Siddhartha Das

Interactions between nanoparticles (NPs) and liquid drops have been widely utilized to manipulate both drops and NPs. However, achieving their separation without altering the intrinsic state of either component has remained a significant challenge in NP-laden drop systems. In our previous work (ACS Appl. Mater. Interfaces 2024, 16, 64253–64263), we addressed this challenge by extracting magnetic nanoparticles (MNPs) from the interior of an MNP-laden drop as a compact aggregate under the influence of an external magnetic field. In the present paper, we develop a technique that enables the seamless transfer of this extracted MNP aggregate from one liquid drop (the source drop) into the interior of a second liquid drop (the receiver drop). Remarkably, the aggregate, which can be tracked throughout the transfer process, is transferred nearly in its entirety, while neither drop undergoes any permanent deformation throughout the process. To the best of our knowledge, this is the first demonstration of a seamless drop-to-drop transfer of NPs, or, more broadly, the transfer of one drop’s functionality to another (here magnetic activity) without permanently altering the morphology of either drop. Beyond facilitating the NP transfer, the process transforms the receiver drop into a heterogeneous system that exhibits a variety of intriguing phenomena. For example, the transferred NPs can self-assemble into either skyscraper-like solid structures or magnetically responsive liquid layers. The nature of these assemblies depends on whether the MNP transfer occurs once or multiple times, and these assemblies display markedly different dynamics when subjected to magnetic fields of varying orientations, as well as oscillating and rotating magnetic fields. Furthermore, the transfer process converts the receiver drop into a highly stable microreactor for chemical reactions, enabling reaction products to form and persist while remaining minimally exposed to external disturbances that could otherwise compromise their stability.

In this paper, we extensively report the additive fabrication and characterization of carbon-nanotube and graphene oxide-based 3D printed rectangular patch antennas that can be integrated on curvilinear and deformable surfaces. Our fabricated antennas can be as thin as few microns and demonstrate reflection coefficient ( S ₁₁ ) values that confirm their super-high-frequency (in tens of GHz) and ultra-wideband characteristics. For example, the measured result of S ₁₁ is below −10 dB in the frequency range of 12 GHz–39 GHz (7 GHz–22 GHz or 5 GHz–25 GHz) for printed patch antennas on flat FR4 substrates (or on curvilinear, bendable PLA substrates). In addition, we quantify other characteristics of these printed antenna structures (e.g., radiation efficiency and gain) and also demonstrate good robustness and reliability (in terms of maintaining the performance) of the antenna structures after being subjected to long-term temperature cycling. Additionally, we measure the radiation efficiency of the printed CNT-GO patch antenna on flat FR4 substrates and also obtain simulation results for quantifying the S ₁₁ and the gain of the antenna.

We carry out all-atom molecular dynamics (MD) simulations for capturing the acoustothermal atomization of a refrigerant (Novec 7000) liquid nanofilm placed on a vibrating substrate. Like similar studies on water and refrigerant nanofilms, our findings also reveal three separate regimes of atomization with progressively increasing acoustic energy input: evaporation-driven atomization, nucleate-boiling-driven atomization, and film-boiling-driven atomization. Unlike previous studies, however, we cannot distinguish between the evaporation-driven and nucleate-boiling-driven atomization regimes from the time evolution of the number of atomized molecules; rather, this differentiation can only be made by studying the corresponding time evolution of the film temperature and the temporal variation of bubble volume. Similarly, unlike the case of water and other refrigerant liquids, the maximum of the film temperature during film-boiling-driven atomization can become comparable to or even exceed the maximum film temperature during nucleate-boiling-driven atomization. This stems from a rapid early-time acoustic energy deposition (for conditions that lead to film boiling), while the molecular nature of Novec 7000 and the associated molecule–molecule interactions enforce a much slower coalescence of vapor cavities into a continuous insulating vapor layer. Also, these maximum temperatures signify an unprecedentedly high degree of superheat for a refrigerant liquid. Furthermore, the rise in the film temperature occurs much faster for the case of film-boiling-driven atomization, ensuring that the corresponding heat flux (nearly as high as 6.9 GW/m2 and comparable to the heat flux observed in the atomization of water nanofilms) can be overwhelmingly greater (at early stages of atomization) than the heat flux associated with nucleate-boiling-driven atomization. Finally, we show that the residual number of Novec 7000 molecules (left after atomization) first scales inversely with the parameter dictating the dimensionless acoustic energy input, and then becomes constant with this parameter, indicating the presence of a stable, nonatomizable layer of Novec 7000 nanofilm.

Phyllosilicates, for example, bentonite clay, are among the most abundant natural minerals, yet their brittleness and poor conductivity have limited their use in energy storage applications, typically requiring harmful solvents and synthetic binders to form functional membranes. Here, we report a polymer‐free, aqueous‐based approach in which betaine, a naturally occurring zwitterion, is intercalated into bentonite clay galleries through ionic interactions between its cationic trimethylammonium group and the negatively charged clay surface. This intercalation bridges the clay galleries, creating ion‐conductive pathways within free‐standing, mechanically robust membranes, as confirmed by molecular dynamics simulations, rheo‐impedance, and electrochemical impedance spectroscopy. Incorporation of transition‐metal ions (Fe ^{3 +} , Mn ^{2 +} ) introduces redox‐active sites that contribute to pseudocapacitive charge storage. The resulting membranes function as both electrodes (113 mAh/g vs. NMC811) and separators in all‐clay supercapacitor devices, delivering an energy density of 158 mWh/cm ³ and a power density of 5688 mW/cm ³ , with ∼75% capacitance retention after 30000 cycles. The devices are biocompatible and can power LEDs up to 2.2 V after brief charging. We show, for the first time, that zwitterions and clay provide a simple, scalable, and sustainable route to biocompatible, flexible supercapacitors for low‐power applications requiring mechanical flexibility and material sustainability.

Phyllosilicates, for example, bentonite clay, are among the most abundant natural minerals, yet their brittleness and poor conductivity have limited their use in energy storage applications, typically requiring harmful solvents and synthetic binders to form functional membranes. Here, we report a polymer‐free, aqueous‐based approach in which betaine, a naturally occurring zwitterion, is intercalated into bentonite clay galleries through ionic interactions between its cationic trimethylammonium group and the negatively charged clay surface. This intercalation bridges the clay galleries, creating ion‐conductive pathways within free‐standing, mechanically robust membranes, as confirmed by molecular dynamics simulations, rheo‐impedance, and electrochemical impedance spectroscopy. Incorporation of transition‐metal ions (Fe ^{3 +} , Mn ^{2 +} ) introduces redox‐active sites that contribute to pseudocapacitive charge storage. The resulting membranes function as both electrodes (113 mAh/g vs. NMC811) and separators in all‐clay supercapacitor devices, delivering an energy density of 158 mWh/cm ³ and a power density of 5688 mW/cm ³ , with ∼75% capacitance retention after 30000 cycles. The devices are biocompatible and can power LEDs up to 2.2 V after brief charging. We show, for the first time, that zwitterions and clay provide a simple, scalable, and sustainable route to biocompatible, flexible supercapacitors for low‐power applications requiring mechanical flexibility and material sustainability.

In this paper, we discuss the coupled problem of steady-state activity-gradient-driven flow of active liquids inside a thin circular capillary, with the active liquid being characterized by the presence of active particles demonstrating vortex defects. In this coupled problem, we solve for the flow field that is driven by the activity gradient, while the concentration distribution of the active particles (which provides this activity gradient) is shown to be dependent on the flow field. We identify an equivalent Peclet number governing the problem: an increase in the capillary radius increases this Peclet number, enforcing a deviation of the active particle axial concentration distribution from a simple linear profile. This concentration, which is a radially averaged one, shows a steeper profile at locations near the exit of the capillary. On the other hand, the radial dependence of this concentration is insignificant. Hence, the axial velocity, which varies with the axial gradient of concentration, shows a significant increase as one moves to locations near the end of the capillary, particularly for larger capillary radii. On the other hand, the radial dependence of the velocity field is similar to that previously reported. Thus, via the consideration of the presence of an activity gradient that is not constant, we are able to propose a new type of activity-driven fluid microscale flow, where the flow field is two-dimensional. We anticipate that obtaining such closed-form analytical results for velocity and concentration profiles for active capillary flows will be a useful contribution in the field of active flows.

This paper probes hydration effects in counterion binding and variation in height (radius) of polyelectrolyte (PE) brushes (chains) as a function of ion's nature (chaotropic/kosmotropic) and PE functional group's nature (hydrophobic/hydrophilic).

Counterion-driven bridging in polyelectrolyte (PE) brush layer is a well-investigated localized event and refers to a given counterion strongly influencing (or bridging) closely located multiple monomers of a single or multiple PE chains. In this paper, we employ a combination of all-atom molecular dynamics (MD) simulations, graph theory, and unsupervised machine learning (ML) to propose a paradigm shift in our understanding of such localized counterion-driven bridging. We establish that such bridging demonstrates global characteristics via the formation of bridging networks. Specifically, we consider two systems: SO42--counterion screened PMETA+ [poly(2-(methacryloyloxy)ethyl) trimethylammonium] brushes and SCN--counterion screened PMETA+ brushes. Using all-atom MD simulation results, for both these systems, we first construct a globally connective bridging network structure using graph theory, where both monomers and counterions are represented as nodes, and identify five node-level features characterizing this bridging network, ensuring a five-dimensional (5D) feature space for each node. Subsequently, we employ a two-step machine learning (ML) approach, where first this 5D feature space is mapped into a 2D space, followed by unsupervised clustering of this 2D data revealing that the bridging network is more heterogeneous (homogeneous) for the SO42--counterion (SCN--counterion) screened PMETA+ brushes. Further analysis of these five node-level features reveal that for the case of the kosmotropic SO42--counterion (chaotropic SCN--counterion) screened PMETA+ brushes, the bridging interactions are primarily governed by the specific electrostatic interactions (non-specific solvent-mediated interactions) between the counterions and the apolar {N(CH3)3}+ group of the PMETA+ brushes, and is therefore, less (more) susceptible to thermal fluctuations. The final result is that the global bridging network for the SO42--counterion (SCN--counterion) is longer (shorter) and is characterized by long-ranged and extended (short-ranged but localized) structures.