10 research outputs found
Slippery to Sticky Transition of Hydrophobic Nanochannels
Contrary
to common intuition that hydrophobic surfaces trivially
cause water to slip, we discover a slippery-to-sticky transition in
tunable hydrophobic nanochannels. We demonstrate this remarkable phenomenon
by bringing out hitherto unveiled interplay between ion inclusions
in the water and the interfacial lattice configuration over molecular
scales. The consequent alterations in frictional characteristics illustrate
that so-called hydrophobic nanochannels can be switchable to manifest
features that are otherwise typically associated with hydrophilicity,
causing water to stick. Our proposition may bear immense consequences
toward fluidically functionalizing a hydrophobic interface without
necessitating elaborate surface treatment techniques, bringing in
far-ranging implications in diverse applications ranging from nature
to energy
Combined Effects of Interfacial Permittivity Variations and Finite Ionic Sizes on Streaming Potentials in Nanochannels
In this work, we investigate the effects of local permittivity
variations, induced by a preferential orientation and exclusion of
water dipoles close to channel walls, and the effects of finite-sized
ions on the induced streaming potential in nanochannels. We make a
detailed analysis of the underlying physicochemical interactions by
considering combinations of cases where ions are considered to be
point sized/finite sized and permittivity variation effects to be
present/absent. By accounting for the dielectric friction (which in
turn is a function of the local permittivity) in addition to the classical
Stokes friction, we show that for high interfacial potentials and
narrow confinements, the induced streaming potential field for the
cases in which the polarization effects are considered for finite-sized
ions is remarkably higher than for the cases in which the polarization
effects are neglected. Thus, by coupling the nonlinear effects of
finite-sized ions and water dipole polarization along with the dielectric
friction, we open a new paradigm of streaming potential predictions
for narrow fluidic confinements, bearing far-ranging scientific and
technological consequences in nanoscale science and technology
Spreading of a Droplet over a Nonisothermal Substrate: Multiple Scaling Regimes
We envisage the spreading behavior
of a two-dimensional droplet
under a thin-film-based paradigm, under a perfect wetting condition,
while the droplet is placed over a nonisothermal substrate. Starting
from the onset of thin-film behavior (or equivalently beyond the inertia-dominated
initial stage), we identify the existence of mutually contrasting
multiple scaling regimes defining the spreading behavior at different
time scales. This is attributable to the time-stage-wise upsurge of
capillarity or thermocapillarity over the other. In particular, the
spreading behavior is characterized by the foot-width (<i>w</i>) evolution with time (<i>t</i>) in a power-law fashion <i>w</i> ∼ <i>t</i><sup>α</sup>, with α
being the spreading exponent, defining the rate of spreading. Following
pertinent thin-film and subsequent similarity analysis, we identify
different asymptotes of α over disparate temporal scales, leading
to the characterization of different scaling regimes over the entire
spreading event starting from the inception of thin-film behavior.
Reported literature data are found to correspond well to the present
interpretations and estimations
Combined Effects of Surface Roughness and Wetting Characteristics on the Moving Contact Line in Microchannel Flows
The present study investigates moving
contact lines in microfluidic
confinements with rough topographies modeled with random generating
functions. Using matched asymptotic expansion, the description of
the whole contact line is obtained and the dynamic contact angle is
extracted by extrapolating the bulk meniscus to the channel wall.
Significant variations are observed in the contact angle because of
the heterogeneities of the confining walls of the microfluidic channel.
The effects of the surface wetting condition also play a crucial role
in altering the description of the contact line bearing particular
nontrivial interactions with the topological features of the solid
boundaries. In an effort to assess the underlying consequences, two
different surface wetting conditions are studied; namely, complete
wetting substrate and partial wetting substrate. Our studies reveal
that the consequent wetting characteristics are strongly influenced
by action of intermolecular forces in presence of surface roughness.
The effect of slip, correlation length, and roughness parameters on
the dynamic contact angle have been also investigated
Contact Line Dynamics during the Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size
Exercising control
over the evaporation of colloidal suspensions
is pivotal to modulate the coating characteristics for specific uses,
wherein the interactions among the liquid, the particles, and the
substrate control the process. In the present study, the contact line
dynamics of a receding colloidal liquid film consisting of particles
of distinctly different sizes (nominal diameters 0.055 and 1 μm
and surface unmodified) during evaporation is analyzed. The role of
the liquid polarity is also investigated by replacing the polar liquid
(water) with a relatively nonpolar liquid (isopropyl alcohol) in the
colloidal suspension. The characteristics of the evaporating receding
meniscus, namely, the film thickness and the curvature are experimentally
evaluated using an image-analyzing interferometry technique. The experimental
results are assessed in conjunction with the augmented Young–Laplace
equation, highlighting the roles of the relevant components of the
disjoining pressure and the polarity of the liquid involved in the
colloidal suspension
Effect of Surface Wettability on Crack Dynamics and Morphology of Colloidal Films
The
effect of surface wettability on the dynamics of crack formation
and their characteristics are examined during the drying of aqueous
colloidal droplets (1 μL volume) containing nanoparticles (53
nm mean particle diameter, 1 w/w %). Thin colloidal films, formed
during drying, rupture as a result of the evaporation-induced capillary
pressure and exhibit microscopic cracks. The crack initiation and
propagation velocity as well as the number of cracks are experimentally
evaluated for substrates of varying wettability and correlated to
their wetting nature. Atomic force and scanning electron microscopy
are used to examine the region in the proximity of the crack including
the particle arrangements near the fracture zone. The altered substrate–particle
Derjaguin–Landau–Verwey–Overbeek (DLVO) interactions, as a consequence of the changed wettability,
are theoretically evaluated and found to be consistent with the experimental
observations. The resistance of the film to cracking is found to depend
significantly on the substrate surface energy and quantified by the
critical stress intensity factor, evaluated by analyzing images obtained
from confocal microscopy. The results indicate the possibility of
controlling crack dynamics and morphology by tuning the substrate
wettability
Contact Line Dynamics during the Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size
Exercising control
over the evaporation of colloidal suspensions
is pivotal to modulate the coating characteristics for specific uses,
wherein the interactions among the liquid, the particles, and the
substrate control the process. In the present study, the contact line
dynamics of a receding colloidal liquid film consisting of particles
of distinctly different sizes (nominal diameters 0.055 and 1 μm
and surface unmodified) during evaporation is analyzed. The role of
the liquid polarity is also investigated by replacing the polar liquid
(water) with a relatively nonpolar liquid (isopropyl alcohol) in the
colloidal suspension. The characteristics of the evaporating receding
meniscus, namely, the film thickness and the curvature are experimentally
evaluated using an image-analyzing interferometry technique. The experimental
results are assessed in conjunction with the augmented Young–Laplace
equation, highlighting the roles of the relevant components of the
disjoining pressure and the polarity of the liquid involved in the
colloidal suspension
Dynamics of Electrically Modulated Colloidal Droplet Transport
Electrically actuated transport dynamics
of colloidal droplets,
on a hydrophobic dielectric film covering an array of electrodes,
is studied here. Specifically, the effects of the size and electrical
properties (zeta-potential) of the colloidal particles on such transport
characteristics are investigated. For the colloidal droplets, the
application of an electrical voltage leads to additional attenuation
of the local dielectric-droplet interfacial tension. This is due to
the electrically triggered enhanced colloidal particle adsorption
at the dielectric-droplet interface, in the immediate vicinity of
the droplet three-phase contact line (TPCL). The extent of such interfacial
particle adsorption, and hence, the extent of the consequential reduction
in the interfacial tension, is dictated by the combined effects of
the three-phase contact line spreading, particle size, the interfacial
electrostatic interaction between the colloidal particles (if charged)
and the charged dielectric surface above the activated electrode,
and the interparticle electrostatic repulsion. The electrical driving
force of varying magnitude, stemming from this altered solid–liquid
interfacial tension gradient in the presence of the colloidal particles,
culminates in different droplet transport velocity and droplet transfer
frequency for different colloidal droplets. We substantiate the inferences
from our experimental results by a quasi-steady state force balance
model for colloidal droplet transport. We believe that the present
work will provide an accurate framework for determining the optimal
design and operational parameters for digital microfluidic chips handling
colloidal droplets, as encountered in a plethora of applications
Is the nexus between capital structure and firm performance asymmetric? An emerging market perspective
The nature of the relationship between leverage and firm performance has been a subject of investigation in extant literature. We re-examine the nature of the association by using a sample of 78 non-financial firms listed in the Nifty 100 index during the 2013-2023 period by applying the quantile regression technique and comparing the result with the linear regression approach (system GMM technique). Our empirical analysis demonstrates that leverage negatively impacts the performance of firms. Further, results show that the association is non-homogeneous among firms of different quantiles: leverage withers the performance of highly profitable firms (upper quantile) than low profitable firms (lower quantile). The identified concave relationship highlights the prominence of optimal capital structure and the role of finance managers in designing a sound financial policy that matches firm characteristics and borrowing requirements. The findings of our study draw insightful implications for managers and policymakers while contributing to the ongoing leverage and firm performance debate reported in previous studies. Since the pioneering work of Modigliani and Miller, the debate on the relationship between Capital Structure (CS) and Firm Performance (FP) has been a subject of discussion. Consequently, the CS and FP linkage has garnered the attention of several academic scholars. However, the majority of the empirical studies have demonstrated a linear link between CS and FP, whereas the studies on the nonlinear relationship are scant in the existing scholarly studies. Thus, to provide more insights, we used quantile regression techniques, and our results corroborate that the CS and FP relationship is non-homogeneous among Indian firms. To succinctly put, the magnitude of the negative impact of leverage is found to be more around highly profitable firms. Our regression result highlights the importance of maintaining the right capital mix and suggests that large firms should refrain from excessive borrowing. Further, we contend that policymakers must strengthen corporate governance mechanisms and restrict the earnings management activities of the management. Overall, our robust findings enhance the existing body of knowledge while drawing significant implications for management, policymakers, and other stakeholders.</p
Droplet Impact Dynamics on Biomimetic Replica of Yellow Rose Petals: Rebound to Micropinning Transition
Rose petals exhibit a phenomenal wetting property of
being sticky
and superhydrophobic simultaneously. A recent study has shown that
for short timescales, associated with drop impact phenomenon, lotus
leaf and rose petal replicas exhibit similar wettability, thereby
highlighting the difference between long and short time wettability.
Also, short time wetting on rose petals of different colors remains
completely unaddressed, as almost all existing study on wetting of
rose petals have been performed with the classical red rose (Rosa chinensis). In this paper, we compare the drop
impact studies on replicas of a yellow rose petal, with those on extensively
studied red rose petal replicas and the lotus leaf over a wide range
of Weber number (We), by varying the height of fall
(h) from 10 to 375 mm. Our results reveal that over
the replica of a yellow rose petal, the initial impact outcome varies
from complete rebound to micro pinning and eventually complete pinning
depending on the kinetic energy of the impacting drop, in contrast
to that on red rose petal replica on which the droplet always pinned.
Based on experimental finding, we present a comprehensive regime phase
map of the post impact behavior of the drop on different surfaces
as a function of impact height. We also present a simple scaling analysis
to understand the combined effect of pattern height and periodicity
on the critical h corresponding to wetting regime
transition. Additionally, variation of maximum spreading diameter
and spreading time with the h for the different surfaces
is also discussed. The results highlight that the initial impact dynamics
of a water drop over a topographically patterned substrate is a strong
function of the topographical parameters and can be very different
from the equilibrium wetting state