213 research outputs found
Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition
We report surface shear rheological measurements on dense insoluble
monolayers of micron sized colloidal spheres at the oil/water interface and of
the protein -lactoglobulin at the air/water surface. As expected, the
elastic modulus shows a changing character in the response, from a viscous
liquid towards an elastic solid as the concentration is increased, and a change
from elastic to viscous as the shear frequency is increased. Surprisingly,
above a critical packing fraction, the complex elastic modulus curves measured
at different concentrations can be superposed to form a master curve, by
rescaling the frequency and the magnitude of the modulus. This provides a
powerful tool for the extrapolation of the material response function outside
the experimentally accessible frequency range. The results are discussed in
relation to recent experiments on bulk systems, and indicate that these two
dimensional monolayers should be regarded as being close to a soft glass state.Comment: to appear in PR
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Multiphase flow of miscible liquids: jets and drops
Drops and jets of liquids that are miscible with the
surrounding bulk liquid are present in many processes from
cleaning surfaces with the aid of liquid soaps to the creation
of biocompatible implants for drug delivery. Although the
interactions of immiscible drops and jets show similarities
to miscible systems, the small, transient interfacial tension
associated with miscible systems create distinct outcomes
such as intricate droplet shapes and breakup resistant jets.
Experiments have been conducted to understand several
basic multiphase flow problems involving miscible liquids.
Using high-speed imaging of the morphological evolution of
the flows, we have been able to show that these processes are
controlled by interfacial tensions. Further multiphase flows
include investigating miscible jets, which allow the creation
of fibers from inelastic materials that are otherwise difficult
to process due to capillary breakup. This work shows
that stabilization from the diminishing interfacial tensions
of the miscible jets allow various elongated morphologies to
be formed.Keywords: miscible, transient interfacial tension, drop
Interfacial Stresses on Droplet Interface Bilayers Using Two Photon Fluorescence Lifetime Imaging Microscopy
Response of lipid bilayers to external mechanical stimuli is an active area
of research with implications for fundamental and synthetic cell biology.
However, there is a lack of tools for systematically imposing mechanical
strains and non-invasively mapping out interfacial (membrane) stress
distributions on lipid bilayers. In this article, we report a miniature
platform to manipulate model cell membranes in the form of droplet interface
bilayers (DIBs), and non-invasively measure spatio-temporally resolved
interfacial stresses using two photon fluorescence lifetime imaging of an
interfacially active molecular flipper (Flipper-TR). We established the
effectiveness of the developed framework by investigating interfacial stresses
accompanying three key processes associated with DIBs: thin film drainage
between lipid monolayer coated droplets, bilayer formation, and bilayer
separation. Interestingly, the measurements also revealed fundamental aspects
of DIBs including the existence of a radially decaying interfacial stress
distribution post bilayer formation, and the simultaneous build up and decay of
stress respectively at the bilayer corner and center during bilayer separation.
Finally, utilizing interfacial rheology measurements and MD simulations, we
also reveal that the tested molecular flipper is sensitive to membrane fluidity
that changes with interfacial stress - expanding the scientific understanding
of how molecular motors sense stress.Comment: 8 pages, 4 figure
Spreading of rinsing liquids across a horizontal rotating substrate
‘‘Rinsing” liquids and their dynamics are interesting both fundamentally in the interaction of several classic modes of spreading, and industrially in a variety of cleaning applications, such as in the manufacturing of silicon wafers. In this paper, we investigate the time-dependent spreading behavior of a rinsing liquid across a horizontal, rotating substrate; the rinsing liquid is applied to the center of the rotating substrate as an orthogonal impinging jet of constant volumetric flow. We present experimental findings on the azimuthally averaged outer radius of the spreading liquid, in which we observed four distinct growth behaviors in time. We use lubrication theory to explain these phenomena and to define boundaries within the explored parameter space where each was observed. In the absence of rotation, capillarity dominates and the spreading radius grows as t^(4/10). When centrifugal forces dominate the spreading process, several time dependencies of the spreading radius are possible, with lubrication theory predicting exponential growth as well as power laws of t^(3/4) and t^(3/2)
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