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Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2008.

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Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2007.

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Presentation on theme: "Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2008."— Presentation transcript:

1 Why Size Matters Adapted from Nanosense STEM ED/CHM Nanotechnology 2008

2 Relative sizes (review) Atomic nuclei ~ meters = nanometers Atoms ~ meters = 0.1 nanometers Nanoscale ~ 1 to 100 nanometers ~ 10 to 1000 atoms Everyday world ~ 1 meter = 10 9 nanometers

3 The Basic Physics At the everyday scale, Newtons 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

4 The Basic Forces Strong Nuclear Force – huge, hold nuclei together; act only at nuclear distances, nm Weak Nuclear Force – small, responsible for nuclear beta decay, act only at nuclear distances, nm Electric and Magnetic – dominant at atomic and nanotech scales; × gravitational forces; long ranged, 1/r 2 Gravitational – long ranged, 1/r 2 ; dominant at everyday world scale, since most objects lack a substantial net electrical charge

5 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 (~10 23 ) aggregations of atoms or molecules

6 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: 12 nanometer gold particles look red Bulk gold looks yellow

7 Traditional ZnO sunscreen is white Zinc oxide nanoparticles Nanoscale ZnO sunscreen is clear Sources: 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 dont scatter it –Appear clear Application to sunscreen

8 Source: 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

9 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 mv 2 ~ temperature Average particle speeds decrease as mass increases, so more massive particles diffuse more slowly

10 Sources: and image adapted from 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

11 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

12 Size Matters in Biology Metabolism (heat generation) is limited by the number of cells, or volume, L 3 Heat loss to the environment is proportional to the surface area, L 2 As we look at smaller and smaller organisms, the surface to volume ratio L 2 / L 3 = 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

13 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

14 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

15 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

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