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Title: Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers

Abstract

We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependentmore » monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed.« less

Authors:
;  [1];  [2];  [3];  [4]
  1. School of Chemistry, University of Bristol, Bristol BS8 1TS (United Kingdom)
  2. Department of Mathematics, University of Leicester, Leicester LE1 7RH (United Kingdom)
  3. Mathematics Institute, University of Warwick, Coventry CV4 7AL (United Kingdom)
  4. Department of Chemistry, University of Leicester, Leicester LE1 7RH (United Kingdom)
Publication Date:
OSTI Identifier:
22679028
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 145; Journal Issue: 5; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; COALESCENCE; DROPLETS; EXPERIMENTAL DATA; HOLOGRAPHY; RELAXATION TIME; SIMULATION; SURFACE TENSION; TIME DEPENDENCE; TIME RESOLUTION; VISCOSITY

Citation Formats

Bzdek, Bryan R., Reid, Jonathan P., E-mail: j.p.reid@bristol.ac.uk, Collard, Liam, Sprittles, James E., and Hudson, Andrew J. Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers. United States: N. p., 2016. Web. doi:10.1063/1.4959901.
Bzdek, Bryan R., Reid, Jonathan P., E-mail: j.p.reid@bristol.ac.uk, Collard, Liam, Sprittles, James E., & Hudson, Andrew J. Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers. United States. doi:10.1063/1.4959901.
Bzdek, Bryan R., Reid, Jonathan P., E-mail: j.p.reid@bristol.ac.uk, Collard, Liam, Sprittles, James E., and Hudson, Andrew J. Sun . "Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers". United States. doi:10.1063/1.4959901.
@article{osti_22679028,
title = {Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers},
author = {Bzdek, Bryan R. and Reid, Jonathan P., E-mail: j.p.reid@bristol.ac.uk and Collard, Liam and Sprittles, James E. and Hudson, Andrew J.},
abstractNote = {We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependent monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed.},
doi = {10.1063/1.4959901},
journal = {Journal of Chemical Physics},
number = 5,
volume = 145,
place = {United States},
year = {Sun Aug 07 00:00:00 EDT 2016},
month = {Sun Aug 07 00:00:00 EDT 2016}
}
  • Direct measurement of the forces experienced by micro-spheres in an acoustic standing wave device have been obtained using calibrated optical traps generated with holographic optical tweezers. A micro-sphere, which is optically trapped in three dimensions, can be moved through the acoustic device to measure forces acting upon it. When the micro-sphere is subjected to acoustic forces, it's equilibrium position is displaced to a position where the acoustic forces and optical forces are balanced. Once the optical trapping stiffness has been calibrated, observation of this displacement enables a direct measurement of the forces acting upon the micro-sphere. The measured forces aremore » separated into a spatially oscillating component, attributed to the acoustic radiation force, and a constant force, attributed to fluid streaming. As the drive conditions of the acoustic device were varied, oscillating forces (>2.5 pN{sub pp}) and streaming forces (<0.2 pN) were measured. A 5 μm silica micro-sphere was used to characterise a 6.8 MHz standing wave, λ = 220 μm, to a spatial resolution limited by the uncertainty in the positioning of the micro-sphere (here to within 2 nm) and with a force resolution on the order of 10 fN. The results have application in the design and testing of acoustic manipulation devices.« less
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  • Void coalescence and interaction in dynamic fracture of ductile metals have been investigated using three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based onmore » the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. The interaction of the voids is not reflected in the volumetric asymptotic growth rate of the voids, as demonstrated here. Finally, the practice of using a single void and periodic boundary conditions to study coalescence is examined critically and shown to produce results markedly different than the coalescence of a pair of isolated voids.« less