31 research outputs found
Roughness Induced Rotational Slowdown Near the Colloidal Glass Transition
Roughening the surface of spherical colloids can drastically change their
translational and rotational dynamics in dense suspensions. Using 3D confocal
microscopy, we show that roughness not only lowers the concentration of the
translational colloidal glass transition, but also generates a broad
concentration range in which the rotational Brownian motion changes signature
from high-amplitude diffusive to low-amplitude rattling. This hitherto not
reported second glass transition for rough spherical colloids emerges when the
particle intersurface distance becomes comparable to the roughness length
scale. Interlocking contacts are responsible for restricting the particle
rotations
Mixing and condensation in a wet granular medium
We have studied the effect of small amounts of added liquid on the dynamic
behavior of a granular system consisting of a mixture of glass beads of two
different sizes. Segregation of the large beads to the top of the sample is
found to depend in a nontrivial way on the liquid content. A transition to
viscoplastic behavior occurs at a critical liquid content, which depends upon
the bead size. We show that this transition can be interpreted as a
condensation due to the hysteretic liquid bridge forces connecting the beads,
and provide the corresponding phase diagram.Comment: submitted to PR
Spherical probes for simultaneous measurement of rotational and translational diffusion in 3 dimensions
Real time visualization and tracking of colloidal particles with 3D
resolution is essential for probing the local structure and dynamics in complex
fluids. Although tracking translational motion of spherical colloids is
well-known, accessing rotational dynamics of such particles remains a great
challenge. Here, we report a novel approach of using fluorescently labeled
raspberry-like colloids with an optical anisotropy to concurrently track
translational and rotational dynamics in 3 dimensions. The raspberry-like
particles are coated by a silica layer of adjustable thickness, which allows
tuning the surface roughness. The synthesis and applicability of the proposed
method is demonstrated by two types of probes: rough and smoothened. The
accuracy of measuring Mean Squared (Angular) Displacements are also
demonstrated by using these 2 probes dispersed in 2 different solvents. The
presented 3D trackable colloids offer a high potential for wide range of
applications and studies, such as probing crystallization dynamics, phase
transitions and the effect of surface roughness on diffusion
Nanometer-Resolved Collective Micromeniscus Oscillations through Optical Diffraction
We study the dynamics of periodic arrays of micrometer-sized liquid-gas
menisci formed at superhydrophobic surfaces immersed into water. By measuring
the intensity of optical diffraction peaks in real time we are able to resolve
nanometer scale oscillations of the menisci with sub-microsecond time
resolution. Upon driving the system with an ultrasound field at variable
frequency we observe a pronounced resonance at a few hundred kHz, depending on
the exact geometry. Modeling the system using the unsteady Stokes equation, we
find that this low resonance frequency is caused by a collective mode of the
acoustically coupled oscillating menisci.Comment: 4 pages, 5 figure
Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy
We present a combined LangmuirβPockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquidβvapor interfaces in combined LangmuirβPockels trough and APXPS measurements
Electrode-assisted trapping and release of droplets on hydrophilic patches in a hydrophobic microchannel
Droplets Formation and Merging in Two-Phase Flow Microfluidics
Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed