Gel Diffusion Experiment STEM ED/CHM Nanotechnology 2014 Presented by Jennifer Welborn.
Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2008.
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Transcript of Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2008.
Why Size Matters
Adapted from Nanosense
http://nanosense.org/activities/sizematters/properties/SM_PropSlides.ppt
STEM ED/CHM Nanotechnology 2008
Relative sizes (review)
• Atomic nuclei ~ 10-15 meters = 10-6 nanometers
• Atoms ~ 10-10 meters = 0.1 nanometers
• Nanoscale ~ 1 to 100 nanometers ~ 10 to 1000 atoms
• Everyday world ~ 1 meter = 109 nanometers
The Basic Physics
• At the everyday scale, Newton’s laws (F=ma, etc.) work fine
• At the atomic and molecular level, quantum mechanics is needed to describe phenomena and properties– Discrete energy levels, tunneling
• Nanomaterials are in a borderline region where either or both approaches may be appropriate
The Basic Forces
• Strong Nuclear Force – huge, hold nuclei together; act only at nuclear distances, 10-6 nm
• Weak Nuclear Force – small, responsible for nuclear beta decay, act only at nuclear distances, 10-6 nm
• Electric and Magnetic – dominant at atomic and nanotech scales; 1039 × gravitational forces; long ranged, 1/r2
• Gravitational – long ranged, 1/r2; dominant at everyday world scale, since most objects lack a substantial net electrical charge
Properties of a Material• Types of properties
– Optical (e.g. color, transparency)– Electrical (e.g. conductivity)– Physical (e.g. hardness, melting point,
diffusion rate)– Chemical (e.g. reactivity, reaction rates)
• Properties are usually measured by looking at large (~1023) aggregations of atoms or molecules
Optical Properties Example: Gold
• Bulk gold appears yellow in color
• Nanosized gold appears red in color– The particles are so small that electrons
are not free to move about as in bulk gold– Because this movement is restricted, the
particles react differently with light
Sources: http://www.sharps-jewellers.co.uk/rings/images/bien-hccncsq5.jpghttp://www.foresight.org/Conferences/MNT7/Abstracts/Levi/
12 nanometer gold particles look red“Bulk” gold looks yellow
“Traditional” ZnO sunscreen is white
Zinc oxide nanoparticles
Nanoscale ZnO sunscreen is clear
Sources: http://www.apt powders.com/images/zno/im_zinc_oxide_particles.jpghttp://www.abc.net.au/science/news/stories/s1165709.htmhttp://www.4girls.gov/body/sunscreen.jpg
Optical Properties Example: Zinc Oxide (ZnO)
• Large ZnO particles – Block UV light– Scatter visible light – Appear white
• Nanosized ZnO particles– Block UV light– So small compared to the
wavelength of visible light that they don’t scatter it
– Appear clear
• Application to sunscreen
Source: http://www.weizmann.ac.il/chemphys/kral/nano2.jpg
Electrical Properties Example: Conductivity of Nanotubes
• Nanotubes are long, thin cylinders of carbon– They are 100 times stronger than steel, very flexible,
and have unique electrical properties
• Their electrical properties change with diameter, “twist”, and number of walls– They can be either conducting or semi-conducting in
their electrical behavior
Electric current varies by tube
structure
Multi-walled
Physical Properties: Diffusion
• Small particles (molecules in suspensions, dust particles in air) move randomly in zigzag paths (Brownian motion) due to collisions
• Particles spread out or diffuse when introduced into a medium at one point– Perfume in a room
• Average kinetic energy ½ mv2 ~ temperature• Average particle speeds decrease as mass
increases, so more massive particles diffuse more slowly
Sources: http://puffernet.tripod.com/thermometer.jpg and image adapted from http://serc.carleton.edu/usingdata/nasaimages/index4.html
Physical Properties Change:Melting Point of a Substance
• Melting Point (Microscopic Definition)– Temperature at which the atoms, ions, or
molecules in a substance have enough energy to overcome the intermolecular forces that hold the them in a “fixed” position in a solid
In contact with 3 atoms
In contact with 7 atoms
– Surface atoms require less energy to move because they are in contact with fewer atoms of the substance
A flower or a person at the edge of a crowd has fewer neighbors than one in the middle
People at the edge can move more easily
Size Matters in Biology
• Metabolism (heat generation) is limited by the number of cells, or volume, L3
• Heat loss to the environment is proportional to the surface area, L2
• As we look at smaller and smaller organisms, the surface to volume ratio L2/ L3 = 1/L gets larger and larger, making it harder to maintain body temperature (even with feathers, fur)
• Smallest warm blooded organisms are hummingbirds and the shrew, a small mouse-like mammal
What Does This All Mean?• Key factors for understanding nanoscale-
related properties– Dominance of electromagnetic forces– Importance of quantum mechanical models– Higher surface area to volume ratio– Random (Brownian) motion
• It is important to understand these four factors when researching new materials and properties
Surface to Volume Ratio Experiments
• As a sample is made larger, a smaller fraction of the atoms (or molecules) are on the surface
• Atoms on the surface have fewer neighbors than those on the interior– Students at the edge of the classroom have fewer
neighbors than those in the center• Explore this with two activities – cards, blocks• Only atoms on the surface can interact with
another material and take part in a chemical reaction
• Explore this with Alka Seltzer tablets and powder
Activities
• Groups of 3 people• Write ups, cards, blocks, Alka Seltzer materials• Explore the effects of increasing size with the
cards • Explore the effects of increasing size with the
blocks• Do the Alka- Seltzer experiment to see the
effect of particle size on chemical processes