Tomasz Glawdel

Microwave sensing and heating of individual droplets in microfluidic devices

Droplet-based microfluidics is an emerging high-throughput screening technology finding applications in a variety of areas such as life science research, drug discovery and material synthesis. In this paper we present a cost-effective, scalable microwave system that can be integrated with microfluidic devices enabling remote, simultaneous sensing and heating of individual nanoliter-sized droplets generated in microchannels. The key component of this microwave system is an electrically small resonator that is able to distinguish between materials with different electrical properties (i.e. permittivity, conductivity). The change in these properties causes a shift in the operating frequency of the resonator, which can be used for sensing purposes. Alternatively, if microwave power is delivered to the sensing region at the frequency associated with a particular material (i.e. droplet), then only this material receives the power while passing the resonator leaving the surrounding materials (i.e. carrier fluid and chip material) unaffected. Therefore this method allows sensing and heating of individual droplets to be inherently synchronized, eliminating the need for external triggers. We confirmed the performance of the sensor by applying it to differentiate between various dairy fluids, identify salt solutions and detect water droplets with different glycerol concentrations. We experimentally verified that this system can increase the droplet temperature from room temperature by 42 °C within 5.62 ms with an input power of 27 dBm. Finally we employed this system to thermally initiate the formation of hydrogel particles out of the droplets that are being heated by this system.

Passive droplet trafficking at microfluidic junctions under geometric and flow asymmetries

When droplets enter a junction they sort to the channel with the highest flow rate at that instant. Transport is regulated by a discrete time-delayed feedback that results in a highly periodic behavior where specific patterns can continue to cycle indefinitely. Between these highly ordered regimes are chaotic structures where no pattern is evident. Here we develop a model that describes droplet sorting under various asymmetries: branch geometry (length, cross-section), droplet resistance and pressures. First, a model is developed based on the continuum assumption and then, with the assistance of numerical simulations, a discrete model is derived to predict the length and composition of the sorting pattern. Furthermore we derive all unique sequences that are possible for a given distribution and develop a preliminary estimation of why chaotic regimes form. The model is validated by comparing it to numerical simulations and results from microfluidic experiments in PDMS chips with good agreement.

Bilinear Temperature Gradient Focusing in a Hybrid PDMS/Glass Microfluidic Chip Integrated with Planar Heaters for Generating Temperature Gradients

Temperature gradient focusing (TGF) is a counterflow gradient focusing technique, which utilizes a temperature gradient across a microchannel or capillary to separate analytes. With an appropriate buffer, the temperature gradient creates a gradient in both the electric field and electrophoretic velocity. Combined with a bulk counter flow, ionic species concentrate at a unique point where the total velocity sums to zero and separate from each other. Scanning TGF uses varying bulk flow so that a large number of analytes that have large differences in electrophoretic mobility can be sequentially focused and passed by a single detection point. Up to now, scanning TGF examples have been performed using a linear temperature gradient which has limitations in improving peak capacity and resolution at the same time. In this work, we develop a bilinear temperature gradient along the separation channel that improves both peak capacity and separation resolution simultaneously. The temperature profile along the channel consists of a very sharp gradient used to preconcentrate the sample followed by a shallow gradient that increases separation resolution. A specialized design is developed for the heaters to achieve the bilinear profile using both analytical and numerical modeling. The heaters are integrated onto a hybrid PDMS/glass chip fabricated using conventional sputtering and soft-lithography techniques. Separation performance is characterized by separating several different dyes and amino acids that have close electrophoretic mobilities. Experiments show a dramatic improvement in peak capacity and resolution in comparison to the standard linear temperature gradient.

Integration of Dialysis Membranes into a Poly(dimethylsiloxane) Microfluidic Chip for Isoelectric Focusing of Proteins Using Whole-Channel Imaging Detection

A poly(dimethylsiloxane) microfluidic chip-based cartridge is developed and reported here for protein analysis using isoelectic focusing (IEF)-whole-channel imaging detection (WCID) technology. In this design, commercial dialysis membranes are integrated to separate electrolytes and samples and to reduce undesired pressure-driven flow. Fused-silica capillaries are also incorporated in this design for sample injection and channel surface preconditioning. This structure is equivalent to that of a commercial fused-silica capillary-based cartridge for adapting to an IEF analyzer (iCE280 analyzer) to perform IEF-WCID. The successful integration of dialysis membranes into a microfluidic chip significantly improves IEF repeatability by eliminating undesired pressure-driven hydrodynamics and also makes sample injection much easier than that using the first-generation chip as reported recently. In this study, two microfluidic chips with a 100-microm-high, 100-microm-wide and a 200-microm-high, 50-microm-wide microchannel, respectively, were applied for qualitative and quantitative analysis of proteins. The mixture containing six pI markers with a pH range of 3-10 was successfully separated using IEF-WCID. The pH gradient exhibited a good linearity by plotting the pI value versus peak position, and the correlation coefficient reached 0.9994 and 0.9995 separately for the two chips. The separation of more complicated human hemoglobin control sample containing HbA, HbF, HbS, and HbC was also achieved. Additionally, for the quantitative analysis, a good linearity of IEF peak value versus myoglobin concentration in the range of 20-100 microg/mL was obtained.

Side-by-side comparison of disposable microchips with commercial capillary cartridges for application in capillary isoelectric focusing with whole column imaging detection

Simple-structured, well-functioned disposable poly(dimethylsiloxane) (PDMS) microchips were developed for capillary isoelectric focusing with whole column imaging detection (CIEF-WCID). Side-by-side comparison of the developed microchips with well-established commercial capillary cartridges demonstrated that the disposable microchips have comparable performance as well as advantages such as absence of lens effect and possibility of high-aspect-ratio accompanied with a dramatic reduction in cost.

Combination of ac electroosmosis and dielectrophoresis for particle manipulation on electrically-induced microscale wave structures

This work presents a simple method to fabricate controllable microscale wave structures on the top of regular interdigitated electrode (IDE) arrays using electrically-assisted lithography techniques. Smooth wave structures are extremely difficult, if not impossible, to fabricate using traditional multilayer photolithography technology. The fabricated wave structures were carefully measured using an optical profiler and the measured wave profiles were used in the numerical simulation of electrical field and for evaluating the parameters influencing the fabricated wave structure. It is demonstrated that the combined smooth wave structure and IDE array offer unique capability for particle manipulation including particle concentration, aggregation and separation. Particle motion manipulated via the combined wave structure and IDE array is governed by ac electroosmosis (ACEO), dielectrophoresis (DEP) or a combination of both depending on the applied frequency. At lower frequencies (~30 kHz), ACEO dominates and particles are driven to move along the valleys of the wave structures; while at higher frequencies (~200 kHz), DEP force dominates which concentrates particles at the peaks of the wave structures. In addition, varying the ac waveform from sine-wave to square-wave allows for dynamic control of particle motion. Size-dependent particle separation over the wave structure is also demonstrated for a mixture of 0.5 µm and 2 µm particles that are separated into two populations by the joint effects of drag and DEP forces when being pumped to flow via ACEO.

Micellar affinity gradient focusing in a microfluidic chip with integrated bilinear temperature gradients

Micellar affinity gradient focusing (MAGF) is a microfluidic counterflow gradient focusing technique that combines the favorable features of MEKC and temperature gradient focusing. MAGF separates analytes on the basis of a combination of electrophoretic mobility and partitioning with the micellar phase. A temperature gradient is produced along the separation channel containing an analyte/micellar system to create a gradient in interaction strength (retention factor) between the analytes and micelles. Combined with a bulk counterflow, species concentrate at a unique point where their total velocity sums to zero. MAGF can be used in scanning mode by varying the bulk flow so that a large number of analytes can be sequentially focused and passed by a single detection point. In this work, we develop a bilinear temperature gradient along the separation channel that improves separation performance over the conventional linear designs. The temperature profile along the channel consists of a very sharp gradient used to preconcentrate the sample followed by a shallow gradient that increases resolution. We fabricated a hybrid PDMS/glass microfluidic chip with integrated micro heaters that generate the bilinear profile. Performance is characterized by separating several different samples including fluorescent dyes using SDS surfactant and pI markers using both SDS and poly‐SUS surfactants as the micellar phase. The new design shows a nearly two times improvement in peak capacity and resolution in comparison to the standard linear temperature gradient.

Non-linear, non-monotonic effect of nano-scale roughness on particle deposition in absence of an energy barrier: Experiments and modeling.

Deposition of colloidal- and nano-scale particles on surfaces is critical to numerous natural and engineered environmental, health, and industrial applications ranging from drinking water treatment to semi-conductor manufacturing. Nano-scale surface roughness-induced hydrodynamic impacts on particle deposition were evaluated in the absence of an energy barrier to deposition in a parallel plate system. A non-linear, non-monotonic relationship between deposition surface roughness and particle deposition flux was observed and a critical roughness size associated with minimum deposition flux or “sag effect” was identified. This effect was more significant for nanoparticles (<1 μm) than for colloids and was numerically simulated using a Convective-Diffusion model and experimentally validated. Inclusion of flow field and hydrodynamic retardation effects explained particle deposition profiles better than when only the Derjaguin-Landau-Verwey-Overbeek (DLVO) force was considered. This work provides 1) a first comprehensive framework for describing the hydrodynamic impacts of nano-scale surface roughness on particle deposition by unifying hydrodynamic forces (using the most current approaches for describing flow field profiles and hydrodynamic retardation effects) with appropriately modified expressions for DLVO interaction energies, and gravity forces in one model and 2) a foundation for further describing the impacts of more complicated scales of deposition surface roughness on particle deposition.

Microchip with an open tubular immobilized ph gradient for UV whole column imaging detection.

This study reports a new method for establishing an open tubular IPG in a microchip coupled with a whole column image detection (WCID) system for protein separation applications. This method allows a wider range of immobilized pH (2.6–9.5) to be established in a PDMS/quartz channel by controlling the diffusion of acidic and basic polymer solutions into the channel through well‐designed channel dimensions. The developed pH gradient was experimentally validated by performing the separation of a mixture of standard pI markers. It was further validated by the separation of the hemoglobin control AFSC sample. This method is advantageous over existing IPG methods because it has a wider range of pH and maintains the open tubular feature that matches the UV WCID to improve the sensitivity.

Robust thin-film fluorescence thermometry for prolonged measurements in microfluidic devices

This work presents a robust method for whole chip temperature mapping in microfluidic devices using a photostable fluorescent-polymer thin film that can be incorporated during the bonding stage. Temperature measurements are based on the ratio of two bands in the fluorescence spectrum of N,N-bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedi carboximide (BTBP) dye. Spectral bands were carefully chosen to minimize errors caused by photobleaching of the dye which results in a perceived drift in the temperature with time. The improvements result in a useful working time for this type of measurement to >20 h. We achieve a temperature resolution <2 °C with drift less than 0.58 °C h−1 in thin-films of common polymers used in microfluidic device fabrication (polystyrene, polydimethylsiloxane, and polymethylmethacrylate) Finally, we demonstrate the usefulness of this technique by presenting examples of temperature fields mapped by the thin-films in several thermal microfluidic devices.