Apratim Sanyal

Precision control of drying using rhythmic dancing of sessile nanoparticle laden droplets

This work analyses the unique spatio-temporal alteration of the deposition pattern of evaporating nanoparticle laden droplets resting on a hydrophobic surface through targeted low frequency substrate vibrations. External excitation near the lowest resonant mode (n = 2) of the droplet initially de-pins and then subsequently re-pins the droplet edge creating pseudo-hydrophilicity (low contact angle). Vibration subsequently induces droplet shape oscillations (cyclic elongation and flattening) resulting in strong flow recirculation. This strong radially outward liquid flow augments nanoparticle transport, vaporization, and agglomeration near the pinned edge resulting in much reduced drying time under certain characteristic frequency of oscillations. The resultant deposit exhibits a much flatter structure with sharp, defined peripheral wedge topology as compared to natural drying. Such controlled manipulation of transport enables tailoring of structural and topological morphology of the deposits and offers pos...

Genetic comparison of large fragment of the 5'untranslated region among foot-and-mouth disease viruses with special reference to serotype Asia1

Summary.Foot-and-mouth disease (FMD), the most economically important disease of cloven-hoofed animals, is endemic in India. Sequence analysis revealed that phylogenetic grouping of type Asia1 field isolates on the basis of the large fragment of the 5′untranslated region (5′LF-UTR) was quite similar to that based on the sequences of the capsid-coding (VP1) region of the same viruses. The existence of two distinct lineages of type Asia1 suggested by the study on the VP1 region was further supported by the detection of a difference in length and predicted secondary structure of the 5′LF-UTR between the two lineages. Sequence variability between the isolates of the two lineages was also observed within the different domains of the internal ribosome entry site (IRES) around conserved motifs like the GNRA,- RAAA,- and the polypyrimidine tract. Certain group and lineage-specific signature nucleotides pertaining to FMDV type Asia1 in the 5′LF-UTR have been identified. The present study shows that the 5′LF-UTR of FMDV serotype Asia1 field isolates are variable in relation to the length and probable secondary structure of the IRES.

How Natural Evaporation Temporally Self-Tunes an Oscillating Sessile Droplet To Resonate at Different Modes.

We report the dynamics and underlying physics of evaporation driven transitions and autotuning of oscillation modes in sessile droplets subject to substrate perturbations. We have shown that evaporation controls temporal transition of the oscillation mode with a spatially downward shift of nodes (surface locations with zero displacement) toward the three-phase contact line. We have explained the physical mechanism using two parameters: the first quantifies evaporation driven tuning for resonance detection, and the second parameter characterizes mode lifetime which is found to be governed by evaporation dynamics. It is desirable to achieve autotuning of the oscillation modes in sessile droplets that essentially self-evolves in a spatiotemporal manner with continued evaporation. The insights suggest control of mode resonances is possible, which in turn will allow precision manipulations at droplet scale crucial for many applications such as surface patterning and others.

Experimental Study of Shape Transition in an Acoustically Levitated and Externally Heated Droplet

Experimental analyses of surface oscillations are reported in acoustically levitated, radiatively heated bicomponent droplets with one volatile and other being nonvolatile. Two instability pathways are observed: one being acoustically driven observed in low-vapor pressure fluid droplets and other being boiling driven observed in high-vapor pressure fluid droplets. The first pathway shows extreme droplets deformation and subsequent breakup by acoustic pressure and externally supplied heat. Also transition of instabilities from acoustically activated shape distortion regime to thermally induced boiling regime is observed with increasing concentration of volatile component in bicomponent droplets. Precursor phases of instabilities are investigated using Legendres polynomial.

Precision stacking of nanoparticle laden sessile droplets to control solute deposit morphology

Stacking pure solvent droplets on a solid substrate is apparently impossible in the absence of an external force as the second droplet will invariably spill over the first leading to a large wetted area. However, the unique feature that emerges during the drying of a nanoparticle laden droplet is the progressively enlarging thin solid film along the evaporating sessile droplet liquid periphery. This solid interface: the edge of which we shall refer to as the agglomeration front comprises of a thin layer of nanoparticle assembly and can support a carefully dispensed second droplet thereby allowing droplet stacking. It will be shown that the growth of this agglomeration front can also be effectively controlled by the dispensing time difference and the nanoparticle concentration in the two droplets. So far, we are commonly aware of material stacking in solid phase. This letter demonstrates stacking in the liquid phase and control over the thin solid interface growth.

Engineering Interfacial Processes at Mini-Micro-Nano Scales Using Sessile Droplet Architecture

Evaporating sessile functional droplets act as the fundamental building block that controls the cumulative outcome of many industrial and biological applications such as surface patterning, 3D printing, photonic crystals, and DNA sequencing, to name a few. Additionally, a drying single sessile droplet forms a high-throughput processing technique using low material volume which is especially suitable for medical diagnosis. A sessile droplet also provides an elementary platform to study and analyze fundamental interfacial processes at various length scales ranging from macroscopically observable wetting and evaporation to microfluidic transport to interparticle forces operating at a nanometric length scale. As an example, to ascertain the quality of 3D printing we must understand the fundamental interfacial processes at the droplet scale. In this article, we review the coupled physics of evaporation flow-contact-line-driven particle transport in sessile colloidal droplets and provide methodologies to control the same. Through natural alterations in droplet vaporization, one can change the evaporative pattern and contact line dynamics leading to internal flow which will modulate the final particle assembly in a nontrivial fashion. We further show that control over particle transport can also be exerted by external stimuli which can be thermal, mechanical oscillations, vapor confinement (walled or a fellow droplet), or chemical (surfactant-induced) in nature. For example, significant augmentation of an otherwise evaporation-driven particle transport in sessile droplets can be brought about simply through controlled interfacial oscillations. The ability to control the final morphologies by manipulating the governing interfacial mechanisms in the precursor stages of droplet drying makes it perfectly suitable for fabrication-, mixing-, and diagnostic-based applications.