Dynamics of Wet Granular Matter

Introduction

For dry granular matter such as a pile of sand, there exists a certain angle of repose above which avalanche might happen. By adding a certain amount of water into the sand pile, the limit of angle becomes not so crucial anymore which enables us to build sand sculptures. What happens is that the liquid added tends to form capillary bridges between particles which introduce cohesion to the granular material. To understand experimentally how the cohesive force (F_b) influence the dynamic behaviors of wet granular matter is the focus of my current research.

Cohesive capillary force makes wet granular matter enables building of a sand sculpture.

The granular particles we are dealing with are macroscopic particles, or to say the thermoenergy doesn't play a role in the granular dynamics. The size of granular particles might vary from tens of microns to millions of kilometers (eg. icebergs), covering 12 order of magnitude. They are ubiquitous in nature, such as sand dunes, volcanic debris, etc.; in various industries including minding, construction, chemical engineering and food industries; and in our daily lives, eg. the spicies sitting in your kitchen. Due to this ubiquity, understanding the dynamics of wet granular matter will be helpful for various industries to avoid accident, increase efficiency and reduce energy consumption.

Granular matter can behave like solids, liquids or gases, but it belongs to neither of the normal states of matter. Because the inelastic collisions or rupture of liquid bridges between particles cost energy, granular matter is dissipative. As a matter of fact, more than 1/10 of the total energy consumption in the whole world is spent in processing granular materials. Due to this dissipation, continuous energy injection is necessary to drive granular matter into different states. And the balance between energy injection and dissipation determines different states of granular matter. This characterizes driven granular matter as a model for understanding non-equilibrium systems.

For driven granular system, could it be possible to have a well defined physical properties (eg. temperature, pressure, interfacial tension) as we use in equilibrium systems? If so, what is the relation between those properties and how those relations governs the dynamical behaviors of granular matter? Since granular matter is composed of macroscopic particles, could it be possible to trace those properties down to the mobility of individual particles? These are challenges for the study of granular matter, and these are also opportunities for us to understand non-equilibrium systems in general.

Research Topics

Various driving methods are used to study the dynamics of wet granular matter in the sand lab:

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'Wet granular matter under SHAKING'

Melting	Pattern formation	Gas bubbles
Bright patterns are from the light emitted from Ruby tracers, which are embedded in the wet granular sample. The driving acceleration increases step by step.
Surface melting has been a topic of interest since Michael Faraday's observations on regelation, welding of two blocks of ice after contact below 0 degree. After more than a century's investigations, it becomes clear that melting is a continuous process that tends to start from the free surface (see e.g. a review of premelted ice). In a non-equilibrium system such as wet granular matter, it is obvious from the above video that melting also tends to starts from the surface. The question associated here is: is the melting mechanisms of a non-equilibrium system comparable to its equilibrium counterpart?	Spirals are widely existing in nature: from galaxies to hurricanes, from the inner structure of a seashell to the atomic scale magnetic ordering. The above video shows a three armed rotating spirals in an agitated thin layer of wet granular matter. In the current stage, we learned from experiments that the spiral arms correspond to kink waves and the rotation of spiral arms arises from the spatiotemporal chirality caused by period tripling. Our ongoing work is to understand what determines the rotation speed and shape of the spiral arms, and the synchronization of multiple spirals, aiming at a better understanding of wet granular flow in general.	Merging of wet granular 'gas bubbles' suggests the existence of interfacial tension. The interesting question associated with this phenomenon is what could we learn about the surface tension and viscous force for fluidized wet granular matter?

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'Wet granular matter under SHEARING'

Except for vertical vibrations, shear is widely used in industries such as chemical engineering, mining and waste water treatment. Different from researchers in the colloidal community, we are interested in particles within the granular material regime, i.e. 10 to 1000 microns in diameter, where van der Wall's force doesn't play an important role. For these particles trapped at the air-liquid interface, forces acting on them usually include mono-polar forces (gravity, buoyancy force), dipolar forces (due to the electric charge between the interface) and quadrapole force (capillary interaction due to the distortion of liquid interface). Here we introduce a hysteretic and short ranged attractive force by using a second liquid which is immiscible with the bulk liquid to wet particles and form liquid bridges between them. In such a way we create a 2D wet granular media. By applying a shear force, we study the aggregation of 2D wet granular material. Comparison between dry and wet sample under shear (as the two sample videos shown below) indicates that the capillary force dominates the others. Two sample videos are shown below. For dry sample, particles aggregation due to the quadrapole force could easily be broken by the viscous drag force. While at the same shear rate, the aggregation formed by wet sample still persists.

Experimental setup for 2D wet granular matter under shear. The image on lower-right is a microscopic top view of the wet glass spheres floating on bulk liquid.

Video:2D dry glass spheres under shear.

Video:2D wet glass spheres under the same shear rate.

The movement of particles under shear flow is captured by a high speed camera and the images captured are in turn subjected to image processing which enables tracing of individual particles. After image processing, we could get the location, velocity and corresponding cluster index of all the particles found in the field of view, based on which the location, velocity, equivalent diameter, orientation of the clusters could be obtained. Below is a video clip after image processing. The particles are color coded by which cluster they are belonging to.

Video:2D wet glass spheres under the same shear rate, after image processing.

With this setup, the formation, growing and merging of clusters as well as its dependence on the driving shear force are studied and compared to numerical simulations. We also study the cluster size distribution and its dependence on shear rate and the fractal dimension of the clusters formed at different shear rate. Further study will focus on the high area fraction regime and study the melting, jamming and rigidity of 2D wet granular matter under shear.

Video:2D wet glass spheres under shear, original video.

Video:2D wet glass spheres under shear, video after image processing.

This project is in collaboration with the 'Dynamics of Complex Fluid' group in the Max Planck Institute for Dynamics and Self-organization.

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'Wet granular matter under SWIRLING'

Gold miners in the 19th centurary knew that gold nuggets would slowly expose by applying a swirling motion.

Swirling is motion composed of two horizontal vibration components. It is not only used in the gold panning and stirring up the bouquet of a glass of wine, but also employed in various industrial processes, eg. to enhance the efficiency of pneumatic conveying. There are three types of questions arising from the preliminary experiment: 1. Regarding the crystallization process: How could isolated particles form the crystalized structure? What is the time scale associated with the process? 2. Concerning the melting process: How does the melted film thickness scale with the driving? Will the melting also follow the two-stage melting scenario as the KTHNY theory predicted? 3. The initially static crystal starts to move as the driving force is higher enough, which indicates a certain frictional force to overcome. How to define and determine the frictional coefficient for the 2D wet granular crystal? What is the relation between this parameter and the underlying driving mechanisms for the horizontal swirling setup? We plan to answer those questions by both `macroscopically' measure the phase transition and morphological behavior of the cluster and also `microscopically' trace the mobility of individual particles.

The wet particles, initially randomly distributed, will self organize into a structure with hexagonal packing, after swirling at a frequency 1.7 Hz for about 10 minutes (a). This crystalized structure will not move in the co-moving frame, indicating that the driving from the container is not enough to overcome the ``static frictional force'' acting on it. As the driving is turned higher via increasing the frequency, this structure starts to move and reshape itself into a perfect hexagonal packing crystal covered by a shell with less packing density (snapshots (b) and (c)). Even stronger driving leads to the melting of particles from the surface of the shell (d).

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