3 research outputs found
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
Combing of Genomic DNA from Droplets Containing Picograms of Material
Deposition of linear DNA molecules is a critical step in many single-molecule genomic approaches including DNA mapping, fiber-FISH, and several emerging sequencing technologies. In the ideal situation, the DNA that is deposited for these experiments is absolutely linear and uniformly stretched, thereby enabling accurate distance measurements. However, this is rarely the case, and furthermore, current approaches for the capture and linearization of DNA on a surface tend to require complex surface preparation and large amounts of starting material to achieve genomic-scale mapping. This makes them technically demanding and prevents their application in emerging fields of genomics, such as single-cell based analyses. Here we describe a simple and extremely efficient approach to the deposition and linearization of genomic DNA molecules. We employ droplets containing as little as tens of picograms of material and simply drag them, using a pipet tip, over a polymer-coated coverslip. In this report we highlight one particular polymer, Zeonex, which is remarkably efficient at capturing DNA. We characterize the method of DNA capture on the Zeonex surface and find that the use of droplets greatly facilitates the efficient deposition of DNA. This is the result of a circulating flow in the droplet that maintains a high DNA concentration at the interface of the surface/solution. Overall, our approach provides an accessible route to the study of genomic structural variation from samples containing no more than a handful of cells
Photophysical Investigation of Cyano-Substituted Terrylenediimide Derivatives
Two new terrylenediimide (TDI) chromophores
with cyano substituents
in the bay and core area (BCN-TDI and OCN-TDI, respectively) have
been characterized by a wide range of techniques, and their applicability
for stimulated emission depletion (STED) microscopy has been tested.
By cyano substitution an increase of the fluorescence quantum yield
and a decrease of the nonradiative rate constant is achieved and attributed
to a reduced charge-transfer character of the excited state due to
a lower electron density of the TDI core. For BCN-TDI, the substitution
in the bay area induces a strong torsional twist in the molecule which,
similar to phenoxy bay-perylenediimide (PDI), has a strong effect
on the fluorescence lifetime but appears to prevent the aggregation
that is observed for OCN-TDI. The single-molecule photobleaching stability
of BCN- and OCN-TDI is lower than that of a reference TDI without
cyano substitution (C7-TDI), although less so for OCN-TDI. The photophysical
properties of the excited singlet state are only slightly influenced
by the cyano groups. The observed intense stimulated emission, the
pump–dump–probe experiments, and STED single-molecule
imaging indicate that STED experiments with the cyano-substituted
TDIs are possible. However, because of aggregation and more efficient
photobleaching, the performance of BCN- and OCN-TDI is worse than
that of the reference compound without cyano groups (C7-TDI). Bay-substituted
TDIs are less suitable for STED microscopy