Thermal analysis of nanofluids in microfluidics using an infrared camera

Hydrogel Micropillar Array for Temperature Sensing in Fluid

Localized temperature sensing and control on a micron-scale have diverse applications in biological systems. We present a micron-sized hydrogel pillar array as potential temperature probes and actuators by exploiting sensitive temperature dependence of their volume change. Soft lithography-based molding processes were presented to fabricate poly N-isopropyl acrylamide (p-NIPAAm)-based hydrogel pillar array on a glass substrate. Au nanorods as light-induced heating elements were embedded within the hydrogel pillars, and near-infrared (NIR) light was used to modulate temperature in a local area. First, static responses of the micro-pillar array were characterized as a function of its temperature. It was shown that the hydrogel had a sensitive volume transition near its low critical solution temperature (LCST). Furthermore, we showed that LCST could be readily adjusted by utilizing copolymerizing with acrylamide (AAM). To demonstrate the feasibility of spatiotemporal temperature mapping and modulation using the presented pillar array, pulsed NIR light was illuminated on a local area of the hydrogel pillar array, and its responses were recorded. Dynamic temperature change in water was mapped based on the abrupt volume change characteristics of the hydrogel pillar, and its potential actuation using NIR light was successfully demonstrated. Considering that the structure can be arrayed in a two-dimensional pixel format with high spatial resolution and high sensitive temperature characteristics, the presented method and the device structure can have diverse applications to change and sense local temperatures in liquid. This is particularly useful in biological systems, where their physiological temperature can be modulated and mapped with high spatial resolution.

Colloid thermophoresis in the dilute electrolyte concentration regime: from theory to experiment

Colloid thermophoresis in aqueous media is vital for numerous applications in nanoscience and life sciences. To date, a general description of colloid thermophoresis in DI water has not been determined. Here, we describe a theoretical model within the framework of the Fokker-Planck formalism and the flickering cluster concept to describe the hydration entropy effect on the thermophoretic behaviour of colloids suspended in DI water and compare this to new experimental results. We built an experimental platform to allow for rapid and robust temperature control and investigate the thermophoretic behaviour of silica microspheres with different sizes at various background temperatures for comparison. In this work, the ionic shielding effect is accounted for by using the well-known Duhr-Dhont's model, and the hydration layer effect is determined using the developed theoretical model. For the latter, our model reveals that the sign of the Soret coefficient is governed by the interplay between the binding energy and the chemical potential of water molecules, which were found to be in the same order of magnitude. We show that our analysis accurately describes the experimental behaviour of colloidal particles that opens a new avenue for developing versatile trapping and separation techniques for various colloidal particles in aqueous systems according to their size and background temperature.

Dielectrophoresis of Amyloid-Beta Proteins as a Microfluidic Template for Alzheimer's Research

We employed dielectrophoresis to a yeast cell suspension containing amyloid-beta proteins (Aβ) in a microfluidic environment. The Aβ was separated from the cells and characterized using the gradual dissolution of Aβ as a function of the applied dielectrophoretic parameters. We established the gradual dissolution of Aβ under specific dielectrophoretic parameters. Further, Aβ in the fibril form at the tip of the electrode dissolved at high frequency. This was perhaps due to the conductivity of the suspending medium changing according to the frequency, which resulted in a higher temperature at the tips of the electrodes, and consequently in the breakdown of the hydrogen bonds. However, those shaped as spheroidal monomers experienced a delay in the Aβ fibril transformation process. Yeast cells exposed to relatively low temperatures at the base of the electrode did not experience a positive or negative change in viability. The DEP microfluidic platform incorporating the integrated microtip electrode array was able to selectively manipulate the yeast cells and dissolve the Aβ to a controlled extent. We demonstrate suitable dielectrophoretic parameters to induce such manipulation, which is highly relevant for Aβ-related colloidal microfluidic research and could be applied to Alzheimer’s research in the future.

A label-free infrared opto-fluidic method for real-time determination of flow rate and concentration with temperature cross-sensitivity compensation

The ability to accurately measure the flow rate, concentration, and temperature in real-time in micro total analysis systems (μTAS) is crucial when improving their practical sensing capabilities within extremely small volumes. Our label-free infrared (1500-1600 nm) opto-fluidic method, presented in this study, utilizes a cantilever-based flow meter integrated with two parallel optical fiber Fabry-Perot interferometers (FPIs). The first FPI serves as an ultra-sensitive flow meter and includes a Fiber Bragg Grating (FBG) tip for localized temperature sensing. The second FPI has a fabricated photopolymer micro-tip for highly sensitive refractive index (RI) determination. In this work, we performed 3-D simulation analysis to characterize cantilever deflection as well as temperature distribution and its effect on the RI. The experimental results from temperature cross-sensitivity analysis lead to real-time measurement resolutions of 5 nL min^-1, 1 × 10^-6 RIU and 0.05 °C, for the flow rate, refractive index, and temperature, respectively.

Active bioparticle manipulation in microfluidic systems

The motion of bioparticles in a microfluidic environment can be actively controlled using several tuneable mechanisms, including hydrodynamic, electrophoresis, dielectrophoresis, magnetophoresis, acoustophoresis, thermophoresis and optical forces.

Heat transfer and friction characteristics of the microfluidic heat sink with variously-shaped ribs for chip cooling

This paper experimentally and numerically investigated the heat transfer and friction characteristics of microfluidic heat sinks with variously-shaped micro-ribs, i.e., rectangular, triangular and semicircular ribs. The micro-ribs were fabricated on the sidewalls of microfluidic channels by a surface-micromachining micro-electro-mechanical system (MEMS) process and used as turbulators to improve the heat transfer rate of the microfluidic heat sink. The results indicate that the utilizing of micro-ribs provides a better heat transfer rate, but also increases the pressure drop penalty for microchannels. Furthermore, the heat transfer and friction characteristics of the microchannels are strongly affected by the rib shape. In comparison, the triangular ribbed microchannel possesses the highest Nusselt number and friction factor among the three rib types.

Organic synthesis: march of the machines

Organic synthesis is changing; in a world where budgets are constrained and the environmental impacts of practice are scrutinized, it is increasingly recognized that the efficient use of human resource is just as important as material use. New technologies and machines have found use as methods for transforming the way we work, addressing these issues encountered in research laboratories by enabling chemists to adopt a more holistic systems approach in their work. Modern developments in this area promote a multi-disciplinary approach and work is more efficient as a result. This Review focuses on the concepts, procedures and methods that have far-reaching implications in the chemistry world. Technologies have been grouped as topics of opportunity and their recent applications in innovative research laboratories are described.

PDMS nanocomposites for heat transfer enhancement in microfluidic platforms

Increasing the thermal conductivity of PDMS (polydimethylsiloxane) based microfluidics is an important issue for the thermal management of hot spots produced by embedding electronic circuits in such systems. This paper presents a solution for enhancing the thermal conductivity of such PDMS based microfluidics by introducing thermally conductive alumina (Al2O3) nanoparticles, forming PDMS/Al2O3 nanocomposites. The materials are fully characterized for different concentrations of Al2O3 in PDMS for experiments which are conducted at different flow rates. Our results suggest that incorporation of Al2O3 nanoparticles at 10% w/w in the PDMS based nanocomposite significantly enhances the heat conduction from hot spots by enhancing the thermal conductivity, while maintaining the flexibility and decreasing the specific heat capacity of the developed materials. This proof-of-concept study offers potential for a practical solution for the cooling of future embedded electronic systems.

Investigation of different nanoparticles for magnetophoretically enabled nanofin heat sinks in microfluidics

Assembled nanofin heat sinks, nanostructures which are formed via external forces in a cooling microfluidic to remove heat from hot spots, are a new concept that has recently been introduced. In this work, we investigate nanofin structures formed by CrO2 and Fe2O3 magnetic nanoparticles and compare their performance. Thermal imaging is used for comparison of three cases including: (i) DI water as the coolant liquid, (ii) suspension of magnetic particles in DI water, and (iii) suspension of magnetic particles in DI water in the presence of a magnetic field. For each case, the experiments are conducted at three different flow rates of 10, 40 and 120 μl min(-1). Our results suggest that the high thermal conductivity of the nanofins composed of CrO2 significantly enhances the heat exchange across the microchannel. The proof-of-concept magnetophoretic system can offer a practical solution for the cooling of future compact electronics.

Microfluidics and Raman microscopy: current applications and future challenges

Raman microscopy systems are becoming increasingly widespread and accessible for characterising chemical species. Microfluidic systems are also progressively finding their way into real world applications. Therefore, it is anticipated that the integration of Raman systems with microfluidics will become increasingly attractive and practical. This review aims to provide an overview of Raman microscopy-microfluidics integrated systems for researchers who are actively interested in utilising these tools. The fundamental principles and application strengths of Raman microscopy are discussed in the context of microfluidics. Various configurations of microfluidics that incorporate Raman microscopy methods are presented, with applications highlighted. Data analysis methods are discussed, with a focus on assisting the interpretation of Raman-microfluidics data from complex samples. Finally, possible future directions of Raman-microfluidic systems are presented.