Microlfuidics provides a platform for a high level of control over small volumes of liquid. Properites of liquids at the microscale change as compared to the macrosclae; gravity has no effect, flow is laminar instead of turbulent, and interfacial interactions dominate. Surface enhanced Raman spectroscopy (SERS) is a highly sensitive and discriminant detection technique that can increase the normal Raman signal of an analyte as much as 1010 times through the utilization of nanostructured metals. We employ a flow merging microfluidic device in the laminar flow regime to allow for optimal aggregation of silver nanoparticles (AgNP). Since flow is laminar, mixing does not occur, while diffusion does. AgNPs, aggregating salt (LiCl), and the analyte molecules eventually diffuse into each other creating SERS "hotspots". The the merging channel is then interrogated using a Raman microscope system to gather analyte molecule spectra. This method has previously been used to detect molecules in nM concentrations. (2013, Andreou et al.)
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Dielectrophoretic Control of SERS Particle Aggregation
Surface Enhanced Raman Spectroscopy (SERS)-based microfluidic platforms show great potential for detection of trace concentrations of chemicals such as narcotics, explosives, etc. Enhancement is typically mediated by salt-induced aggregation of silver/gold nanoparticles. Uncontrolled aggregation can lead to diminished sensor sensitivity, clogging of channels, and fouling. In this project dielectrophoresis is explored as a modality for spatial control of SERS-hot particles with the goal of making a reusable SERS-based microfluidic trace chemical detector.
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Microfluidics for Fundamental Biological Research
The Meinhart lab enjoys an on going collaboration with the Rothman Lab in the department of Molecular and Cellular Biology at UCSB. This collaboration is working on creating microfluidic devices to enable biological experiments that would be impossible with current tools available to biology researchers. The current focus of this collaboration is a temperature gradient device in which the 50 µm long embryos of the nematode C. elegans can be delivered to and oriented in a temperature gradient of 5 oC across their 50 µm length. The internal capture region of the microfluidic device is shown in the center of the figure below. The embryos are captured and oriented by the pillars which are on top of the dark strip shown. The dark strip is a platinum electrode that heats the immediate surroundings when current flows, which creates a tunable temperature gradient.
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PIV Optimization
Knowledge of three dimensional, three component velocity fields is central to the understanding and development of effective microfluidic devices for lab-on-chip mixing applications. In this project, we develop a hybrid experimental-numerical method for generating 3D flow information from 2D particle image velocimetry (PIV) experimental data and finite element simulations of an alternating current electrothermal (ACET) micromixer. We implement the hybrid technique by applying a numerical least-squares optimization algorithm to a 3D multiphysics simulation in conjunction with 2D PIV data to generate an improved estimation of the steady state velocity field. This 3D velocity field can be used to assess mixing phenomena more accurately than would be possible through simulation alone. Our approach can also be used to estimate uncertain quantities in experimental situations by fitting the gathered field data to a simulated physical model.
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Acoustofluidic Simulations
The research domain of acoustofluidics is concerned with the effects of acoustic fields inside fluidic devices. The arising time-averaged phenomena called acoustic streaming and acoustic radiation forces can be used to manipulate fluid-suspended micro-particles in a contactless fashion. It has been shown that acoustofluidic particle handling has some clear advantages over competing technologies like hydrodynamic, dielectrophoretic, magnetophoretic, or optical manipulation strategies. Acoustoflu- idic particle manipulation does not require specific electric, magnetic, or optical particle properties and it is known for excellent cell viability when processing living biological samples like cells, bacteria, or larger organisms. Massively parallel particle manipulation can be achieved easily because the acoustic force fields spread over the volume of the fluid cavity, enabling the simultaneous manipulation of hundreds or thousands of particles at a time.
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Free Surface Microfluidics
A free-surface microfluidic device has been developed for continuous analysis of airborne molecules. Surface tension at the free-surface interface is used to confine the pressure-driven flow through the microchannel. The free surface allows airborne species to be absorbed directly into the fluid flowing in the microchannel. SERS was used to detect the presence of gaseous 4-ABT molecules that became entrained into the liquid phase.
Free Surface Microfluidics form the basis of a sensitive sensor for real-time, continuous monitoring of water-soluble gas-phase or airborne agents..
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Sessile Droplets
Chemical detection based on Surface Enhanced Raman Spectroscopy (SERS) has received much attention due to its ability to positively identify molecular species based on their vibrational spectra even at trace concentrations. In this work, we investigate the absorption and mixing kinetics of airborne analytes in sessile droplets subject to a specified temperature gradient and evaporation rate. Airborne analyte detection is achieved by the absorption of analyte into the microscale droplet through the gas-liquid interface, and subsequent mixing of the analyte with SERS-active colloid due to the Marangoni effect. The presence of analyte within the droplet stimulates a colloid aggregation process that results in the creation of areas with enhanced plasmonic activity, called ‘SERS hotspots’. Upon droplet evaporation, the colloid aggregates are deposited onto the substrate forming a circular stain. The remaining stain is interrogated with a Raman spectrometer to obtain spectra enabling analyte detection. The experiments and numerical simulations are performed to find the optimal conditions for detection based on the reaction rate, concentration of nanoparticles, and evaporation time.
