Javed Ally

Detachment Force of Particles from Air-Liquid Interfaces of Films and Bubbles

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.

Adhesion of Particles with Sharp Edges to Air-Liquid Interfaces

In order to study the effect of sharp edges on solid particle adhesion to air-liquid interfaces, spherical colloidal probes were modified with a circumferential cut by focused ion beam milling. The interaction of the modified particles with water drops and bubbles was studied using the colloidal probe technique. When the modified particles were brought into contact with air-liquid interfaces, the contact line was pinned at the edge of the cut. Contact hysteresis between the approach and retraction components of the measured force curves was eliminated. The contact angle at the edge takes a range of values within the limits defined by the Gibbs inequality. These limits determine the adhesion force. As such, the adhesion force is a function of the particle wettability and edge geometry.

Interaction of a microsphere with a solid-supported liquid film.

The interaction between particles with thin liquid films on solid surfaces was studied by sintering polystyrene microspheres of 4 to 5 microm diameter to the end of atomic force microscope cantilevers. Films of three silicone oils (viscosity 4.6, 9.2, and 9700 mPa s) and water of thickness 0.2-1.8 microm were formed on glass. The interaction between a particle and the film was measured at different particle approach/retraction velocities. The interaction is dominated by capillary and hydrodynamic forces. It depends on the surface tension and the viscosity of the liquid. The film thickness can be determined from the force curves. In addition, the meniscus formation of a film wetting a particle was demonstrated experimentally by solidifying a liquid polystyrene film as it wetted glass particles.

Direct visualization of the interfacial position of colloidal particles and their assemblies

A method for direct visualization of the position of nanoscale colloidal particles at air-water interfaces is presented. After assembling hard (polystyrene, poly(methyl methacrylate), silica) or soft core-shell gold-hydrogel composite (Au@PNiPAAm) colloids at the air-water interface, butylcyanoacrylate is introduced to the interface via the gas phase. Upon contact with water, an anionic polymerization reaction of the monomer is initiated and a film of poly(butylcyanoacrylate) (PBCA) is generated, entrapping the colloids at their equilibrium position at the interface. We apply this method to investigate the formation of complex, binary assembly structures directly at the interface, to visualize soft, nanoscale hydrogel colloids in the swollen state, and to visualize and quantify the equilibrium position of individual micro- and nanoscale colloids at the air-water interface depending of the amount of charge present on the particle surface. We find that the degree of deprotonation of the carboxyl group shifts the air-water contact angle, which is further confirmed by colloidal probe atomic force microscopy. Remarkably, the contact angles determined for individual colloidal particles feature a significant distribution that greatly exceeds errors attributable to the size distribution of the colloids. This finding underlines the importance of accessing soft matter on an individual particle level.

Forces between a monolayer at an air/water interface and a particle in solution: influence of the sign of the surface charges and the subphase salt concentration

The way the rigidity, molecular packing density, and charge of an insoluble monolayer at an air/aqueous interface and the type of substrate affect the deposition of a monolayer on a substrate is still unclear. In this study, we aimed to better understand the forces involved in the deposition process, when a solid surface is brought near a monolayer. We achieved this by using the Monolayer Particle Interaction Apparatus to directly measure the interaction forces between a monolayer at an air/aqueous interface and a hard particle in solution. The monolayer surface pressure, the monolayer and particle surface charge, and the concentration of salt in the subphase were varied. An adhesion was observed between a particle and a monolayer in an unlike-charged monolayer–particle system, or when a particle with a non-zero contact angle was used in the measurement. The addition of salt changed the packing of the monolayer, increasing the stiffness of the monolayer and generally decreasing the adhesion. Increasing the surface pressure of the monolayer decreased the interfacial stiffness and generally decreased the adhesion.

Condensation in Nanoporous Packed Beds

In materials with tiny, nanometer-scale pores, liquid condensation is shifted from the bulk saturation pressure observed at larger scales. This effect is called capillary condensation and can block pores, which has major consequences in hydrocarbon production, as well as in fuel cells, catalysis, and powder adhesion. In this study, high pressure nanofluidic condensation studies are performed using propane and carbon dioxide in a colloidal crystal packed bed. Direct visualization allows the extent of condensation to be observed, as well as inference of the pore geometry from Bragg diffraction. We show experimentally that capillary condensation depends on pore geometry and wettability because these factors determine the shape of the menisci that coalesce when pore filling occurs, contrary to the typical assumption that all pore structures can be modeled as cylindrical and perfectly wetting. We also observe capillary condensation at higher pressures than has been done previously, which is important because many applications involving this phenomenon occur well above atmospheric pressure, and there is little, if any, experimental validation of capillary condensation at such pressures, particularly with direct visualization.

Hydrodynamic force between a sphere and a soft, elastic surface.

The hydrodynamic drainage force between a spherical silica particle and a soft, elastic polydimethylsiloxane surface was measured using the colloidal probe technique. The experimental force curves were compared to finite element simulations and an analytical model. The hydrodynamic repulsion decreased when the particle approached the soft surface as compared to a hard substrate. In contrast, when the particle was pulled away from the surface again, the attractive hydrodynamic force was increased. The hydrodynamic attraction increased because the effective area of the narrow gap between sphere and the plane on soft surfaces is larger than on rigid ones.

Model studies of magnetic particle retention in the conducting airways

This paper reports initial findings of using magnetic force to target and retain particles in airways using an ex vivo model, i.e. palate of bullfrogs (Rana catesbeiana). The purpose of this study was to investigate the feasibility of using magnetic force in targeted chemotherapy agents inhaled in an aerosolized form for lung cancer therapy. The results show that the clearance mechanism of airways will overcome magnetic force to clear aggregates of small particles (1-3 /spl mu/m) deposited, whereas aggregates of larger particles (180 /spl mu/m) can be retained at the targeted site when a magnetic strength of 80 mT and gradient of -6 T/m was used.

Automatic Focusing for Objects with Large Displacement at High Magnification

Current systems for automatic focusing of microscopes have a limited focal range and require piezoelectric actuators to achieve the necessary precision. Using only one microscope, the algorithms are generally based on the deviation of the pixel grey level or the pixel energy. These techniques have difficulties when the microscope is far out of focus1-3. In ref1, a focusing criterion called the Dynamic Focus Criterion (DFC) was proposed. This method is based on the calculation of grey level in two images detected through the dynamic contrast histogram. However, when the object is far out of focus, there is little or no contrast to allow efficient application of the DFC. In ref2, an auto-focusing system is designed which follows a drifting focus or compensate for a shifted focus while the sample is changing size and position. This is a useful algorithm, but it is more suited for slowchanging events such as division of yeast cells that take about 90 min. In ref3, the standard deviation of pixel grey levels is used to generate the feedback signal in an auto-focusing system. Although the technique can be used for submicron focusing, it needs to know the direction that the optics should be moved or it has to be initially nearly focused. Such limitation will not be helpful in application discussed in this paper (see below). To show the effectiveness of our automatic focusing strategy, we applied it to a scientific instrument used for simulating human alveoli to study its interfacial properties (e.g. surface tension). Alveoli (sac-shaped final branching of respiratory system) are traditionally simulated by the generation of an air bubble in a chamber filled with an aqueous solution4,5 (see Figure 1). The experimentalist is generally interested in the study of surface active agents’ behaviour at the air-liquid interface. This is typically done by observing the meridian profile of the bubble and analysing its shape using software such as Axisymmetric Drop Shape Analysis-Profile (ADSA-P)4 to obtain interfacial tension information, and using fluorescence microscopy to closely examine the structural formations of the surfactant molecules at the air-liquid interface. Such experiments often require expanding and contracting the air bubble to simulate breathing to study the response of the interfacial tension or molecular formations to the simulated breathing action. At all times during expanding and contracting of the bubble, its apex needs to remain at the focus of the fluorescent microscope to allow the study of molecular formations at the air-liquid interface. To date the focusing is done manually and here we propose an automated focusing system based on a PID control algorithm combining both feedforward and feedback control elements.