Why does paper curl on burning?

The upper or flame side of the paper loses water faster compared with the lower side. This is because paper is not a good conductor of heat and hence both sides are not heated to the same extent by the flame. That only the upper side curls can be further tested by the turning the burning paper by 180 degrees. The curling reverses and does not continue to be in the direction of the side that is now the lower side.Another interesting way to check the direction of curling is by wetting only one side of the paper. The side of the paper that is dry will tend to curl.Paper tends to curl basically because the expansion of the fibers on both sides of the paper is different when it either brunt or wetted on one side only. In the case of the burning paper, the upper side loses water faster and hence the curling is seen on that side. In the case where the paper is wetted on one side only, the side that is dry tends to curl. Curling is not seen when both sides of the paper gets wet at the same time as when it is immersed in water. One can control the way direction of curling in a paper that is kept with neither sides being the upper or lower side by gently curling it in one direction. The curling is seen in the direction that is concave.The nature and extend of curling when the paper is burnt can be checked using papers of different thickness. The thinner the paper, the lesser the curling seen in one direction.

Source: thehindu.com

If increase of viscosity increases the stability of an emulsion, why is milk more stable and show less settling even though the viscosity is less?

An emulsion is a dispersion of one liquid in another immiscible one (for example, oil and water). One phase is called continuous and other is called disperse phase. Oil droplets are dispersed in water by fragmenting it in presence of a surface-active species such as surfactants or polymers, which provide stability. The diameter of the disperse phase varies from a few nanometres to several microns. Milk is a natural emulsion where each fat globule is stabilized by a membrane of phospholipids and proteins. Milk contains 88 per cent water, 3.3 per cent protein (casein 82 per cent ), 3.3 per cent fat , 4.7 per cent carbohydrate, 0.7 per cent ash. The percentage of disperse phase in milk is very low compared to some of the industrial products. Now coming to the answer to the question, what happens when the viscosity of the continuous phase (water) goes up? The settling velocity decreases due to viscous resistance and hence the settling time increases. Therefore, we may be right to say that the stability of an emulsion goes up with viscosity. However, the answer is not fully correct. The density mismatch between the two phases, the diameter of the dispersed droplets and the electrical charges of the surfactant molecules also play a crucial role in the stability of emulsions.

Source: thehindu.com

At times raindrops are bigger when it just begins to rain and sometimes smaller? What is the reason?

Rain happens when warm, moist air rapidly cools at high altitudes in the presence of surrounding colder air. At any given temperature, dry air can accommodate only a certain amount of water vapour. As the moist air temperature drops, due to cooling, it becomes super-saturated with water vapour. When air can accommodate no more water vapour under equilibrium conditions, it is saturated. Under super-saturated conditions, air holds more moisture than it possibly can under equilibrium conditions. Hence, there is the tendency of air to precipitate out any excess moisture present and it rains. Rain drops form due to condensation of water vapour around preferential sites such as microscopic dust particles, pollen grains etc. that are present in air. As the drop-wise condensation proceeds, microscopic droplets of water initially formed grow in size and the water droplet begins to fall under the action of gravity. While falling, the droplets can collide with other droplets and coalesce to form bigger droplets. Alternatively, droplets can collide with other droplets and break apart into smaller droplets due to the impact. There is another reason for the break-up of water drops. Due to the relative velocity between the falling water drop and the air around it, viscous shearing forces develop on the surface of the drop. These drag forces cause the drop to distort in shape. During its fall the waterdrop accelerates, and the magnitude of the drag forces increases roughly in proportion to the square of the fall speed of the drop. Due to shape distortions and the rapidly increasing effects of the shear stresses on the air-water interface of the drop, the drop may break apart. Large drops of water have a tendency to break apart more than the smaller ones. Due to these reasons growth of drop size due to coalescence, and break up due to collisions and viscous shear rain drops come in varying sizes, from the smallest to the largest, by the time they hit the ground.

Source: thehindu.com

Why is the nib of a fountain pen split?

A fountain pen allows a controlled flow of ink because of its specially designed flow system for ink and air. The flow system consists of an ink reservoir, a feed bar (also called feed) and a metal nib. 

The nib is split starting from a breather hole till the tip to form two tines (branches). The feed has a longitudinal flow channel on its top surface starting from the inside end almost up to the forward end and it is covered by the nib. The channel is a broad groove and along its floor are cut two or more narrower grooves (called fissures). The broad channel brings ink up to the breather hole and the split of the nib. The split carries the ink to the tip and paper. The channel allows an occasional flow of air in the opposite direction into the reservoir so that pressure there does not decrease too much due to emptying of the ink. When such an air-flow takes place the fissures still contain ink and enable immediate filling of the channel. The nib tip has a special shape consisting of two hemispherical ends at the bottom of the tines. Ink forms a meniscus between these two rounded parts and spreads on paper as soon as a contact is established. The split width will change if pressure is applied and this will change the flow. The feed often has slots, combs or other design elements so that the capillary action can draw ink and hold it to have a steady flow or to avoid an overflow. Even when a pen is not used for a while, capillarity replenishes through the split, the evaporated ink and the pen is kept ready. The capillary force that maintains a continuous ink flow also helps to support ink from dripping due to gravity while not writing.

Source: thehindu.com

Why do bats hang upside down from tree brances?

Bats hang upside down primarily to help them get in to flight ("take off") quickly. Unlike birds, they do not have strong wings that provide enough lift for taking off directly from the ground. Their hind legs are also not strong enough to run fast to get the necessary take off speed. When they hang upside down they can get in to flight right away just by releasing their grip. If a bat accidentally falls to the ground, it has to climb up to a tall spot such as a tree branch to take off. 

Bats also obtain additional advantages due to hanging upside down such as safe guarding themselves from predators. The unique ability to hang upside down from tree branches and on the inner roof of caves makes it difficult for predators to reach them. Newborn bat pups can cling on to the mother's body and get transported by her. Usually, they stay in the bat roost where several bats nurse the newborns. The muscle arrangement in their legs is such that when they hang, their claws are kept closed due to their body weight. This makes it easier for them to hang spending minimal energy. They have to use energy only to open the grip and fly. Their grip is so strong that if a bat dies while hanging, it will keep hanging until forcibly knocked off by another bat or something.

Source: thehindu.com

What makes a mushroom sprout out of soil soon after monsoon but not after sprinkling water?

A majority of mushrooms that sprout directly from soil during the monsoons belong to the Tricholomataceae family. Calocybe indica, a beautiful and white coloured umbrella shaped mushroom is the most prominent among them. This genus originated from the forests of West Bengal. Today, its variants are cultivated commercially under the popular name 'Milky mushroom'.

Mushroom spores are hyaline, broadly ellipsoidal, thin-walled with prominent apiculis. Under favourable conditions such as low temperature, high relative humidity and faded light, the spores germinate and the mycelial spread resumes. It takes nearly 50 days for the completion of the mycelial spread and subsequent emergence of the sporophores. During the entire period of mycelial spread, very high relative humidity, 25-30 degree Celsius temperature and faint light are necessary. Unlike sprinkling water, monsoon changes the entire microclimate. Sprinkling cannot reduce the atmospheric temperature, control sunshine or maintain high relative humidity for long time whereas the monsoon's prominent effects will be the high or stable relative humidity coupled with decrease in the intensity of sunshine, which are necessary for spawn spread. Mushroom beds, after inoculation with mushroom spawn, should be kept for 30 days under darkness with high atmospheric humidity. After the mycelial spread the plastic cover of the bed is removed, sterilised casing mixture composed of equal quantities of soil, sand and cowdung is loosely spread over the bed. Only faded light is permitted but high humidity is maintained. After about two weeks mushroom start sprouting.

Source: The Hindu

Does the permanent tooth form only after the milk tooth falls down or is it formed already?

The permanent tooth is not formed after the loss of primary tooth. Normally there are tooth buds in the developing foetus itself. These tooth buds, sometimes called the tooth germs, are an aggregation of cells, which eventually form the tooth. A whole lot of processes happen inside the tooth bud to form a tooth. This phenomenon is generally referred to as calcification and this calcification of primary teeth takes place between 13 and 16 months of postnatal life. There are ten sets of primary tooth buds in both the jaws. After complete calcification the primary dentition takes two to three years to emerge into the oral cavity.

The transition from primary to permanent teeth begins at about 6 years of age. The permanent tooth buds stay beneath the primary teeth. The calcification process starts in the permanent tooth too. For the permanent teeth there are 16 sets of tooth buds, each forming at different years of age. The crown portion of the permanent teeth starts to form and this crown portion is formed by enamel. There are four centres for formation of each tooth. All these centres coalesce to form a lobe, which forms the crown of the tooth. After the crown is formed, root formation begins. At the cervix of the crown, cemetum accumulates for the root to be formed. Once root development takes place in the permanent teeth underneath the primary teeth, this initiates resorption or eroding of the roots of the primary teeth by cells called osteoclasts. This resorption of primary teeth roots causes exfoliation of the primary teeth, which is normally referred to as tooth shake. Once the primary tooth falls off, the permanent tooth starts to emerge through the gingival or the gums. The first permanent tooth that erupts is the lower first molar at the age of 6 years. Gradually all primary teeth begin to exfoliate and permanent teeth emerge into the oral cavity.

Source: The Hindu

Why do raindrops falling on a windowpane not always run down straight?

A water drop on a vertical windowpane is pulled down by gravity and should run down straight if there is no other force. Adhesive forces between the glass and water keep a small drop stuck to the glass surface. There is also the surface tension acting at the free surface between the drop and air. Due to surface tension the free surface behaves like a stretched membrane and this gives shape to the drop. Similar surface energies are present between the windowpane and water and also between windowpane and air. These three surface energies fix the contact angle of the drop surface at the contact line. Contact angle, surface tension and gravity decide the shape of the drop. As a drop rolls down there is a hairpin shaped contact line on the windowpane. Most of the water is concentrated at the head with a tail of thin film between the two boundaries of the contact line. When the drop rolls down further there is an increase in the surface area. Creating more surface area needs additional work. If there is already a water-film or drop, this additional work is not needed and water takes this path of least resistance. If such a film downstream does not exist, the complex shape of the drop's head affects the direction of progress. This direction need not always be straight. Surface waviness, dust particles acting as barriers, impurities present, which locally change the contact angle and surface tension, thus modify the direction in which the water stream progresses. Path of raindrops on a windowpane is also affected by the wind.

Source: The Hindu

Why does a coin go in a zigzag fashion and not straight down when dropped in a bucket of water?

When a coin is dropped in a bucket of water, the free fall is resisted by friction or drag due to the water. This force consists of two parts: 
  1. Due to pressure acting perpendicular to the coin surface
  2. Due to viscous forces acting along the coin surface.
The first part is the pressure drag and the second part is called the skin friction drag. Note that the pressure acting on the coin is not just hydrostatic (which generates the Archimedes buoyancy force) but pressure modified by the motion of the coin itself. Skin friction also depends on the motion of the coin. It is usually smaller than the pressure drag unless the body under consideration is highly streamlined like an aircraft wing. Vortices are formed behind (lee side) the falling coin and they grow in size and get detached periodically. This formation and shedding of the vortices drastically influences the pressure distribution on the coin, especially on the leeward side. This may lead to a sudden change in the motion of the coin including its rotation and hence a zigzag motion. 

The skin friction drag is small on the coin but it affects the vortex shedding and hence the pressure distribution on the coin. It is easy to imagine how complicated this water motion can be. A coin dropped in air, say from a high-rise building, is subjected to similar motion but the frictional forces are smaller due to low air density. We observe similar motion when the leaves are falling from a tree, especially in the absence of wind.Even a coin released carefully in a bucket of water starting from a vertical position soon turns inclined and then tumbles. The laws governing the motion of the coin and the fluid (Newton's laws for a fluid are known as the Navier-Stokes equations) are known but it is difficult to solve them accurately even using a supercomputer to predict the exact motion of this coin. But it is fun to release a coin gently in a bucket of water and try to hit a particular spot at the bottom.

Source: The Hindu

Why does a candle flame take a teardrop shape?

The flame in a candle is caused by the burning of the wax, a process that liberates a large amount of energy in the form of heat. This heat, in turn, excites the molecules and atoms in the air and the carbonaceous combustion products of the burnt wax. These excited atoms and molecules get de-excited and emit the light we see. The flame is, thus, a collection of highly heated gas atoms and molecules, which having lower density than the surrounding air, lifts itself up. During this, it goes farther from the source of heat and gets cooled by transferring heat to the surrounding air, which rushes from the neighbourhood. As the height increases, the cool air exerts a transverse pressure on the flame from all sides making it teardrop-shaped. 

The progressively increased effective cooling of the flame at higher levels from the tip of the candle can be demonstrated by a simple experiment. Take a flat-bottomed plate with some ordinary water and hold it for a short while at about half the length of the original flame. Take care not to hold the plate for too long. You observe that the central portion has no mark but the outer regions have the black mark of the carbon soot indicating the cooler gas being at the periphery of the flame compared with the inner region. The hotter gas in the inner region converts all the carbon into CO{-2} but not at the periphery. Hence black soot is formed. Next, hold the plate above the flame and you will collect a smaller soot mark without the clear middle region. This is because the hot gas has got cooled by the time it has reached the top and the carbon in the gas escapes conversion into CO{-2}. The teardrop shape of the flame is mainly because of the transverse air movement around the flame.On Earth, gravity-driven buoyant convection causes the teardrop shape. In microgravity, where convective flows are absent, the flame is spherical.

Source: The Hindu