transfer for the range of parameters studied in the experiments.
Controlled observation of nondegenerate cavity modes in a microdroplet on a superhydrophobic surface
We demonstrate controlled lifting of the azimuthal degeneracy of the whispering gallery modes (WGMs) of single glycerol–water microdroplets standing on a superhydrophobic surface by using a uniform electric field. A good agreement is observed between the measured spectral positions of the nondegenerate WGMs and predictions made for a prolate spheroid. Our results reveal fewer azimuthal modes than expected from an ideal spherical microdroplet due to the truncation by the surface. We use this difference to estimate the contact angles of the microdroplets.