32 research outputs found
Reversible Phase Transitions in the Phospholipid Monolayer
The
polymorphism of phospholipid monolayers
has been extensively studied because of its importance in surface
thermodynamics, soft matter physics, and biomembranes. To date, the
phase behavior of phospholipid monolayers has been nearly exclusively
studied with the classical Langmuir-type film balance. However, because
of experimental artifacts caused by film leakage, the Langmuir balance
fails to study the reversibility of two-dimensional surface phase
transitions. We have developed a novel experimental methodology called
the constrained drop surfactometry capable of providing a leakage-proof
environment, thus allowing reversibility studies of two-dimensional
surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC)
as a model system, we have studied the reversibility of isothermal
and isobaric phase transitions in the monolayer. It is found that
not only the compression and expansion isotherms but also the heating
and cooling isobars, completely superimpose with each other without
hysteresis. Microscopic lateral structures of the DPPC monolayer also
show reversibility not only during the isothermal compression and
expansion processes but also during the isobaric heating and cooling
processes. It is concluded that the two-dimensional surface phase
transitions in phospholipid monolayers are reversible, which is consistent
with the reversibility of phase transitions in bulk pure substances.
Our results provide a better understanding of surface thermodynamics,
phase change materials, and biophysical studies of membranes and pulmonary
surfactants
Droplet Oscillation as an Arbitrary Waveform Generator
Oscillating droplets and bubbles have
been developed into a novel experimental platform for a wide range
of analytical and biological applications, such as digital microfluidics,
thin film, biophysical simulation, and interfacial rheology. A central
effort of developing any droplet-based experimental platform is to
increase the effectiveness and accuracy of droplet oscillations. Here,
we developed a novel system of droplet-based arbitrary waveform generator
(AWG) for feedback-controlling single-droplet oscillations. This AWG
was developed through closed-loop axisymmetric drop shape analysis
and based on the hardware of constrained drop surfactometry. We have
demonstrated the capacity of this AWG in oscillating the volume and
surface area of a millimeter-sized droplet to follow four representative
waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating
the surface area of a droplet across the frequency spectrum makes
the AWG an ideal tool for studying interfacial rheology. The AWG was
used to determine the surface dilational modulus of a commonly studied
nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based
AWG developed in this study is expected to achieve accuracy, versatility,
and applicability in a wide range of research areas, such as thin
film and interfacial rheology
Droplet Oscillation as an Arbitrary Waveform Generator
Oscillating droplets and bubbles have
been developed into a novel experimental platform for a wide range
of analytical and biological applications, such as digital microfluidics,
thin film, biophysical simulation, and interfacial rheology. A central
effort of developing any droplet-based experimental platform is to
increase the effectiveness and accuracy of droplet oscillations. Here,
we developed a novel system of droplet-based arbitrary waveform generator
(AWG) for feedback-controlling single-droplet oscillations. This AWG
was developed through closed-loop axisymmetric drop shape analysis
and based on the hardware of constrained drop surfactometry. We have
demonstrated the capacity of this AWG in oscillating the volume and
surface area of a millimeter-sized droplet to follow four representative
waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating
the surface area of a droplet across the frequency spectrum makes
the AWG an ideal tool for studying interfacial rheology. The AWG was
used to determine the surface dilational modulus of a commonly studied
nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based
AWG developed in this study is expected to achieve accuracy, versatility,
and applicability in a wide range of research areas, such as thin
film and interfacial rheology
Accuracy of Axisymmetric Drop Shape Analysis in Determining Surface and Interfacial Tensions
Axisymmetric
drop shape analysis (ADSA) has been used in a broad range of applications
for determining surface tensions of air–liquid surfaces and
interfacial tensions of liquid–liquid interfaces. However,
it is well-known that the accuracy of ADSA deteriorates upon the reduction
of drop volume. Here, we systematically compared different criteria
and parameters in evaluating the accuracy of ADSA upon reducing drop
volume. By scrutinizing the dependence of ADSA accuracy on various
parameters, including the capillary constant (<i>c</i>),
the Bond number (<i>Bo</i>), the Worthington number (<i>Wo</i>), and the shape parameter (<i>P</i><sub>s</sub>), we concluded that the classical Bond number failed to predict
the accuracy of drop shape analysis at very small drop volumes. Thus,
we proposed a replacement of the classical Bond number, called the
Neumann number <i>Ne</i> ≡ (Δρ<i>gR</i><sub>0</sub><i>H</i>)/γ. The design rationale
of this new dimensionless number lies in the use of the geometric
mean of the radius of curvature at the drop apex (<i>R</i><sub>0</sub>) and the drop height (<i>H</i>) as the new
characteristic length (<i>L</i>) to represent the drop size,
that is, <i>L</i> = (<i>R</i><sub>0</sub><i>H</i>)<sup>1/2</sup>. It is found that the Neumann number is
capable of evaluating the accuracy of ADSA. Moreover, we have demonstrated
the usefulness of the local Neumann number, <i>Ne</i><sub><i>z</i></sub> ≡ (Δρ<i>gR</i><sub>0</sub>/γ)<i>z</i>, in evaluating the contribution
of the local drop profile to the accuracy of ADSA
Droplet Oscillation as an Arbitrary Waveform Generator
Oscillating droplets and bubbles have
been developed into a novel experimental platform for a wide range
of analytical and biological applications, such as digital microfluidics,
thin film, biophysical simulation, and interfacial rheology. A central
effort of developing any droplet-based experimental platform is to
increase the effectiveness and accuracy of droplet oscillations. Here,
we developed a novel system of droplet-based arbitrary waveform generator
(AWG) for feedback-controlling single-droplet oscillations. This AWG
was developed through closed-loop axisymmetric drop shape analysis
and based on the hardware of constrained drop surfactometry. We have
demonstrated the capacity of this AWG in oscillating the volume and
surface area of a millimeter-sized droplet to follow four representative
waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating
the surface area of a droplet across the frequency spectrum makes
the AWG an ideal tool for studying interfacial rheology. The AWG was
used to determine the surface dilational modulus of a commonly studied
nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based
AWG developed in this study is expected to achieve accuracy, versatility,
and applicability in a wide range of research areas, such as thin
film and interfacial rheology
Droplet Oscillation as an Arbitrary Waveform Generator
Oscillating droplets and bubbles have
been developed into a novel experimental platform for a wide range
of analytical and biological applications, such as digital microfluidics,
thin film, biophysical simulation, and interfacial rheology. A central
effort of developing any droplet-based experimental platform is to
increase the effectiveness and accuracy of droplet oscillations. Here,
we developed a novel system of droplet-based arbitrary waveform generator
(AWG) for feedback-controlling single-droplet oscillations. This AWG
was developed through closed-loop axisymmetric drop shape analysis
and based on the hardware of constrained drop surfactometry. We have
demonstrated the capacity of this AWG in oscillating the volume and
surface area of a millimeter-sized droplet to follow four representative
waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating
the surface area of a droplet across the frequency spectrum makes
the AWG an ideal tool for studying interfacial rheology. The AWG was
used to determine the surface dilational modulus of a commonly studied
nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based
AWG developed in this study is expected to achieve accuracy, versatility,
and applicability in a wide range of research areas, such as thin
film and interfacial rheology
Automated Droplet Manipulation Using Closed-Loop Axisymmetric Drop Shape Analysis
Droplet manipulation plays an important
role in a wide range of
scientific and industrial applications, such as synthesis of thin-film
materials, control of interfacial reactions, and operation of digital
microfluidics. Compared to micron-sized droplets, which are commonly
considered as spherical beads, millimeter-sized droplets are generally
deformable by gravity, thus introducing nonlinearity into control
of droplet properties. Such a nonlinear drop shape effect is especially
crucial for droplet manipulation, even for small droplets, at the
presence of surfactants. In this paper, we have developed a novel
closed-loop axisymmetric drop shape analysis (ADSA), integrated into
a constrained drop surfactometer (CDS), for manipulating millimeter-sized
droplets. The closed-loop ADSA generalizes applications of the traditional
drop shape analysis from a surface tension measurement methodology
to a sophisticated tool for manipulating droplets in real time. We
have demonstrated the feasibility and advantages of the closed-loop
ADSA in three applications, including control of drop volume by automatically
compensating natural evaporation, precise control of surface area
variations for high-fidelity biophysical simulations of natural pulmonary
surfactant, and steady control of surface pressure for <i>in
situ</i> Langmuir–Blodgett transfer from droplets. All
these applications have demonstrated the accuracy, versatility, applicability,
and automation of this new ADSA-based droplet manipulation technique.
Combining with CDS, the closed-loop ADSA holds great promise for advancing
droplet manipulation in a variety of material and surface science
applications, such as thin-film fabrication, self-assembly, and biophysical
study of pulmonary surfactant
Biophysical Influence of Airborne Carbon Nanomaterials on Natural Pulmonary Surfactant
Inhalation of nanoparticles (NP), including lightweight airborne carbonaceous nanomaterials (CNM), poses a direct and systemic health threat to those who handle them. Inhaled NP penetrate deep pulmonary structures in which they first interact with the pulmonary surfactant (PS) lining at the alveolar air–water interface. In spite of many research efforts, there is a gap of knowledge between <i>in vitro</i> biophysical study and <i>in vivo</i> inhalation toxicology since all existing biophysical models handle NP–PS interactions in the liquid phase. This technical limitation, inherent in current <i>in vitro</i> methodologies, makes it impossible to simulate how airborne NP deposit at the PS film and interact with it. Existing <i>in vitro</i> NP–PS studies using liquid-suspended particles have been shown to artificially inflate the no-observed adverse effect level of NP exposure when compared to <i>in vivo</i> inhalation studies and international occupational exposure limits (OELs). Here, we developed an <i>in vitro</i> methodology called the constrained drop surfactometer (CDS) to quantitatively study PS inhibition by airborne CNM. We show that airborne multiwalled carbon nanotubes and graphene nanoplatelets induce a concentration-dependent PS inhibition under physiologically relevant conditions. The CNM aerosol concentrations controlled in the CDS are comparable to those defined in international OELs. Development of the CDS has the potential to advance our understanding of how submicron airborne nanomaterials affect the PS lining of the lung
Melting of the Dipalmitoylphosphatidylcholine Monolayer
Langmuir
monolayer self-assembled at the air–water interface
represents an excellent model for studying phase transition and lipid
polymorphism in two dimensions. Compared with numerous studies of
phospholipid phase transitions induced by isothermal compression,
there are very scarce reports on two-dimensional phase transitions
induced by isobaric heating. This is mainly due to technical difficulties
of continuously regulating temperature variations while maintaining
a constant surface pressure in a classical Langmuir-type film balance.
Here, with technological advances in constrained drop surfactometry
and closed-loop axisymmetric drop shape analysis, we studied the isobaric
heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer.
It is found that temperature and surface pressure are two equally
important intensive properties that jointly determine the phase behavior
of the phospholipid monolayer. We have determined a critical point
of the DPPC monolayer at a temperature of 44 °C and a surface
pressure of 57 mN/m. Beyond this critical point, no phase transition
can exist in the DPPC monolayer, either by isothermal compression
or by isobaric heating. The melting process of the DPPC monolayer
studied here provides novel insights into the understanding of a wide
range of physicochemical and biophysical phenomena, such as surface
thermodynamics, critical phenomena, and biophysical study of pulmonary
surfactants
Unveiling the Molecular Structure of Pulmonary Surfactant Corona on Nanoparticles
The growing risk of human exposure
to airborne nanoparticles (NPs) causes a general concern on the biosafety
of nanotechnology. Inhaled NPs can deposit in the deep lung at which
they interact with the pulmonary surfactant (PS). Despite the increasing
study of nano-bio interactions, detailed molecular mechanisms by which
inhaled NPs interact with the natural PS system remain unclear. Using
coarse-grained molecular dynamics simulation, we studied the interaction
between NPs and the PS system in the alveolar fluid. It was found
that regardless of different physicochemical properties, upon contacting
the PS, both silver and polystyrene NPs are immediately coated with
a biomolecular corona that consists of both lipids and proteins. Structure
and molecular conformation of the PS corona depend on the hydrophobicity
of the pristine NPs. Quantitative analysis revealed that lipid composition
of the corona formed on different NPs is relatively conserved and
is similar to that of the bulk phase PS. However, relative abundance
of the surfactant-associated proteins, SP-A, SP-B, and SP-C, is notably
affected by the hydrophobicity of the NP. The PS corona provides the
NPs with a physicochemical barrier against the environment, equalizes
the hydrophobicity of the pristine NPs, and may enhance biorecognition
of the NPs. These modifications in physicochemical properties may
play a crucial role in affecting the biological identity of the NPs
and hence alter their subsequent interactions with cells and other
biological entities. Our results suggest that all studies of inhalation
nanotoxicology or NP-based pulmonary drug delivery should consider
the influence of the PS corona