Monitoring and analysis of pendant droplets evaporation using bare and monolayer-coated optical fiber facets

Abstract

The monitoring of sub nano-liter pendant liquid droplets, during their evaporation from the cleaved facet of a standard optical fiber, is proposed and demonstrated. The combined reflections of incident light from the two boundaries, between fiber and liquid and between liquid and air, give rise to interference fringes as the fluid evaporates. The analysis of the fringe pattern allows for the reconstruction of the instantaneous size and evaporation rate of the droplets. These, in turn, provide information regarding the properties of the liquid itself, and the surface to which it is applied. The sensor readout is validated against direct video observation of evaporating droplets. Several examples illustrate the potential of the proposed sensor. Evaporation dynamics measurements identify the ethanol contents in binary ethanol-water mixtures with 2% certainty. The evaporation dynamics are modified by the application of a hydrophobic self-assembled monolayer coating to the tip of the fiber. Ten different organic solvents are accurately classified by clustering analysis of their evaporation data, collected using bare and coated fibers. Potential applications of the sensors could include quality control of water, beverages and oils, recognition of flexible fuel blends and fuel dilutions, mobile pointof-care diagnostics, and laboratory analysis of surface treatments.

Optomechanical sensing of liquids outside standard fibers using forward stimulated Brillouin scattering

“…To go around this difficulty, the fiber-optic sensors of liquids developed to date either rely on hollow-core, photonic crystal fibers (PCFs) [5][6][7][8] or involve considerable structural modifications of standard fibers. Specific forms of the latter include long-period fiber Bragg gratings for the excitation of cladding modes [9], etching or polishing fibers down to the core [10,11], tapering fibers down to few-microns diameters [12], the application of transducer coating layers [13], the fabrication of inline cavities [14], and the use of cleaved facets [15,16]. Many of these devices and setups are extremely sensitive.…”

Chemical and biochemical sensing applications of microstructured optical fiber‐based systems

“…This simple sensor was used to measure the rate of evaporation of pL droplets, and results were in good agreement compared to direct visual examination. This sensor was also adequate to study the composition of binary systems (water/ethanol) with 2% accuracy .…”

Optomechanical time-domain reflectometry

“…Known protocols for chemical sensing of surrounding media mandate spatial overlap between light and the substance under test. Hence fibre-based chemical sensors involve non-standard geometries such as photonic-crystal or micro-structured fibres 9 – 12 , make use of reactive materials 13 – 16 or require significant structural modifications of standard fibres 17 – 24 . Most realizations support only point sensors.…”