76,746 research outputs found
Contact Angle Measurement of Dental Restorative Materials by Drop Profile Image Analysis
The capability of initial microbial adhesion to dental restorative composites surface is influenced by the surface wettability of the materials. The common method to evaluate surface wettability of materials is contact angle measurement. The existing conventional method to measure contact angle is by means of a contact angle (CA)-Goniometer device, which is less practically applicable in clinical circumstances. Therefore, a more practical and applicable method is needed to measure contact angle in clinical circumstances. This research was performed to compare between contact angles measured by means of a CA-Goniometer device and a new practical method of drop profile image analysis. In addition, since there were two different formulas that can be used to calculate contact angle value from a drop profile image, then we also need to evaluate which formula is more reliable to be used. Tests were carried out using three composite discs (Clearfill-Kuraray Medical, Inc.) sample and deionised water for different measurement procedures. One drop of 3µl liquid was dropped onto the surface of the composite discs, and the drop profile image was captured by means of a customized home-made device connected to a digital camera. Two different formulas were used to calculate the contact angle value from the drop profile image, namely the “linier gradient equation” and the “tangential line”. The contact angle values obtained from the two different formulas were compared with the value obtained from the conventional method descriptively. Tests were carried out using three composite discs (Clearfill-Kuraray Medical, Inc.) sample and deionised water for different measurement procedures. One drop of 3µl liquid was dropped onto the surface of the composite discs, and the drop profile image was captured by means of a customized home-made device connected to a digital camera. Two different formulas were used to calculate the contact angle value from the drop profile image, namely the “linier gradient equation” and the “tangential line”. The contact angle values obtained from the two different formulas were compared with the value obtained from the conventional method descriptively. The differences in percentage between the contact angle value calculated by the “linier gradient equation” and “tangential line” formulas, and those calculated by means of the CA-Goniometer are 20,56% and 3,51%, respectively. It is obviously demonstrated that the value obtained by the “tangential line” formula has a smaller difference compared to those obtained by the “linier equation gradient” formula. Among the two different formulas, it is confirmed that the contact angle value calculated with the “tangential line” formula has closer similarity with the value obtained from the CA-Goniometer. This result confirms that the new practical method of drop profile image analysis is promising for measuring contact angle values in clinical circumstances. Related to the drop profile image analysis, the “tangential line” formula is more accurate compared to the “linier gradient equation” formula
Contact Angle Hysteresis on Superhydrophobic Stripes
We study experimentally and discuss quantitatively the contact angle
hysteresis on striped superhydrophobic surfaces as a function of a solid
fraction, . It is shown that the receding regime is determined by a
longitudinal sliding motion the deformed contact line. Despite an anisotropy of
the texture the receding contact angle remains isotropic, i.e. is practically
the same in the longitudinal and transverse directions. The cosine of the
receding angle grows nonlinearly with , in contrast to predictions of
the Cassie equation. To interpret this we develop a simple theoretical model,
which shows that the value of the receding angle depends both on weak defects
at smooth solid areas and on the elastic energy of strong defects at the
borders of stripes, which scales as . The advancing
contact angle was found to be anisotropic, except as in a dilute regime, and
its value is determined by the rolling motion of the drop. The cosine of the
longitudinal advancing angle depends linearly on , but a satisfactory
fit to the data can only be provided if we generalize the Cassie equation to
account for weak defects. The cosine of the transverse advancing angle is much
smaller and is maximized at . An explanation of its value can
be obtained if we invoke an additional energy due to strong defects in this
direction, which is shown to be proportional to . Finally, the
contact angle hysteresis is found to be quite large and generally anisotropic,
but it becomes isotropic when .Comment: 17 pages, 8 figure
Pinning, de-pinning and re-pinning of a slowly varying rivulet
The solutions for the unidirectional flow of a thin rivulet with prescribed volume flux down an inclined planar substrate are used to describe the locally unidirectional flow of a rivulet with constant width (i.e. pinned contact lines) but slowly varying contact angle as well as the possible pinning and subsequent de-pinning of a rivulet with constant contact angle and the possible depinning and subsequent re-pinning of a rivulet with constant width as they flow in the azimuthal direction from the top to the bottom of a large horizontal cylinder. Despite being the same locally, the global behaviour of a rivulet with constant width can be very different from that of a rivulet with constant contact angle. In particular, while a rivulet with constant non-zero contact angle can always run from the top to the bottom of the cylinder, the behaviour of a rivulet with constant width depends on the value of the width. Specifically, while a narrow rivulet can run all the way from the top to the bottom of the cylinder, a wide rivulet can run from the top of the cylinder only to a critical azimuthal angle. The scenario in which the hitherto pinned contact lines of the rivulet de-pin at the critical azimuthal angle and the rivulet runs from the critical azimuthal angle to the bottom of the cylinder with zero contact angle but slowly varying width is discussed. The pinning and de-pinning of a rivulet with constant contact angle, and the corresponding situation involving the de-pinning and re-pinning of a rivulet with constant width at a non-zero contact angle which generalises the de-pinning at zero contact angle discussed earlier, are described. In the latter situation, the mass of fluid on the cylinder is found to be a monotonically increasing function of the constant width
Controlling wetting with electrolytic solutions: phase-field simulations of a droplet-conductor system
The wetting properties of immiscible two-phase systems are crucial in a wide
range of applications, from lab-on-a-chip devices to field-scale oil recovery.
It has long been known that effective wetting properties can be altered by the
application of an electric field; a phenomenon coined as electrowetting. Here,
we consider theoretically and numerically a single droplet sitting on an
(insulated) conductor, i.e., within a capacitor. The droplet consists of a pure
phase without solutes, while the surrounding fluid contains a symmetric
monovalent electrolyte, and the interface between them is impermeable. Using
nonlinear Poisson--Boltzmann theory, we present a theoretical prediction of the
dependency of the apparent contact angle on the applied electric potential. We
then present well-resolved dynamic simulations of electrowetting using a
phase-field model, where the entire two-phase electrokinetic problem, including
the electric double layers (EDLs), is resolved. The simulations show that,
while the contact angle on scales smaller than the EDL is unaffected by the
application of an electric field, an apparent contact angle forms on scales
beyond the EDL. This contact angle relaxes in time towards a saturated apparent
contact angle. The dependency of the contact angle upon applied electric
potential is in good compliance with the theoretical prediction. The only
phenomenological parameter in the prediction is shown to only depend on the
permeability ratio between the two phases. Based on the resulting unified
description, we obtain an effective expression of the contact angle which can
be used in more macroscopic numerical simulations, i.e. where the
electrokinetic problem is not fully resolved
Contact angle of sessile drops in Lennard-Jones systems
Molecular dynamics simulation is used for studying the contact angle of
nanoscale sessile drops on a planar solid wall in a system interacting via the
truncated and shifted Lennard-Jones potential. The entire range between total
wetting and dewetting is investigated by varying the solid--fluid dispersive
interaction energy. The temperature is varied between the triple point and the
critical temperature. A correlation is obtained for the contact angle in
dependence of the temperature and the dispersive interaction energy. Size
effects are studied by varying the number of fluid particles at otherwise
constant conditions, using up to 150 000 particles. For particle numbers below
10 000, a decrease of the contact angle is found. This is attributed to a
dependence of the solid-liquid surface tension on the droplet size. A
convergence to a constant contact angle is observed for larger system sizes.
The influence of the wall model is studied by varying the density of the wall.
The effective solid-fluid dispersive interaction energy at a contact angle of
90 degrees is found to be independent of temperature and to decrease linearly
with the solid density. A correlation is developed which describes the contact
angle as a function of the dispersive interaction, the temperature and the
solid density. The density profile of the sessile drop and the surrounding
vapor phase is described by a correlation combining a sigmoidal function and an
oscillation term
The contact angle in inviscid fluid mechanics
We show that in general, the specification of a contact angle condition at
the contact line in inviscid fluid motions is incompatible with the classical
field equations and boundary conditions generally applicable to them. The
limited conditions under which such a specification is permissible are derived;
however, these include cases where the static meniscus is not flat. In view of
this situation, the status of the many `solutions' in the literature which
prescribe a contact angle in potential flows comes into question. We suggest
that these solutions which attempt to incorporate a phenomenological, but
incompatible, condition are in some, imprecise sense `weak-type solutions';
they satisfy or are likely to satisfy, at least in the limit, the governing
equations and boundary conditions everywhere except in the neighbourhood of the
contact line. We discuss the implications of the result for the analysis of
inviscid flows with free surfaces.Comment: 13 pages, no figures, no table
- …