62nd Annual Report on Research 2017

The goal of this project is to leverage the analogy between long-range particle-wall interactions acting on colloidal particles and on floating millimeter scale particles to better understand aggregation processes under flow. For colloidal particles, the formation of aggregates results from the competition between attractive particle-surface interactions and hydrodynamic forces. Millimeter scale objects that float at the surface of a liquid bath locally deform the air/liquid interface, which results in attractive capillary interactions. When the liquid bath is flowing, drag forces are exerted on the floating particles. In vicinity of attractive fixed surfaces, one can therefore expect the deposition of particles on surfaces and even formation of aggregates. These aggregates can grow until they span across the width of the channel forming a clog.

During the first year of the project, a new experimental system was developed to study the capture of individual advected floating particles by a fixed cylindrical obstacle. The particles are advected at the surface of an open channel flow (30cm wide and 1 meter long). The flow is driven by gravity to avoid the fluctuations that a pump would introduce. The flow rate is further controlled with a valve system. We have worked with Reynolds number from 10 to 100. The fixed obstacles are cylinders of diameter of the order of a few centimeters. We varied the material from hydrophilic to hydrophobic to obtain a range of contact angles. The floating spheres are millimeter-sized and are made of different polymers. By changing the composition of the sphere, we control the contact angle and density, which determine the sign of the water surface curvature. Menisci of identical curvature (positive or negative) attract whereas menisci of different curvature repel. Particles advected down the channel can be captured by direct interception and because of the attractive capillary forces. Figure 1 shows an example of such capture.

Figure1: Time series showing the trajectory of an advected particle and captured by a vertical obstacle (top view). The attraction between the particle and the cylinder is made clear by the capture on the back of the cylinder.

During this second year of funding, my team and I have extended the scope of our experimental investigation and developed a new numerical tool to describe the aggregation and clogging processes in 2D suspension flows. The new numerical method allows taking into account the coupling between the flow and the forming aggregate. Going beyond the proof of concept established in the first year, this second year has led to (1) the preparation of a publication on the capture of individual particles, (2) a conference proceedings on the filtration of particles by an array of obstacles, (3) the development of an advanced numerical framework to address the couplings between the flow and the growing aggregate and (4) preliminary tests on the capture of geometrical and mechanically complex particles and the clogging of a channel.

Outcome of Year 2:

-       Experimental results

The experimental system has been modified to explore lower Reynolds number flows, in which the capillary effects on the transport of particles are most important. Increasing the viscosity of the solution is a good strategy. We could record the capture of individual particles on a single obstacle and the growth of aggregates under flow.

-       Numerical results

The numerical work focused on the capture of particles by an array of obstacles. To better capture the aging of such filter, we developed a new method to describe the evolution of the flow as more particles get captured. This method, which requires the combined use of Matlab software and Comsol models, is promising for the study of aggregate growth.

-       Theoretical work

Experiments show that it is important to describe the shape of the meniscus accurately, shape that depends on the diameter of the cylinder.

-       Scientific publications and conferences

A first publication comparing numerical results and experiments is in preparation. The numerical study of particle capture by an array of cylinders was accepted for publication as a Proceedings of the 2017 ASME IMECE conference in Tampa, Florida. The presentation will take place in November 2017.

This year, the work was presented in an invited talk at local workshop (The 7th Northeast Complex Fluids and Soft Matter Workshop at Princeton University), in an invited seminar at the University of Cambridge, UK and at international conferences (APS-DFD 2016 in Portland, OR and the APS March meeting in New Orleans, LA)

-       Student participation

The student participation did not require financial support from the grant for different reasons (the work was part of their M. Sc thesis or the student had her own funding).

Federico Gregori completed his M. Sc. thesis on the study of single particle capture, both experimental and numerical. Federico presented a poster at the 7th Northeast Complex Fluids and Soft Matter Workshop at Princeton University, wrote a conference proceedings for the 2017 ASME IMECE conference where he will give a presentation next month.  In addition to his thesis, Federico drafted a long paper on single particle capture..

DeeAnn Vasquez Medrano started her PhD work with the experimental study of aggregate formation. Under the mentorship of Federico Gregori she learned about the experimental system and further automated it.

Vachitar Judge is working on transport through granular media for his M. Sc. thesis. He built on the experimental system to study jamming in 2D suspension flows.

-       Impact on faculty career

The work supported by this grant has been a great opportunity to develop my expertise in capillary effects and assembly. I have spoken of this study in different settings: invited presentation in prestigious universities such as the University of Cambridge and Princeton University. I have also organized an invited session at the APS March meeting in the Spring 2017 on the topic of colloidal assembly, sponsored by the newly formed group on Soft Matter. Invited speakers included Kate Stebe from UPenn and Vinothan Manoharan of Harvard University.

The work will result in several publications that will be communicated with the ACS-PRF team when published.