Emanating Jets As Shaped by Surface Tension Forces

We show that emanating jets can be regarded as growing liquid towers, which are shaped by the twofold action of surface tension: first the emanated fluid is being accelerated back by surface tension force, herewith creating the boundary conditions to solve the shape of the liquid tower as a solution of an equation mathematically related to the hydrostatic Young-Laplace equation, known to give solutions for the shape of pending and sessile droplets, and wherein the only relevant forces are gravity g and surface tension γ. We explain that for an emanating jet under specific constraints all mass parts with density ρ will experience a uniform time dependent acceleration a(t). An asymptotic solution is subsequently numerically derived by making the corresponding Young-Laplace type equation dimensionless and by dividing all lengths by a generalized time dependent capillary length λc(t) = γ(t)/ρ(a(t)-g). The time dependent surface tension γ(t) can be derived by measuring both time dependent acceleration a(t) and time dependent capillary length λc(t). Jetting experiments with water and coffee show that the dynamic surface tension behavior according to the emanating jet method and with the well-known maximum bubble pressure method are the same, herewith verifying the proposed model.</p

Gas-shell-encapsulation of Activated Carbon to Reduce Fouling and Increase the Efficacy of Volatile Organic Compound Removal

A method to encapsulate activated carbon particles is presented that reduces fouling of these particles with Natural Organic Matter (NOM) to preserve their adsorption capacity for Volatile Organic Compounds (VOCs) from water in the presence of NOM. The encapsulation method uses an oil-in-water emulsion template to encapsulate the activated carbon particles within a gas-filled porous shell of hydrophobic silica particles. This new ‘gas-shell-encapsulation’ method has little influence on the adsorption capacity of the activated carbon for a representative VOC (toluene). The adsorption of NOM components (humic acid) onto the encapsulated activated carbon is however strongly reduced such that preloading of the encapsulated activated carbon with humic acid hardly reduces its adsorption capacity for toluene, whereas for unencapsulated activated carbon preloading with humic acid reduces the adsorption capacity by roughly a factor of three.</p

Droplet Formation by Confined Liquid Threads inside Microchannels

A confined liquid thread can form monodisperse droplets near the exit of a microchannel, provided the continuous phase is able to enter the microchannel. A general model that accurately predicts the droplet size including the breakup position inside the microchannel is presented and is verified with experimental observations; breakup occurs as long as the capillary number (Ca) of the liquid thread is below a critical capillary number (Cacr); for cylindrical microchannels, it is derived that Cacr = 1/16. Below Cacr, the formed droplets at the exit of the microchannel have a diameter approximately two times the diameter of the liquid thread; around and above Cacr, the liquid thread remains stable and the formed droplets grow infinitely large. The presented controlled droplet generation method is a useful tool for producing monodisperse emulsions and has great potential for the food and pharmaceutical industry.</p

Viscous Liquid Threads with Inner Fluid Flow Inside Microchannels

Forming droplets are often accompanied by an interconnecting liquid thread. It is postulated that this phenomenon can only exist as long as a pressure gradient exists within the thread, for instance, when a viscous liquid is conveyed via the liquid thread to the forming droplet. We have built a microfluidic setup to form and sustain a liquid thread, which after a length L ends in a droplet. To prevent the droplet from moving up too fast due to buoyancy, we force the droplet to shift along a tilted ceiling, which can be positioned at three different angles. This enables us to keep the gradual lengthening of the liquid thread under control. Based on the Navier-Stokes equation, we are able to predict the axial shape of such a liquid thread as a function of fluid mass density, initial thread radius, initial fluid velocity at the nozzle, fluid viscosity, and surface tension. Although an explicit solution of the governing differential equations is not known, we managed to find an explicit approximating expression for the shape function, which shows excellent agreement with both the measured and the numerically calculated shape functions. An intriguing phenomenon observed in the experiments is the breakup of the thread. This breakup always occurs close to the droplet. Using our approximating solution, we derive a relation that connects, for any time in the development of the thread, its length and the pressure gradient stemming from, among other effects, the shear at the interface of the liquid thread due to motion of the inner liquid. For relatively short thread lengths, this relation is linear on a log-log scale, due to the fact that in this regime, viscosity effects are dominant. However, if the thread length increases, this relation starts to deviate from linear behavior, due to surface tension effects. We show from the experimental results that the thread starts to show unstable behavior as soon as these capillary effects come into play. We show how to predict the thread length at which the capillary instability sets in for any liquid thread system. It is found that the predicted maximum dimensionless thread length is given by Lmax,pred ≈ 12Ca with Ca the capillary number.</p

Biosensor-based detection of tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M.tb.), is one of the most prevalent and serious infectious diseases worldwide with an estimated annual global mortality of 1.4 million in 2010. Diagnosis of TB in the developing world is very challenging due to the limited suitability of currently available techniques under tropical field conditions. M. tb. is a slowly growing Mycobacterium that takes around six to eight weeks to be detected via sensitive culture methods. There is also hardly any clinical symptom at an early stage of infection, thereby causing a delay in diagnosis and treatment, and the complexity of the disease is further increased by the emergence of multiple drug resistant (MDR) strains. A lot of work has been done over the last few decades to develop effective point of care diagnostic techniques that are cheap, robust and can be performed at high throughput in rural areas. However, despite considerable technical improvements reported from the lab, such economical fool-proof diagnostic assays are still lacking on the market. The objective of this review is to evaluate currently available biosensing techniques that are either already in use or under development for detection of TB. The focus of the review is on the emerging field of diagnostic biosensors that combine ligand capture and detection in a one-step assay. A comparison will also be made with conventional multistep techniques.</p

Stable Protein-Repellent Zwitterionic Polymer Brushes Grafted from Silicon Nitride

Zwitterionic poly(sulfobetaine acrylamide) (SBMAA) brushes were grafted from silicon-rich silicon nitride (SixN4, x > 3) surfaces by atom transfer radical polymerization (ATRP) and studied in protein adsorption experiments. To this aim ATRP initiators were immobilized onto SixN4 through stable Si−C linkages via three consecutive reactions. A UV-induced reaction of 1,2-epoxy-9-decene with hydrogen-terminated SixN4 surfaces was followed by conversion of the epoxide with 1,2-ethylenediamine resulting in primary and secondary amine-terminated surfaces. A reaction with 2-bromoisobutyryl bromide led to ATRP initiator-covered surfaces. Zwitterionic polymer brushes of SBMAA were grown from these initiator-coated surfaces (thickness ∼30 nm), and the polymer-coated surfaces were characterized in detail by static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and an atomic force microscope (AFM). The adsorption of proteins onto zwitterionic polymer coated surfaces was evaluated by in situ reflectometry, using a fibrinogen (FIB) solution of 0.1 g·L−1, and compared to hexadecyl-coated SixN4 surfaces (C16−SixN4), uncoated air-based plasma oxidized SixN4 surfaces (SiOy−SixN4), and hexa(ethylene oxide)-coated SixN4 surfaces (EO6−SixN4). Excellent protein repellence (>99%) was observed for these zwitterionic polymer-coated SixN4 surfaces during exposure to FIB solution as compared to C16−SixN4 surfaces. Furthermore, the stability of these zwitterionic polymer-coated SixN4 surfaces was surveyed by exposing the surfaces for 1 week to phosphate buffered saline (PBS) solution at room temperature. The zwitterionic polymer-coated SixN4 surfaces before and after exposure to PBS solution were characterized by XPS, AFM, and water contact angle measurements, and their protein-repelling properties were evaluated by reflectometry. After exposure to PBS solution, the zwitterionic polymer coating remained intact, and its thickness was unchanged within experimental error. No hydrolysis was observed for the zwitterionic polymer after 1 week exposure to PBS solution, and the surfaces still repelled 98% FIB as compared to C16−SixN4 surfaces, demonstrating the long-term efficiency of these easily prepared surface coatings

Microsieves made with laser interference lithography for micro-filtration applications

A microsieve with a very uniform pore size of 260 nm and a pore to pore spacing of 510 nm has been fabricated using multiple exposure interference lithography and (silicon) micro-machining technology.\ud \ud The sieve consists of a 0.1 µm thick silicon nitride membrane perforated with sub-micron diameter pores and a macro perforated silicon support. The calculated clean water flux is at least one to two orders higher than that of conventional inorganic membranes