This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.

https://iopscience.iop.org/article/10.1088/0960-1317/15/2/R01/pdf

Micromixers?a review

Nanotechnology is the science of understanding matter and the control of matter at dimensions of 100 nm or less. Encompassing nanoscale science, engineering, and technology, nanotechnology involves imaging, measuring, modeling, and manipulation of matter at this level of precision. An important aspect of research in nanotechnology involves precision control and manipulation of devices and materials at a nanoscale, i.e., nanopositioning. Nanopositioners are precision mechatronic systems designed to move objects over a small range with a resolution down to a fraction of an atomic diameter. The desired attributes of a nanopositioner are extremely high resolution, accuracy, stability, and fast response. The key to successful nanopositioning is accurate position sensing and feedback control of the motion. This paper presents an overview of nanopositioning technologies and devices emphasizing the key role of advanced control techniques in improving precision, accuracy, and speed of operation of these systems.

/pdf/a-survey-of-control-issues-in-nanopositioning-1zgjre9s1l.pdf

A Survey of Control Issues in Nanopositioning

This article reviews acoustic microfiuidics: the use of acoustic fields, principally ultrasonics, for application in microfiuidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.

/pdf/microscale-acoustofluidics-microfluidics-driven-via-5bet39j13p.pdf

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

The aim of microfluidic mixing is to achieve a thorough and rapid mixing of multiple samples in microscale devices. In such devices, sample mixing is essentially achieved by enhancing the diffusion effect between the different species flows. Broadly speaking, microfluidic mixing schemes can be categorized as either “active”, where an external energy force is applied to perturb the sample species, or “passive”, where the contact area and contact time of the species samples are increased through specially-designed microchannel configurations. Many mixers have been proposed to facilitate this task over the past 10 years. Accordingly, this paper commences by providing a high level overview of the field of microfluidic mixing devices before describing some of the more significant proposals for active and passive mixers.

/pdf/microfluidic-mixing-a-review-3egj61w5ox.pdf

Microfluidic Mixing: A Review

The ability to tailor the chemical composition and structure of a surface at the sub-100-nm length scale is important for studying topics ranging from molecular electronics to materials assembly, and for investigating biological recognition at the single biomolecule level. Dip-pen nanolithography (DPN) is a scanning probe microscopy-based nanofabrication technique that uniquely combines direct-write soft-matter compatibility with the high resolution and registry of atomic force microscopy (AFM), which makes it a powerful tool for depositing soft and hard materials, in the form of stable and functional architectures, on a variety of surfaces. The technology is accessible to any researcher who can operate an AFM instrument and is now used by more than 200 laboratories throughout the world. This article introduces DPN and reviews the rapid growth of the field of DPN-enabled research and applications over the past several years.

Applications of dip-pen nanolithography.

This paper describes the work done in an attempt to integrate ultrasonic sensors in micro-fluidic channels. We have developed a system with fluidic channels having embedded zinc oxide transducers. A zinc oxide film is deposited on a glass substrate and is sandwiched between two electrodes. The transducers made are found to resonate at 400 MHz in thickness mode. The channels are made both from PDMS using a silicon master and from glass. These channels are aligned on top of the transducer forming a narrow path for fluid flow. The fluid of interest is pumped in the microchannels by means of a syringe pump and pulse echo measurements are done with the necessary circuitry. This paper will also show the results of the first experiments on mixing induced by the transducer and the measurement of physical dimensions, temperature of fluid in the channel using broad band pulse echo techniques.

Micro-fluidic channels with integrated ultrasonic transducers

The results of coating capacitive micromachined ultrasonic transducer (CMUT) arrays with two different biocompatible materials, parylene-c and polydimethylsiloxane (PDMS), are reported. These materials were characterized for use with CMUTs to enable direct contact transcutaneous and in vivo imaging. A passivation coating is required to provide electrical isolation to the active areas of the device and to protect it from a corrosive environment. It must also provide good mechanical characteristics to void imaging artifacts. The coated devices were compared side by side with uncoated devices for testing in air. The resonant frequency, collapse voltage and crosstalk were sampled. Parylene coated CMUTs were also tested underwater using pulse excitation. The parylene coating provided electrical insulation to the aqueous solution for 14 days. Both coatings showed a decrease in device resonant frequency and an increase in collapse voltage, as expected from the proposed theory.

Biocompatible coatings for CMUTs in a harsh, aqueous environment

Electrostatic transducers are usually operated under a DC bias below their collapse voltage. The same scheme has been adopted for capacitive micromachined ultrasonic transducers (cMUTs). DC bias deflects the cMUT membranes toward the substrate, so that their centers are free to move during both receive and transmit operations. In this paper, we present time-domain, finite element calculations for cMUTs using LS-DYNA, a commercially available finite element package. In addition to this DC bias mode, other new cMUT operations (collapse and collapse-snapback) have recently been demonstrated. Because cMUT membranes make contact with the substrate in these new operations, modeling of these cMUTs should include contact analysis. Our model was a cMUT transducer consisting of many hexagonal membranes; because it was symmetrical, we modeled only one-sixth of a hexagonal cell loaded with a fluid medium. The finite element results for both conventional and collapse modes were compared to measurements made by an optical interferometer; a good match was observed. Thus, the model is useful for designing cMUTs that operate in regimes where membranes make contact with the substrate.

/pdf/dynamic-analysis-of-capacitive-micromachined-ultrasonic-4poaysu63r.pdf

Dynamic analysis of capacitive micromachined ultrasonic transducers

The collapse voltage of micromachined capacitive ultrasonic transducers (CMUT) depends on the size, thickness, type, and position of the metal electrode within the membrane. This paper reports the result of a finite element study of this effect. The program (ANSYS 5.7) is used to model a circular membrane on top of a Si substrate covered by a Si3N4 insulation layer. We find that the collapse voltage increases in proportion to the metal thickness for constant membrane thickness. The collapse voltage of a membrane with a thin metal electrode decreases as the metal plate moves closer to the bottom of the membrane; whereas, for electrodes with larger metal thickness, the collapse voltage has a peak intermediate value. Decreasing the outer radius of the metal plate results in an asymptotic increase of the collapse voltage. For a finite metal thickness, an initial decrease in the collapse voltage is seen as the outer radius decreases. The collapse voltages of half-metallized and full-metallized structures are almost equal for typical metal plate thickness. The asymptotic increase of the collapse voltage is seen for ring shaped metal plates as the inner radius is varied from the center to the outer radius. In summary, we find that the influence of the metal electrode on the collapse voltage is a very important parameter in determining optimum performance of a CMUT.

/pdf/influence-of-the-electrode-size-and-location-on-the-3pqq98p35b.pdf

Influence of the Electrode Size and Location on the Performance of a CMUT

High-density through-wafer interconnects are incorporated in a two-dimensional (2D) micromachined cantilever array. The design addresses alignment and density issues associated with 2D arrays. Each cantilever has piezoresistive deflection sensors and high-aspect ratio silicon tips. The fabrication process and array operation are described. The integration of cantilevers, tips, and interconnects enables operation of a high-density 2D scanning probe array over large areas.

/pdf/integration-of-through-wafer-interconnects-with-a-two-45w5doxdae.pdf

Goksen G. Yaralioglu

Papers

Micro-fluidic channels with integrated ultrasonic transducers

Biocompatible coatings for CMUTs in a harsh, aqueous environment

Dynamic analysis of capacitive micromachined ultrasonic transducers

Influence of the Electrode Size and Location on the Performance of a CMUT

Integration of through-wafer interconnects with a two-dimensional cantilever array