During the ball milling process of laboratory samples, the phenomenon of settling at the bottom and sticking to the jar is usually related to multiple factors such as material properties, ball milling process parameters, equipment selection, and operating methods. The specific reasons are as follows:

I. Material Property Factors

1. Excessively High Material Viscosity

•Principle: Samples containing high organic matter (e.g., certain polymers, clay minerals) or moisture (e.g., wet-milled samples) exhibit strong interparticle forces, making them prone to agglomeration into lumps and adhesion to the jar wall or grinding ball surface.

•Typical Scenarios: Biological samples (e.g., plant tissues, bacterial cells), colloidal solutions, undried soil/slurry, etc.

2. Significant Differences in Particle Size

•Phenomena:

•Fine particles (e.g., nano-scale powder) have a large specific surface area and high surface energy, easily adsorbing onto the jar wall to form a "sticky layer";

•Coarse particles (e.g., larger than 1mm) have a fast sedimentation rate, depositing at the bottom of the jar to form a dense layer that hinders the movement of grinding balls.

•Case Example: When co-grinding quartz sand (coarse) and graphene (fine), the layered sticking phenomenon of "coarse at the bottom and fine on top" is likely to occur.

3. Material Wettability Issues

•Dry Milling Scenarios: Hydrophobic materials (e.g., graphite, certain polymer materials) are not easily driven by grinding balls and tend to accumulate at the bottom of the jar;

•Wet Milling Scenarios: Mismatched polarity between the solvent and the material (e.g., grinding oily samples with water) leads to uneven dispersion, local agglomeration, and sticking to the jar.

II. Inappropriate Ball Milling Process Parameters

1. Unreasonable Ball-to-Material Ratio

•Insufficient Ball-to-Material Ratio: An inadequate number of grinding balls fails to effectively disperse the material, leading to mutual extrusion and adhesion between particles (Recommended ball-to-material ratio: 5:1 ~ 10:1, which should be adjusted according to material hardness);

•Excessive Ball-to-Material Ratio: Too many grinding balls occupy the internal space of the jar, preventing sufficient material movement. Additionally, heat generated by friction may exacerbate the adhesion of viscous materials.

2. Unsuitable Rotational Speed

•Too Low Rotational Speed: Insufficient kinetic energy of grinding balls cannot break up agglomerates, resulting in fine particles settling at the bottom to form a "dead layer";

•Too High Rotational Speed: Centrifugal force causes grinding balls to adhere to the jar wall and rotate, losing the grinding effect. Meanwhile, frictional heat may soften and stick the material (especially for thermosensitive materials such as plastics and asphalt).

3. Excessively Long or Short Grinding Time

•Too Short Grinding Time: Materials are not fully dispersed, and large particles settle and stick to the bottom;

•Too Long Grinding Time: Over-refinement of fine particles leads to a sharp increase in specific surface area and enhanced adsorption capacity, forming a dense sticky layer (e.g., prone to occur when clay minerals are ground to the nano-scale).

4. Improper Use of Solvents/Additives

•Insufficient Wet Milling Solvent: Failure to fully wet the material results in dry powder agglomeration and jar sticking;

•No Dispersant Added: Hydrophilic materials (e.g., oxide powders) tend to form hydrogen-bonded agglomerates in water. Dispersants such as sodium hexametaphosphate should be added to reduce surface energy.

III. Equipment and Operation Factors

1. Mismatched Ball Mill Jar Material

•Strong Material Adsorptivity: Stainless steel jar walls are prone to cold welding and adhesion with certain metal powders (e.g., iron, copper);

•High Surface Roughness: Unpolished jar walls or severely worn jars easily retain materials, forming sticky spots.

2. Single Type and Size of Grinding Balls

•Single Grinding Ball Size: Using only large balls results in low grinding efficiency for fine particles, which tend to settle at the bottom; using only small balls leads to insufficient crushing capacity for coarse particles, causing accumulation and agglomeration;

•Material Mismatch: Using glass beads for grinding hard materials (e.g., ceramics) is prone to wear and debris generation, exacerbating jar sticking.

3. Excessive or Insufficient Loading Capacity

•Overloading (Exceeding 2/3 of Jar Volume): Limited movement space for materials prevents effective tumbling, and bottom materials are compacted and stuck to the jar;

•Underloading (Less Than 1/3 of Jar Volume): Grinding balls directly impact the jar wall, generating significant heat that causes local overheating and adhesion of materials.

4. Non-Standard Operating Procedures

•No Pre-Dispersion Before Dry Milling: Agglomerated materials are directly added to the jar, forming large lumps that stick to the bottom at the initial grinding stage;

•Incorrect Feeding Sequence in Wet Milling: Adding powder first and then solvent easily forms "dry powder clumps". It is necessary to first add a portion of solvent to wet the jar wall, then gradually add the powder.

IV. Environmental Factors

1. Insufficient Temperature Control

•Significant Heat Generation During Grinding: For high-hardness materials or long-duration grinding, failure to use cooling devices (e.g., ice water bath, circulating cooling fluid) causes material softening, melting, and jar sticking (e.g., polymer blending grinding).

2. Impact of Ambient Humidity

•Hygroscopic Materials: Substances such as anhydrous copper sulfate and cement clinker absorb moisture and agglomerate in high-humidity environments, forming viscous lumps during dry milling.

•Settling and jar sticking during ball milling are comprehensive results of mismatched material properties and process conditions. It is recommended to conduct a systematic analysis from four dimensions: material dispersibility, grinding ball kinetic energy, interface adsorption, and temperature control. Through pretreatment optimization, parameter adjustment, and equipment adaptation, the risk of jar sticking can be significantly reduced, while grinding efficiency and sample uniformity are improved.