Papers by Dileep Mampallil
Evaporative phase separation in polymer microdroplets with confinement and internal flow
JCIS Open, Mar 31, 2024
Spheroids formation in large drops suspended in superhydrophobic paper cones
Biomicrofluidics, Mar 1, 2024
Electrochemical IFN-γ Immunosensor Based on a Nanocomposite of Gold Nanorods and Reduced Graphene Oxide
Social Science Research Network, 2022
Electrochemical IFN-γ immunosensor based on a nanocomposite of gold nanorods and reduced graphene oxide
Journal of Applied Electrochemistry, Jul 14, 2023
Resonance, Feb 1, 2014
His research interests include micronanofluidics and nanoscale electrochemistry. Spreading of a d... more His research interests include micronanofluidics and nanoscale electrochemistry. Spreading of a droplet placed on a solid surface (sessile droplet) depends on the wetting properties of the liquid on that surface. Droplets are of great scientific interest because studying their formation and dynamics is important in many technologies such as in microfluidics [1, 2]. Evaporation of droplets is another interesting aspect that has caught much scientific attention. An example of an evaporating droplet is a coffee droplet spilled on a table. After evaporation the coffee droplet leaves a ring-like stain called a coffee ring or coffee stain [3]. This phenomenon, known as the 'coffee ring effect', occurs not only with coffee droplets but also with all droplets containing non-volatile solutes. There are a number of interesting physical processes going on within the droplets during evaporation to form the 'coffee stains'. In this article, we will see what they are.
Analytical Chemistry, May 28, 2013
Redox cycling between two electrodes separated by a narrow gap allows dramatic amplification of t... more Redox cycling between two electrodes separated by a narrow gap allows dramatic amplification of the faradaic current. Unlike conventional electrochemistry at a single electrode, however, the mass-transport-limited current is controlled by the diffusion coefficient of both the reduced and oxidized forms of the redox-active species being detected and, counterintuitively, by the redox state of molecules in the bulk solution outside the gap itself. Using a combination of finiteelement simulations, analytical theory, and experimental validation, we elucidate the interplay between these interrelated factors. In so doing, we generalize previous results obtained in the context of scanning electrochemical microscopy and obtain simple analytical results that are generally applicable to experimental situations where efficient redox cycling takes place.
Faraday Discussions, 2016
The diffusive mass transport of individual redox molecules was probed experimentally in microfabr... more The diffusive mass transport of individual redox molecules was probed experimentally in microfabricated nanogap electrodes. The residence times for molecules inside a welldefined detection volume were extracted and the resulting distribution was compared with quantitative analytical predictions from random-walk theory for the time of first passage. The results suggest that a small number of strongly adsorbing sites strongly influence mass transport at trace analyte levels.
Journal of Physical Chemistry Letters, Jan 29, 2014
Adsorption often dominates the response of nanofluidic systems due to their high surface-to-volum... more Adsorption often dominates the response of nanofluidic systems due to their high surface-to-volume ratios. Here we harness this sensitivity to investigate the reversible adsorption of outer-sphere redox species at electrodes, a phenomenon that is easily overlooked in bulk measurements. We find that even though adsorption does not necessarily play a role in the electron-transfer process, such adsorption is nevertheless ubiquitous for the widely used outer-sphere species. We investigate the physical factors driving adsorption and find that this counterintuitive behavior is mediated by the anionic species in the supporting electrolyte, closely following the well-known Hofmeister series. Our results provide foundations both for theoretical studies of the underlying mechanisms and for contriving strategies to control adsorption in micro/nanoscale electrochemical transducers where surface effects are dominant.
Physical Review Letters, Sep 10, 2012
We introduce all-electrical cross-correlation spectroscopy of molecular number fluctuations in na... more We introduce all-electrical cross-correlation spectroscopy of molecular number fluctuations in nanofluidic channels. Our approach is based on a pair of nanogap electrochemical transducers located downstream from each other in the channel. When liquid is driven through this device, mesoscopic fluctuations in the local density of molecules are transported along the channel. We perform a time-offlight measurement of these fluctuations by cross-correlating current-time traces obtained at the two detectors. Thereby we are able to detect ultralow liquid flow rates below 10 pL= min. This method constitutes the electrical equivalent of fluorescence cross-correlation spectroscopy.
ACS Nano, Dec 4, 2013
A. Reduction of single-molecule current due to dead volume and origin of geom We argue in the ma... more A. Reduction of single-molecule current due to dead volume and origin of geom We argue in the main text that the observed single-molecule current, i p , is suppressed by a factor geom due to the existence of a dead volume. Here we explain in more detail the origin of this suppression.
Microfluidics – ALab in Your Palm
(right) Sajan D George is an Associate Professor at Center for Atomic and Molecular Physics, Mani... more (right) Sajan D George is an Associate Professor at Center for Atomic and Molecular Physics, Manipal University. India. His research interests include micro and optofluidics, nanophotonics, biomedical applications of laser and photothermal methods. A few decades ago electronics evolved from huge circuits based on vacuum tubes into small scale semiconducting integrated circuits. In simple words, a calculator that was as huge as a room, now ts into a wrist watch. Nowadays, a similar trend is observed in bio and chemical engineering. Analysis and synthesis involving °uids, which originally could be done only on large scales, are more and more integrated into small devices called micro°uidic chips. Such microsacle or even nanoscale devices, in general called labon-a-chip devices, o®er a higher accuracy and better e±ciency compared to bulk devices. In this article we describe some important aspects about micro°uidics.
Interferon-γ detection in point of care diagnostics: Short review
Talanta, Aug 1, 2022
Interferon (IFN)-γ is a cytokine secreted by immune cells. The elevated levels of IFN-γ are an ea... more Interferon (IFN)-γ is a cytokine secreted by immune cells. The elevated levels of IFN-γ are an early indicator of multiple diseases such as tuberculosis and autoimmune diseases. This short review focuses on different sensing methods based on optical, electrochemical, and mechanical principles. We explain how specific biorecognition molecules such as antibodies and aptamers are employed in the sensing methods. We also compare different surface functionalization methods and their details. Although the review gives an overview of only IFN-γ sensing, the same strategies can be applied to sensing other analytes with appropriate modifications.
Electrochemical IFN-γ Immunosensor Based on a Nanocomposite of Gold Nanorods and Reduced Graphene Oxide
Social Science Research Network, 2022
Advances in Colloid and Interface Science, Feb 1, 2018
Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent l... more Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent leaves behind ring-like solid stains. As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line. This phenomenon, called the coffee-ring effect, compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, manufacturing of nano-structured materials through colloidal and macromolecular patterning. Various approaches have been developed to suppress this phenomenon, which is otherwise difficult to avoid. The coffee-ring effect has also been leveraged to prepare new materials through convection induced assembly. This review underlines not only the strategies developed to suppress the coffee-ring effect but also sheds light on approaches to arrive at novel processes and materials. Working principles and applicability of these strategies are discussed together with a critical comparison.
Microfluidic Droplet Cluster with Distributed Evaporation Rates as a Model for Bioaerosols
Langmuir, Apr 8, 2022
Aerosols and microdroplets are known to act as carriers for pathogens or vessels for chemical rea... more Aerosols and microdroplets are known to act as carriers for pathogens or vessels for chemical reactions. The natural occurrence of evaporation of these droplets has implications for the viability of pathogens or chemical processes. For example, it is important to understand how pathogens survive extreme physiochemical conditions such as confinement and osmotic stress induced by evaporation of aerosol droplets. Previously, larger evaporating droplets were proposed as model systems as the processes in the tiny aerosol droplets are difficult to image. In this context, we propose the concept of evaporation of capillary-clustered aqueous microdroplets dispersed in a thin oil layer. The configuration produces spatially segregated evaporation rates. It allows comparing the consequences of evaporation and its rate for processes occurring in droplets. As a proof of concept, we study the consequences of evaporation and its rate using Escherichia coli (E. coli) and Bacillus subtilis as model organisms. Our experiments indicate that the rate of evaporation of microdroplets is an important parameter in deciding the viability of contained microorganisms. With slow evaporation, E. coli could mitigate the osmotic stress by K+ ion uptake. Our method may also be applicable to other evaporating droplet systems, for example, microdroplet chemistry to understand the implications of evaporation rates.
Drop impact on thin powder layers: pattern formation by air entrapment
Soft Matter, 2020
Drops impacting on thin powder layers can form fascinating patterns of powder particles by the in... more Drops impacting on thin powder layers can form fascinating patterns of powder particles by the interplay of processes such as micro-bubble generation and large air-disc entrapment.
Physics of Fluids, Sep 1, 2020
Evaporation of drops almost always deposits their suspended particles at the drop edge. The dynam... more Evaporation of drops almost always deposits their suspended particles at the drop edge. The dynamics of this process and the resulting patterns depend upon various parameters related to the liquid, substrate, and particles. An interesting scenario is interactions among the particles leading to inhomogeneous depositions characterized by distinct edge-growth dynamics. Here, we study a more complex system with bacteria inside the evaporating drop. Bacteria interact like sticky particles forming inhomogeneous clusters, however, with edge-growth dynamics as that of non-interacting particles. We hypothesis that this contradicting behavior is due to the increased randomness introduced by bacteria-substrate interactions. Our findings have importance in understanding the patterns and their formation in growth systems of soft matter.
arXiv (Cornell University), Aug 8, 2021
Pathogens in droplets on fomites and aerosols go through extreme physiochemical conditions, such ... more Pathogens in droplets on fomites and aerosols go through extreme physiochemical conditions, such as confinement and osmotic stress, due to evaporation. Still, these droplets are the predominant transmission routes of many contagious diseases. The biggest challenge for studying the survival mechanisms of pathogens in extreme conditions is closely observing them, especially in aerosols, due to the small droplet sizes, movements, and fast evaporative dynamics. To mimic evaporating aerosols and microdroplets, we employ microfluidic emulsion droplets. The presence of oil forms a microdroplet cluster with spatially gradient evaporation rates, which helps studying the impact of evaporation and its rate. With Escherichia coli, we show that the viability is adversely affected by the evaporation and its rate. Our method can mimic bioaerosol evaporation with controlled number of cells inside. In general, the droplets can act as microreactors with varying kinetics induced by different evaporation rates.
Electrochemical IFN-γ immunosensor based on a nanocomposite of gold nanorods and reduced graphene oxide
Journal of Applied Electrochemistry
Physics of Fluids
We experimentally demonstrate capillary clustering of microdroplets when a microfluidic water-in-... more We experimentally demonstrate capillary clustering of microdroplets when a microfluidic water-in-oil emulsion drop was placed on a solid substrate. Distributed evaporation rates occur within the cluster manifested by a size gradient of microdroplets. We show that the process depends upon the oil and substrate properties. We study the evaporative dynamics of the cluster and the individual microdroplets. We identify three different evaporation regimes of the microdroplets and model the process. Unlike bare droplets, the evaporation rate varies with time when the droplet size approaches the oil height. Our findings contribute to a better understanding of droplet evaporation in emulsions and on oil-infused surfaces. Also, it may have implications for soft materials, microdroplet chemistry, and biosystems involving droplets.
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Papers by Dileep Mampallil