Ali Hashmi

Oscillating bubbles: a versatile tool for lab on a chip applications.

With the fast development of acoustic and multiphase microfluidics in recent years, oscillating bubbles have drawn more-and-more attention due to their great potential in various Lab on a Chip (LOC) applications. Many innovative bubble-based devices have been explored in the past decade. In this article, we first briefly summarize current understanding of the physics of oscillating bubbles, and then critically summarize recent advancements, including some of our original work, on the applications of oscillating bubbles in microfluidic devices. We intend to highlight the advantages of using oscillating bubbles along with the challenges that accompany them. We believe that these emerging studies on microfluidic oscillating bubbles will be revolutionary to the development of next-generation LOC technologies.

Controlling Porosity in Lignin‐Derived Nanoporous Carbon for Supercapacitor Applications

Low-cost renewable lignin has been used as a precursor to produce porous carbons. However, to date, it has not been easy to obtain high surface area porous carbon without activation processes or templating agents. Here, we demonstrate that low molecular weight lignin yields highly porous carbon with more graphitization through direct carbonization without additional activation processes or templating agents. We found that molecular weight and oxygen consumption during carbonization are critical factors to obtain high surface area, graphitized porous carbons. This highly porous carbon from low-cost renewable lignin sources is a good candidate for supercapacitor electrode materials.

Leidenfrost levitation: beyond droplets

Friction is a major inhibitor in almost every mechanical system. Enlightened by the Leidenfrost effect – a droplet can be levitated by its own vapor layer on a sufficiently hot surface – we demonstrate for the first time that a small cart can also be levitated by Leidenfrost vapor. The levitated cart can carry certain amount of load and move frictionlessly over the hot surface. The maximum load that the cart can carry is experimentally tested over a range of surface temperatures. We show that the levitated cart can be propelled not only by gravitational force over a slanted flat surface, but also self-propelled over a ratchet shaped horizontal surface. In the end, we experimentally tested water consumption rate for sustaining the levitated cart, and compared the results to theoretical calculations. If perfected, this frictionless Leidenfrost cart could be used in numerous engineering applications where relative motion exists between surfaces.

Microbubble array for on-chip worm processing

We present an acoustic non-contact technique for achieving trapping, enrichment, and manipulation of Caenorhabditis elegans using an array of oscillating microbubbles. We characterize the trapping efficiency and enrichment ratio under various flow conditions, and demonstrate a single-worm manipulation mechanism through temporal actuation of bubbles. The reason for oscillating bubbles being versatile in processing worms in a microfluidic environment is due to the complex interactions among acoustic field, microbubbles, fluid flow, and live animals. We explain the operating mechanisms used in our device by the interplay among secondary acoustic radiation force, drag force, and the propulsive force of C. elegans.

Freezing of a liquid marble.

In this study, we present for the first time the observations of a freezing liquid marble. In the experiment, liquid marbles are gently placed on the cold side of a thermo-electric cooler (TEC), and the morphological changes are recorded and characterized thereafter. These liquid marbles are noticed to undergo a shape transition from a spherical to a flying-saucer-shaped morphology. The freezing dynamics of liquid marbles is observed to be very different from that of a freezing water droplet on a superhydrophobic surface. For example, the pointy tip appearing on a frozen water drop could not be observed for a frozen liquid marble. In the end, we highlight a possible explanation of the observed morphology.

On the quantification of mixing in microfluidics.

Methods for quantifying mixing in microfluidics have varied largely in the past, and various indices have been employed to represent the extent of mixing. Mixing between two or more colored liquids is usually quantified using simple mathematical functions operated over a sequence of images. The function, usually termed mixing indices, involves a measure of standard deviation. Here, we first review some mixing indices and then experimentally verify the index most representative of a mixing event. It is observed that the relative mixing index is not affected by the lighting conditions, unlike other known mixing indices. Based on this finding, the use of a relative mixing index is advocated for further use in the lab-on-a-chip community for quantifying mixing events.

A compact lab-on-a-chip nanosensor for glycerol detection

Real-time monitoring of glycerol concentration has significant meaning in many lab-on-a-chip applications. The existing sensors for glycerol detection have complicated fabrication and testing procedures and are not truly compatible with microfluidic systems for on-site detection. In this study, we explore the possibility of using an integrated single-walled carbon nanotubes nanosensor for glycerol detection. Our device enables real-time, in-channel detection of the concentration of static or flowing aqueous glycerol solutions. Sensor resistance is found to increase with an increasing glycerol-to-water weight ratio and is sensitive to flow velocity. We also highlight the sensing mechanisms for both conditions.

Liquid marbles with in-flows and out-flows: Characteristics and performance limits

We characterized the breaking, buckling and deformation of a liquid marble upon liquid exchange via external inflow and outflow. During liquid addition, the breaking of liquid marbles was affected by the existence of a swirl flow pattern. During liquid removal, the liquid marbles buckled and their morphology was controlled using penetrating pins. During a zero-net-flow perfusion process, the shapes of liquid marbles were controlled by the arrangement of the inlets and outlets.

Microfluidic Flow Control and Particle Transport Using Acoustically Actuated Bubbles in Teardrop Shaped Cavities

It is well known that a symmetric microstreaming flow field will present in the vicinity of an acoustically actuated bubble. In this study, we demonstrate that oscillating microbubbles confined in teardrop-shaped cavities can result in a break in the symmetry of a microstreaming flow field. The teardrop cavity controls the size and shape of the bubble, regulating the volume and therefore its resonance frequency. If actuated in an acoustic field, the induced flow field can then be turned on and off by changing the acoustic actuation frequency. By harnessing the flow field directing capabilities of symmetry breaks and the switching properties of selective excitation of microbubbles, we generate and characterize a microfluidic switch for directing flow direction. We also show that a chain of multiple teardrop-shaped cavities can be used as a transport mechanism for directing particles spatially at high flow speeds. Our results demonstrate that teardrop cavities have great potential in future lab-on-a-chip devices by providing simple solutions to complex flow circuits for temporal and spatial flow control.© 2012 ASME

Microbubble Array as a Versatile Tool for On-Chip Worm Processing

In this study, we present a novel technique for achieving precise trapping, enrichment and manipulation of C. elegans using an array of oscillating microbubbles in a simple microfluidic device. We demonstrate a high trapping efficiency and enrichment of C. elegans characterized by the flow conditions. We also demonstrate for the first time an on-chip manipulation of the pathways of a single C. elegans through temporal actuation of bubbles.Copyright