Sticking Process Research Articles

Aggregation modes of colloids in the presence of block copolymer micelles

We investigated the aggregation of colloidal latex induced by the interparticle bridging phenomenon as a function of the surface density of link-forming sites. To perform aggregation experiments on a model system, of reproducible geometry, an unstable colloid was obtained by adsorbing stable spherical diblock copolymer micelles onto latex particles. The conformation of this site, active in the bridging mechanism, did not change either on adsorption or with time, and thus the modified colloid constituted a model to investigate colloid aggregation induced by interparticle polymer bridging. Different aggregation modes were determined in experimental studies of the kinetics of aggregation processes. Dynamic scaling laws were established, which were compared to those determined by computer simulation and theory. By increasing the number of micelles adsorbed onto one latex particle, the following flocculation modes were observed. At very low surface coverage, irreversible reaction-limited processes were found to generate agglomerates having a size frequency function c n ( t) scaling like t − τ with τ equal to 1.50. At a given polymer coating, reversible diffusion-limited aggregation was found to describe the process and a bell-shaped size distribution curve was obtained. At greater surface occupancy, the irreversible reaction-limited aggregation mode was recovered. Due to the presence of adsorbed copolymers, interactions between the colloid/micelle complexes were modified, leading to different reactivity in particle-particle agglomeration. As a result, at each time of the flocculation period, the cluster size distribution c n ( t) scaled like t − τ with a modified exponent τ equal to 1.65. The existence of the unexpected diffusion-limited aggregation process, not evidenced in the presence of the parent flexible polymer, was interpreted in terms of topological constraints governing the collision efficiency in the sticking process. In fact, a minimal surface coverage was found to be necessary to induce this aggregation mode. Above and below this critical domain, the aggregation developed with features characteristic of the classical bridging mechanism induced by water-soluble flexible homopolymers. Diblock copolymers seemed to constitute a new class of very efficient polymeric flocculants.

Oxygen interaction with CoSi(100) and CoSi 2(100) surfaces

The interaction of oxygen with (100) surfaces of single crystalline CoSi and CoSi 2 has been studied using LEED, AES and UPS techniques. Reference experiments at recrystallized Co and (111) oriented Si surfaces are performed to allow comparison with those alloy surfaces. It is found that the oxygen uptake kinetics at CoSi resembles that measured at Co surfaces, whereas at CoSi 2 almost Si-like behaviour is observed. This strongly supports a structure model which exhibits surface reconstruction with enhanced Co concentration at CoSi(100) surfaces. At CoSi 2(100) surfaces the toplayer contains only Si, the second layer may be composed of Si, too. From Auger electron and UP spectra it is seen that the oxygen always is bonded at Si although at CoSi(100) the sticking process occurs via Co atoms.

Model for handling the transmission problem in sticking at cold solid surfaces

The sticking rate is calculated for a simple model Hamiltonian, which assumes a one-dimensional motion of the incident gas atom together with a one- to three-dimensional phonon density of states. The gas-atom-phonon interaction is of short range and treated in a localized basis set. After solving exactly for this short-range interaction the localized wave functions are embedded into the continuum of scattering states. This embedding problem is solved in an approximate way. The calculated sticking coefficient at zero kinetic energy of the incoming particle and zero substrate temperature is unity for a two- and three-dimensional phonon density of states, and finite between zero and one for a one-dimensional phonon density of states. Many phonon events are found to dominate the sticking process.

Sticking Coefficient of Atoms on Solid Surfaces at Low Temperatures

It is shown that because of a polarization effect the sticking coefficient of atoms on surfaces remains finite down to much lower temperatures than has hitherto been supposed on the basis of the Born approximation to the sticking process.

Grain Formation through Nucleation Process in Astrophysical Environment

General picture of grain formation is presented based on the nucleation theory. Grain formation process is described by a growth equation of grain radius and an equation of monomer consumption due to the growth of grains. These equations are characterized by two parameters. One depends on the physical conditions of the system and the other reflects the nature of grain materials. An overall feature of the grain formation process is illustrated by the use of an analytic expression of the solutions. After the vapor cools down to the saturated state, a waiting time is necessary until the grain formation begins effectively. Size distribution is relatively sharp in general. The representative size is closely related to the parameter which depends on the physical conditions. Growth by coalescence is not effective until the monomer sticking process is almost completed. The results are applied to the condensation in the primordial solar nebula. It is shown how the picture based on the chemical equilibrium calculations should be modified.

Plaited Polyhedra

In Cundy and Rollett’s invaluable book on Mathematical Models, the authors begin their Chapter on Polyhedra by remarking that “The most suitable, and in many ways the most attractive, subject for an experiment in the construction of mathematical models is a set of polyhedra.” Various methods are in general use to produce finished models of polyhedra for the showcase, and similar methods form the basis of those considered suitable for constructing the simpler polyhedra in the classroom. That most commonly adopted is to draw the net of the required solid on a sheet of cardboard, allow for tabs as necessary, cut it out, score the creases half-through, fold up, and stick the tabs with some suitable quick-drying cement. For class-room use the major drawback to model making on these lines is the time-consuming and potentially messy process of sticking; and it is the object of this article to develop a method which dispenses with the use of paste or cement altogether. In fact I shall show how, using paper and scissors only, firm, neat models can be made, which in the case of the simpler solids could easily be produced in the classroom, and which in the case of the more complex polyhedra reveal by their construction in a striking manner some of the geometrical relationships of these solids.