Air microjet system for non-contact force application and the actuation of micro-structures

We demonstrate a non-contact technique to apply calibrated and localized forces in the micro-Newton to milli-Newton range using an air microjet. An electromagnetically actuated diaphragm controlled by a signal generator is used to generate the air microjet. With a nozzle diameter of 150 mu m, the microjet diameter was maintained to a maximum of 1 mm at a distance of 5 mm from the nozzle. The force generated by the microjet was measured using a commercial force sensor to determine the velocity profile of the jet. Axial flow velocities of up to 25 m s(-1) were obtained at distances as long as 6 mm. The microjet exerted a force up to 1 mu N on a poly dimethyl siloxane (PDMS) micropillar (50 mu m in diameter, 157 mu m in height) and 415 mu N on a PDMS membrane (3 mm in diameter, 28 mu m thick). We also demonstrate that from a distance of 6 mm our microjet can exert a peak pressure of 187 Pa with a total force of about 84 mu N on a flat surface with 8 V operating voltage. Out of the cleanroom fabrication and robust design make this system cost effective and durable.

Relationship between entropy and diffusion: A statistical mechanical derivation of Rosenfeld expression for a rugged energy landscape

Diffusion-a measure of dynamics, and entropy-a measure of disorder in the system are found to be intimately correlated in many systems, and the correlation is often strongly non-linear. We explore the origin of this complex dependence by studying diffusion of a point Brownian particle on a model potential energy surface characterized by ruggedness. If we assume that the ruggedness has a Gaussian distribution, then for this model, one can obtain the excess entropy exactly for any dimension. By using the expression for the mean first passage time, we present a statistical mechanical derivation of the well-known and well-tested scaling relation proposed by Rosenfeld between diffusion and excess entropy. In anticipation that Rosenfeld diffusion-entropy scaling (RDES) relation may continue to be valid in higher dimensions (where the mean first passage time approach is not available), we carry out an effective medium approximation (EMA) based analysis of the effective transition rate and hence of the effective diffusion coefficient. We show that the EMA expression can be used to derive the RDES scaling relation for any dimension higher than unity. However, RDES is shown to break down in the presence of spatial correlation among the energy landscape values. (C) 2015 AIP Publishing LLC.

Intrinsic Limit for Contact Resistance in Exfoliated Multilayered MoS2 FET

A new method for the separation of contact resistance (R-contact) into Schottky barrier resistance (R-SB) and interlayer resistance (R-IL) is proposed for multilayered MoS2 FETs. While R-SB varies exponentially with Schottky barrier height (Phi(bn)), R-IL essentially remains unchanged. An empirical model utilizing this dependence of R-contact versus Phi(bn) is proposed and fits to the experimental data. The results, on comparison with the existing reports of lowest R-contact, suggest that the extracted R-IL (1.53 k Omega.mu m) for an unaltered channel would determine the lower limit of intrinsic R-contact even for barrierless contacts for multilayered exfoliated MoS2 FETs.

Insight into the Evaporation Dynamics of a Pair of Sessile Droplets on a Hydrophobic Substrate

In this work, we have demonstrated three unique regimes in the evaporation lifecycle of a pair of sessile droplets placed in variable proximity on a hydrophobic substrate. For small separation distance, the droplets undergo asymmetric spatiotemporal,evaporation leading to contact angle hysteresis and suppressed vaporization. The reduced evaporation has been attributed quantitatively to the existence of a constrained vapor-rich dome between the two droplets. However, a dynamic decrease in the droplet radius due to solvent removal marks a return to symmetry in terms of evaporation and contact angle. We have described the variation in evaporation flux using a universal correction factor. We have also demonstrated the existence of a critical separation distance beyond which the droplets in the, droplet pair do not affect each other. The results are crucial to a plethora of applications ranging from surface patterning to lab-on-a-chip devices.

Experimental study on evaporation of pentane from a heated capillary slot

An experimental investigation of evaporation of a pentane meniscus from a heated capillary slot is presented. A novel aspect of this study is that both the wicking height and steady state evaporation mass flow rate are measured simultaneously. Based on a macroscopic force balance, the apparent contact angle of the evaporating meniscus is experimentally estimated from the wicking height and mass flow rate. This is compared with the results obtained using evaporating thin-film theory. The experimentally estimated contact angle is slightly larger than that obtained from the thin-film model but both show similar trends. Further, it is found that the reduction in the meniscus height is primarily due to an increase in the apparent contact angle. The liquid and vapor pressure drops in the capillary are insignificant relative to the capillary pressure. (C) 2015 Elsevier Ltd. All rights reserved.

Insight into the Evaporation Dynamics of a Pair of Sessile Droplets on a Hydrophobic Substrate

In this work, we have demonstrated three unique regimes in the evaporation lifecycle of a pair of sessile droplets placed in variable proximity on a hydrophobic substrate. For small separation distance, the droplets undergo asymmetric spatiotemporal,evaporation leading to contact angle hysteresis and suppressed vaporization. The reduced evaporation has been attributed quantitatively to the existence of a constrained vapor-rich dome between the two droplets. However, a dynamic decrease in the droplet radius due to solvent removal marks a return to symmetry in terms of evaporation and contact angle. We have described the variation in evaporation flux using a universal correction factor. We have also demonstrated the existence of a critical separation distance beyond which the droplets in the, droplet pair do not affect each other. The results are crucial to a plethora of applications ranging from surface patterning to lab-on-a-chip devices.

LOW RESISTANCE LIQUID MOTION FOR ENERGY HARVESTING

Low resistance motion of liquids on a well-defined path is beneficial for several MEMS based applications including energy harvesting and switching. By eliminating the contact line we demonstrate low resistance motion of a liquid bulge on pre-wetted strips. The bulge appears on wetted strips due to a morphological instability. The wetted strip confines the mercury bulge and defines its path of motion. Resistance to initiate motion of the bulge was studied experimentally and compared to other cases. An electret based energy harvesting device using bulge motion has been fabricated and tested.

Modulation of Buckling Dynamics in Nanoparticle Laden Droplets Using External Heating

Dynamics of contact free (levitated) drying of nanofluid droplets is ubiquitous in many application domains ranging from spray drying to pharmaceutics. Controlling the final morphology (macro to micro scales) of the dried out sample poses some serious challenges. Evaporation of solvent and agglomeration of particles leads to porous shell formation in acoustically levitated nanosilica droplets. The capillary pressure due to evaporation across the menisci at the nanoscale pores causes buckling of the shell which leads to ring and bowl shaped final structures. Acoustics plays a crucial role in flattening of droplets which is a prerequisite for initiation of buckling in the shell: Introduction of mixed nanocolloids (sodium dodecyl sulfate + nanosilica) reduces evaporation rate, disrupts formation of porous shell, and enhances mechanical strength of the shell, all of which restricts the process of buckling. Although buckling is completely arrested in such surfactant added droplets, controlled external heating using laser enhances evaporation through the pores in the shell due to thermally induced structural changes and rearrangement of SDS aggregates which reinitializes buckling in such droplets, Furthermore, inclusion of anilinium hydrochloride into the nanoparticle laden droplets produces ions which adsorb and modify the morphology of sodium dodecyl sulfate crystals and reinitializes buckling in the shell (irrespective of external heating conditions). The kinetics of buckling is determined by the combined effect of morphology of the colloidal particles, particle/aggregate diffusion rate within the droplet, and the rate of evaporation of water. The buckling dynamics leads to cavity formation which grows subsequently to yield final structures with drastically different morphological features. The cavity growth is controlled by evaporation through the nanoscate pores and exhibits a universal trend irrespective of heating rate and nanoparticle type.

Atomistic Simulation of Stacked Nucleosome Core Particles: Tail Bridging, the H4 Tail, and Effect of Hydrophobic Forces

We report the first atomistic simulation of two stacked nucleosome core particles (NCPs), with an aim to understand, in molecular detail, how they interact, the effect of salt concentration, and how different histone tails contribute to their interaction, with a special emphasis on the H4 tail, known to have the largest stabilizing effect on the NCP-NCP interaction. We do not observe specific K16-mediated interaction between the H4 tail and the H2A-H2B acidic patch, in contrast with the findings from crystallographic studies, but find that the stacking was stable even in the absence of this interaction. We perform simulations with the H4 tail (partially/completely) removed and find that the region between LYS-16 and LYS-20 of the H4 tail holds special importance in mediating the inter-NCP interaction. Performing similar tail-clipped simulations with the H3 tail removed, we compare the roles of the H3 and H4 tails in maintaining the stacking. We discuss the relevance of our simulation results to the bilayer and other liquid-crystalline phases exhibited by NCPs in vitro and, through an analysis of the histone-histone interface, identify the interactions that could possibly stabilize the inter-NCP interaction in these columnar mesophases. Through the mechanical disruption of the stacked nucleosome system using steered molecular dynamics, we quantify the strength of inter-NCP stacking in the presence and absence of salt. We disrupt the stacking at some specific sites of internucleosomal tail-DNA contact and perform a comparative quantification of the binding strengths of various tails in stabilizing the stacking. We also examine how hydrophobic interactions may contribute to the overall stability of the stacking and find a marked difference in the role of hydrophobic forces as compared with electrostatic forces in determining the stability of the stacked nucleosome system.