Optical Control of Emulsion Drops for Nanofluidics and Microfabrication

Lead Research Organisation: Imperial College London
Department Name: Dept of Chemistry

Abstract

Ground-breaking discoveries at the STFC's Central Laser Facility showed that optical traps can be used to control the shape of micron-sized emulsion droplets, to create droplets connected by stable threads of oil a thousand times thinner than a human hair and to pump liquids from one drop to another through these nanothreads. This project will explore the science behind these discoveries and develop prospective applications. First, we will develop a robust experimental and theoretical framework for manipulating emulsion drops and for creating nanofluidic networks with laser beams. Second, we will use this platform to reveal the physical and chemical principles governing the structure and dynamics of these nanothreads and use the nanothreads to explore the physics of transport in nanofluidic networks. Third, optical deformation of polymerisable emulsions will be developed as a technology for microfabrication of polymer objects with complex 3-D shapes. Such microparticles have potential applications in medical devices, drug delivery, MEMS, photonic materials and ion sources. Finally, we will establish the principles for using these nanofluidic networks to carry out chemical reactions on the attolitre scale.

Publications

10 25 50

publication icon
Bolognesi G. (2014) A microfluidic platform for the production of monodisperse ultralow interfacial tension oil droplets in water in 18th International Conference on Miniaturized Systems for Chemistry and Life Sciences, MicroTAS 2014

 
Description The project was divided into four workstreams on optics, chemistry, microfluidic engineering and theory. These workstreams were then brought together to address the key objectives of the project. We briefly address the technical developments in each of the workstreams and then how the workstreams were integrated.

(i) Optics
Spatial light modulators were used to create multiple traps to shape droplets in 2 and 3 dimensions and to create nanofluidic networks in 2 and 3 dimensions. Two methods for measuring the 3D shape of deformed droplets were implemented: confocal microscopy at the LSF at RAL and structured illumination at Durham. A rig was developed for UV polymerisation of trapped droplets with Raman detection of the progress of the polymerisation reaction. A FRAP (fluorescence recovery after photo-bleaching) experiment was developed to measure the flow rate through nanothreads.
(ii) Chemistry
The phase behaviour of both temperature-sensitive and temperature-insensitive microemulsion formulations was characterised. The effect of polymers on the interfacial tension (IFT) was explored in the ultralow IFT (ULIFT) regime.
(iii) Microfluidic engineering
Microfluidic devices were developed comprising a flow-focussing junction (FFJ) for generation of oil droplets and an observation chip (ObC) for deforming them. The design allows the generation of monodisperse 3-10 µm drops at moderate IFT and the transfer of these droplets to the Peltier-controlled ObC where ULIFT is achieved, based on the phase behaviour determined in the 'chemistry' workstream. Both temperature and salinity can be used to control interfacial tension across four orders of magnitude. In addition, a microfludic tensiometry technique for the quantitative characterisation of the mechanical properties of the ultralow interfacial tension droplets was developed. We combined the microfluidic platform for droplet generation with the analysis of thermally-driven capillary waves/fluctuation analysis for the measurement of ULIFT and the results compared with literature data from spinning drop tensiometry.
(iv) Theory
A numerical 'spokes' model was implemented to compute the shape of a droplet in one or more optical traps. An analytical asymptotic model for droplet deformation in a single trap was developed and compared with the numerical model. The effects of buoyancy were added to the theoretical model. A theoretical model for flow through nanothreads was developed, which places an upper limit of the length of the nanothread through which fluid can be pumped using optical pressure from the trapping lasers.

Integration
(i) A parametric study was completed on the formation of droplets in an FFJ as a function of salinity and temperature in an AOT/brine/heptane system. Droplet generation is possible even in the ULIFT regime if interface is created faster than surfactant can adsorb to the nascent interface.
(ii) The effect of laser heating on trapped droplets was modelled in Comsol and the consequences for the behaviour of temperature-insensitive emulsions in an optical trap was explored experimentally. Intriguing behaviour including the spontaneous generation of new phases was observed and explained in terms of the phase diagram of the quinary mixture.
(iii) The deformed shape of a single droplet in an optical trap was measured experimentally, including the observation of the theoretically predicted hour-glass shape. Experimental shapes were compared with theoretical predictions.
(iv) The shapes of droplets were characterised for three or four traps in a plane as a function of the optocapillary number (a dimensionless parameter that measures the relative strength of optical and capillary forces). The formation of a tetrahedron with out-of-plane traps was attempted. The importance of incorporating buoyancy and spherical aberration in theoretical models and practical implementation of droplet shaping strategies was highlighted.
(v) Initial experiments on the production of connected nanodroplet networks in 3D were successfully conducted
Exploitation Route In the short term, we plan to bring to a conclusion the objectives of the project that were not fully attained: specifically, (i) optimisation of the strategy for shaping droplets in 3-D; (ii) measurement of polymerisation kinetics in trapped droplets and optimisation of strategy for maintaining ULIFT during polymerisation; (iii) generation and characterisation of 3D nanodroplet networks, involving 4-way junctions and closed loops.

In the longer term, shaped microparticles have potential applications in medical devices, specialist coatings, drug delivery, micromechanical systems, photonic materials and ion sources. Nanofluidic networks could also be used as a means of performing chemical reactions using only tiny volumes of the chemicals. Our main route for exploitation is using the optical deformation of polymerisable emulsions of monomers as a technology for microfabrication of objects with complex 3-D shapes. Nanofluidic networks could also be exploited in as a means of performing chemical reactions on the attolitre scale and we are exploring this opportunity through follow-on funding.
Sectors Chemicals,Healthcare,Pharmaceuticals and Medical Biotechnology

 
Description The key aims of the grant have been to develop novel high-throughput strategies for the manufacture of ultra-low tension droplets with control of size and composition. In the presence of optical traps ultra-low tension droplets (ULTDs) can be sculpted; such that their shape can be controlled. We have made excellent progress with respect to the generation of ULTDs and the integration of these systems with the optical trapping assemblies. In parallel excellent progress has been made with respect to modelling the behaviour of ULTDs. Building on this foundation we have been exploring applications of ULTDs as building motifs for chemical reactors and shaped nanoparticles. This has included the construction of 3-D nano-networks that can be used as the basis for controlled sequential reactions exploiting oil based chemistries. The ability to manufacture such oil based "labs on a chip" where reagents can systematically be added to a reaction chamber has already attracted strong interest from the biotech sector. Extended capabilities of these systems by incorporating molecular rotors.
First Year Of Impact 2015
Sector Chemicals,Pharmaceuticals and Medical Biotechnology
Impact Types Economic

 
Description Mass Transport in Ultralow Interfacial Tension Droplet-Nanothread Networks (DNNs)
Amount £30,000 (GBP)
Funding ID 17,330,050 
Organisation Rutherford Appleton Laboratory 
Sector Academic/University
Country United Kingdom
Start 02/2018 
End 06/2018
 
Description Molecular rotors as quantitative probes of interfacial tension
Amount £30,000 (GBP)
Funding ID 16,230,049 
Organisation Rutherford Appleton Laboratory 
Sector Academic/University
Country United Kingdom
Start 02/2017 
End 06/2017
 
Description Translational grant (Pathways to Impact funding)
Amount £132,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2015 
End 11/2016
 
Title Fabrication of nanofluidic networks using optical traps 
Description By using a time-sharing optical trapping system combined to our microfluidic platform, we optically manipulated ultralow interfacial tension droplets (5µm in diameter) in order to create and control in real-time user-defined three-dimensional nanofluidic networks. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2015 
Impact Closed-loop networks as well as nanothread Y junctions were achieved. 
 
Title Image analysis software for characterising ULIFT droplets 
Description We developed a class of image analysis software, based on Python code and Image Java macros, to characterise ULIFT droplet populations. The analysis of images captured through standard and high-speed digital video microscopy enabled the determination of i) distributions of droplet diameters ii) distributions of levels of droplet deformation iii) droplet generation frequencies iv) droplet speeds. 
Type Of Technology Software 
Year Produced 2013 
Impact The analysis of images captured through standard and high-speed digital video microscopy enabled the determination of i) distributions of droplet diameters ii) distributions of levels of droplet deformation iii) droplet generation frequencies iv) droplet speeds. 
 
Title Microfluidic generator for ULIFT droplets 
Description We developed a microfluidic platform, consisting of s droplet generator device coupled to custom built observation/manipulation chamber chip, to generate, manipulate and characterise micron-sized heptane droplets in NaCl-Aerosol OT(surfactant) solutions. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2014 
Impact By varying the geometry of the droplet generator device and the flow rates of the liquid phases, we could obtain i) monodisperse populations of large droplets with diameters ranging from 11µm to 40µm, ii) monodisperse populations of small droplets with diameters ranging from 5µm to 8µm iii) bidisperse population of main and satellite droplets with diameters ranging from 11µm to 20µm and from 2µm to 4µm, respectively. 
 
Title Microfluidic platform for the generation of micron sized ultra-low tension droplets 
Description Microfluidic platform for the generation of micron sized ultra-low tension droplets. Fully automated-able to modulate size and composition of ultra-low tension droplets as well as provide an observation chamber for their manipulation using optical traps. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2013 
Impact User defined capabilities for manufacturing ULITD for the first time. 
 
Title Microfluidic strategy for tuning interfacial tension of droplets 
Description We devised a microfluidic strategy based on on-chip control of the emulsion temperature and salinity level, which enabled the active tuning of the droplet equilibrium and dynamic interfacial tensions over a range spanning four orders of magnitudes (namely, from 10-7 N/m to 10-3 N/m). 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2015 
Impact By adjusting the oil-brine interfacial tension according to the microfluidic operation required (e.g. droplet generation, transport, storage), we could manufacture monodisperse droplet populations in the low (<10-3N/m) and ultralow (<10-3N/m) interfacial tension regimes with high levels of droplet stability with respect to rupture and coalescence rates. 
 
Title Microfluidic tensiometry 
Description We developed and validated a microfludic tensiometry technique for the quantitative characterisation of the mechanical properties of ultralow interfacial tension droplets. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2015 
Impact We combined our microfluidic platform for droplet generation with the analysis of thermally-driven capillary waves, observed at the droplet equatorial plane through bright field microscopy, in order to measure both the droplet interfacial tension and the surfactant film bending rigidity. The microfluidic tensiometry measurements were in good agreement with the literature data. 
 
Title Model for simulating droplet generation in microfluidic chips 
Description Two-dimensional finite-element simulations of droplet generation in cross-junction devices were performed through Comsol Multiphysics®. 
Type Of Technology Software 
Year Produced 2014 
Impact The two-phase flow dynamics and the droplet break-up process were simulated by solving the Stokes equations (Momentum and mass conservation) and the Level set equation through the Comsol microfluidics module. The outcome of the numerical simulations in terms of droplet size and generation frequency were qualitatively in good agreement with the experimental results. 
 
Title model for simulating effect of laser induced heating on ULIFT droplets 
Description Two-dimensional axisymmetric models were developed in Comsol Multiphysics®. to investigate the effect of laser-induced heating on ultralow interfacial tension droplets under optical traps. 
Type Of Technology Software 
Year Produced 2014 
Impact The heat equation was solved for heptane droplets both spherical and deformed, heated by Gaussian laser beams and immersed in either water or deuterium oxide bulk phases. The model inputs were i) the laser beam intensity profile - obtained from numerical solutions of the Maxwell equations (in the modelling and control work stream) and ii) the deformed droplet shapes - experimentally obtained through structured illumination measurements (in the optics work stream). The model outputs were the temperature and the interfacial tension profiles at the droplet interfaces. 
 
Title optical trapping technique to qualitatively assess and compare the deformability of micron-sized ultralow interfacial tension droplets 
Description We introduced an optical method based on time-sharing optical trapping technique to qualitatively assess and compare the deformability of micron-sized ultralow interfacial tension droplets. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2015 
Impact Two optical traps were used to stretch the droplets by pulling them from two opposite ends. By analysing the droplet deformed shapes under varying salinity and laser power levels, the droplet interfacial tension was qualitatively estimated. Our measurements were in good agreement with the literature data. 
 
Description Conference presentation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact "Droplet-based functional materials for biochemical studies and applications" at 1st Meeting of UK Fluids Network Special Interest Group (SIG) on Fluid Mechanics of Nanostructured Materials
Year(s) Of Engagement Activity 2018
 
Description Conference presentation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Royal Society meeting entitled "The artificial cell: biology inspired compartmentalisation of chemical function"
Year(s) Of Engagement Activity 2018
URL https://royalsociety.org/science-events-and-lectures/2018/02/artificial-cell/
 
Description Conference presentation at Chains 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation on Artificial Cells and Compartmentalisation at Chains 2018, December 2017
Year(s) Of Engagement Activity 2017
URL http://www.nwochains.nl/
 
Description International Microfluidics Congress 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact International conference: disseminate findings of research and look for new end users of the technology
Year(s) Of Engagement Activity 2015
 
Description Presentation entitled "A microfluidic platform for the mechanical property characterisation of ultralow interfacial tension droplets"at British Society of Rheology Winter Meeting 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2015
 
Description Presentation entitled "A microfluidic platform for the production of monodisperse ultralow interfacial tension oil droplets in water"at Micro-TAS 2014 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2014
 
Description Presentation entitled "Formation of monodisperse ultra-low interface tension droplets with a microfluidic flow focusing device" at Flow 14 (2014) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2014
 
Description Presentation entitled "Microfluidic generation and optical manipulation of ultra-low interfacial tension droplets" at Compartmentalisation & Confinement in Biological Systems 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2015
 
Description Presentation entitled "Microfluidic generation and optical manipulation of ultra-low interfacial tension droplets" at Global Engage's Microfluidics Congress 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2015
 
Description Presentation entitled "Microfluidic generation and optical manipulation of ultra-low interfacial tension droplets"at SPIE Microtechnologies 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Dissemination of results to new end users

Successful presentation of new microfluidic device to potential end users
Year(s) Of Engagement Activity 2015
 
Description Presentation to the general public (Friends of Imperial College) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Presentation aimed at addressing public concerning associated with manufacturing artificial cells and manipulating biological systems.
Year(s) Of Engagement Activity 2015
 
Description Technology showcase presentation to industry (AstraZeneca) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Workshop and associated talk aimed at translating EPSRC breakthrough to industry.
Year(s) Of Engagement Activity 2015