13 research outputs found
Simple Optical Imaging of Nanoscale Features in Free-Standing Films
Measuring
thicknesses in thin films with high spatial and temporal
resolution is of prime importance for understanding the structure
and dynamics in thin films and membranes. In the present work, we
introduce fluorescence-interferometry, a method that combines standard
reflected light thin film interferometry with simultaneous fluorescence
measurements. We apply this method to the thinning dynamics and phase
separation in free-standing inverse phospholipid bilayer films. The
measurements were carried out using a standard fluorescence microscope
using multichannel imaging and yielded subnanometer resolution, which
is applied to optically measure the discrete thickness variations
across phase-separated membranes
Contact Angles of Microellipsoids at Fluid Interfaces
The
wetting of anisotropic colloidal particles is of great importance
in several applications, including Pickering emulsions, filled foams,
and membrane transduction by particles. However, the combined effect
of shape and surface chemistry on the three-phase contact angle of
anisotropic micrometer and submicrometer colloids has been poorly
investigated to date, due to the lack of a suitable experimental technique
to resolve individual particles. In the present work, we investigate
the variation of the contact angle of prolate ellipsoidal colloids
at a liquid–liquid interface as a function of surface chemistry
and aspect ratio using freeze-fracture shadow-casting cryo-SEM. The
method, initially demonstrated for spherical colloids, is extended
here to the more general case of ellipsoids. The prolate ellipsoidal
particles are prepared from polystyrene and poly(methyl methacrylate)
spheres using a film stretching technique, in which cleaning steps
are needed to remove all film material from the particle surface.
The effects of the preparation protocol are reported, and wrinkling
of the three-phase contact line is observed when the particle surface
is insufficiently cleaned. For identically prepared ellipsoids, the
cosine of the measured contact angle is, in a first approximation,
a linearly decreasing function of the contact line length and thus
a decreasing function of the aspect ratio. Such a trend violates Young–Laplace’s
equation and can be rationalized by adding a correction term to the
ideal Young–Laplace contact angle that expresses the relative
importance of line effects relative to surface effects. From this
term the contribution of an <i>effective line tension</i> can be extracted. This contribution includes the effects that both
surface chemical and topographical heterogeneities have on the contact
line and which become increasingly more important for ellipsoids with
higher aspect ratios, where the contact line length to contact area
ratio increases
Thermocapillary Fingering in Surfactant-Laden Water Droplets
The drying of sessile droplets represents
an intriguing problem,
being a simple experiment to perform but displaying complexities that
are archetypical for many free surface and coating flows. Drying can
leave behind distinct deposits of initially well dispersed colloidal
matter. For example, in the case of the coffee ring effect, particles
are left in a well-defined macroscopic pattern with particles accumulating
at the edge, controlled by the internal flow in the droplet. Recent
studies indicate that the addition of surfactants strongly influences
this internal flow field, even reversing it and suppressing the coffee
ring effect. In this work, we explore the behavior of droplets at
high surfactant loadings and observe unexpected outward fingering
instabilities. The experiments start out with droplets with a pinned
contact line, and fast confocal microscopy is used to quantify a radially
outward surfactant-driven Marangoni flow, in line with earlier observations.
However, the Marangoni flows are observed to become unstable, and
local vortex cells are now observed in a direction along the contact
line. The occurrence of these vortices cannot be explained on the
basis of the effects of surfactants alone. Thermal imaging shows that
thermocapillary effects are superimposed on the surfactant-driven
flows. These local vortex cells acts as little pumps and push the
fluid outward in a fingering instability, rather than an expected
inward retraction of the drying droplet. This leads to a deposition
of colloids in a macroscopical flower-shaped pattern. A scaling analysis
is used to rationalize the observed wavelengths and velocities, and
practical implications are briefly discussed
Adsorption of Ellipsoidal Particles at Liquid–Liquid Interfaces
The
adsorption of particles at liquid–liquid interfaces
is of great scientific and technological importance. In particular,
for nonspherical particles, the capillary forces that drive adsorption
vary with position and orientation, and complex adsorption pathways
have been predicted by simulations. On the basis of the latter, it
has been suggested that the timescales of adsorption are determined
by a balance between capillary and viscous forces. However, several
recent experimental results point out the role of contact line pinning
in the adsorption of particles to interfaces and even suggest that
the adsorption dynamics and pathways are completely determined by
the latter, with the timescales of adsorption being determined solely
by particle characteristics. In the present work, the adsorption trajectories
of model ellipsoidal particles are investigated experimentally using
cryo-SEM and by monitoring the altitudinal orientation angle using
high-speed confocal microscopy. By varying the viscosity and the viscosity
jump across the interfaces, we specifically interrogate the role of
viscous forces
Sorption and Interfacial Rheology Study of Model Asphaltene Compounds
The
sorption and rheological properties of an acidic polyaromatic
compound (C5PeC11), which can be used to further our understanding
of the behavior of asphaltenes, are determined experimentally. The
results show that C5PeC11 exhibits the type of pH-dependent surface
activity and interfacial shear rheology observed in C<sub>6</sub>-asphaltenes
with a decrease in the interfacial tension concomitant with the elastic
modulus when the pH increases. Surface pressure–area (Π–<i>A</i>) isotherms show evidence of aggregation behavior and π–π
stacking at both the air/water and oil/water interfaces. Similarly,
interactions between adsorbed C5PeC11 compounds are evidenced through
desorption experiments at the oil/water interface. Contrary to indigenous
asphaltenes, adsorption is reversible, but desorption is slower than
for noninteracting species. The reversibility enables us to create
layers reproducibly, whereas the presence of interactions between
the compounds enables us to mimic the key aspects of interfacial activity
in asphaltenes. Shear and dilatational rheology show that C5PeC11
forms a predominantly elastic film both at the liquid/air and the
liquid/liquid interfaces. Furthermore, a soft glassy rheology model
(SGR) fits the data obtained at the liquid/liquid interface. However,
it is shown that the effective noise temperature determined from the
SGR model for C5PeC11 is higher than for indigenous asphaltenes measured
under similar conditions. Finally, from a colloidal and rheological
standpoint, the results highlight the importance of adequately addressing
the distinction between the material functions and true elasticity
extracted from a shear measurement and the apparent elasticity measured
in dilatational–pendant drop setups
Interfacial Rheology and Structure of Tiled Graphene Oxide Sheets
The hydrophilic nature of graphene oxide sheets can be
tailored
by varying the carbon to oxygen ratio. Depending on this ratio, the
particles can be deposited at either a water–air or a water–oil
interface. Upon compression of thus-created Langmuir monolayers, the
sheets cover the entire interface, assembling into a strong, compact
layer of tiled graphene oxide sheets. With further compression, the
particle layer forms wrinkles that are reversible upon expansion,
resembling the behavior of an elastic membrane. In the present work,
we investigate under which conditions the structure and properties
of the interfacial layer are such that free-standing films can be
obtained. The interfacial rheological properties of these films are
investigated using both compressional experiments and shear rheometry.
The role of surface rheology in potential applications of such tiled
films is explored. The rheological properties are shown to be responsible
for the efficiency of such layers in stabilizing water–oil
emulsions. Moreover, because of the mechanical integrity, large-area
monolayers can be deposited by, for example, Langmuir–Blodgett
techniques using aqueous subphases. These films can be turned into
transparent conductive films upon subsequent chemical reduction
Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure–Property Relations
In
food-grade emulsions, particles with an appropriate surface
modification can be used to replace surfactants and potentially enhance
the stability of emulsions. During the life cycle of products based
on such emulsions, they can be exposed to a broad range of pH conditions
and hence it is crucial to understand how pH changes affect stability
of emulsions stabilized by particles. Here, we report on a comprehensive
study of the stability, microstructure, and macroscopic behavior of
pH-controlled oil-in-water emulsions containing silica nanoparticles
modified with chitosan, a food-grade polycation. We found that the
modified colloidal particles used as stabilizers behave differently
depending on the pH, resulting in unique emulsion structures at multiple
length scales. Our findings are rationalized in terms of the different
emulsion stabilization mechanisms involved, which are determined by
the pH-dependent charges and interactions between the colloidal building
blocks of the system. At pH 4, the silica particles are partially
hydrophobized through chitosan modification, favoring their adsorption
at the oil–water interface and the formation of Pickering emulsions.
At pH 5.5, the particles become attractive and the emulsion is stabilized
by a network of agglomerated particles formed between the droplets.
Finally, chitosan aggregates form at pH 9 and these act as the emulsion
stabilizers under alkaline conditions. These insights have important
implications for the processing and use of particle-stabilized emulsions.
On one hand, changes in pH can lead to undesired macroscopic phase
separation or coalescence of oil droplets. On the other hand, the
pH effect on emulsion behavior can be harnessed in industrial processing,
either to tune their flow response by altering the pH between processing
stages or to produce pH-responsive emulsions that enhance the functionality
of the emulsified end products
Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure–Property Relations
In
food-grade emulsions, particles with an appropriate surface
modification can be used to replace surfactants and potentially enhance
the stability of emulsions. During the life cycle of products based
on such emulsions, they can be exposed to a broad range of pH conditions
and hence it is crucial to understand how pH changes affect stability
of emulsions stabilized by particles. Here, we report on a comprehensive
study of the stability, microstructure, and macroscopic behavior of
pH-controlled oil-in-water emulsions containing silica nanoparticles
modified with chitosan, a food-grade polycation. We found that the
modified colloidal particles used as stabilizers behave differently
depending on the pH, resulting in unique emulsion structures at multiple
length scales. Our findings are rationalized in terms of the different
emulsion stabilization mechanisms involved, which are determined by
the pH-dependent charges and interactions between the colloidal building
blocks of the system. At pH 4, the silica particles are partially
hydrophobized through chitosan modification, favoring their adsorption
at the oil–water interface and the formation of Pickering emulsions.
At pH 5.5, the particles become attractive and the emulsion is stabilized
by a network of agglomerated particles formed between the droplets.
Finally, chitosan aggregates form at pH 9 and these act as the emulsion
stabilizers under alkaline conditions. These insights have important
implications for the processing and use of particle-stabilized emulsions.
On one hand, changes in pH can lead to undesired macroscopic phase
separation or coalescence of oil droplets. On the other hand, the
pH effect on emulsion behavior can be harnessed in industrial processing,
either to tune their flow response by altering the pH between processing
stages or to produce pH-responsive emulsions that enhance the functionality
of the emulsified end products
Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure–Property Relations
In
food-grade emulsions, particles with an appropriate surface
modification can be used to replace surfactants and potentially enhance
the stability of emulsions. During the life cycle of products based
on such emulsions, they can be exposed to a broad range of pH conditions
and hence it is crucial to understand how pH changes affect stability
of emulsions stabilized by particles. Here, we report on a comprehensive
study of the stability, microstructure, and macroscopic behavior of
pH-controlled oil-in-water emulsions containing silica nanoparticles
modified with chitosan, a food-grade polycation. We found that the
modified colloidal particles used as stabilizers behave differently
depending on the pH, resulting in unique emulsion structures at multiple
length scales. Our findings are rationalized in terms of the different
emulsion stabilization mechanisms involved, which are determined by
the pH-dependent charges and interactions between the colloidal building
blocks of the system. At pH 4, the silica particles are partially
hydrophobized through chitosan modification, favoring their adsorption
at the oil–water interface and the formation of Pickering emulsions.
At pH 5.5, the particles become attractive and the emulsion is stabilized
by a network of agglomerated particles formed between the droplets.
Finally, chitosan aggregates form at pH 9 and these act as the emulsion
stabilizers under alkaline conditions. These insights have important
implications for the processing and use of particle-stabilized emulsions.
On one hand, changes in pH can lead to undesired macroscopic phase
separation or coalescence of oil droplets. On the other hand, the
pH effect on emulsion behavior can be harnessed in industrial processing,
either to tune their flow response by altering the pH between processing
stages or to produce pH-responsive emulsions that enhance the functionality
of the emulsified end products
Shear-Stress-Induced Conformational Changes of von Willebrand Factor in a Water–Glycerol Mixture Observed with Single Molecule Microscopy
The
von Willebrand factor (VWF) is a human plasma protein that plays a
key role in the initiation of the formation of thrombi under high
shear stress in both normal and pathological situations. It is believed
that VWF undergoes a conformational transition from a compacted, globular
to an extended form at high shear stress. In this paper, we develop
and employ an approach to visualize the large-scale conformation of
VWF in a (pressure-driven) Poiseuille flow of water–glycerol
buffers with wide-field single molecule fluorescence microscopy as
a function of shear stress. Comparison of the imaging results for
VWF with the results of a control with λ-phage double-stranded
DNA shows that the detection of individual VWF multimers in flow is
feasible. A small fraction of VWF multimers are observed as visibly
extended along one axis up to lengths of 2.0 μm at high applied
shear stresses. The size of this fraction of molecules seems to exhibit
an apparent dependency on shear stress. We further demonstrate that
the obtained results are independent of the charge of the fluorophore
used to label VWF. The obtained results support the hypothesis of
the conformational extension of VWF in shear flow