ily ▁systems , ▁like ▁when ▁particles ▁are ▁depos ited ▁in ▁the ▁nas al ▁cav ity ▁by ▁in hal ation . ▁The ▁principles ▁of ▁initial ▁ad s or ption ▁of ▁small ▁mole c ule ▁ad sob ates ▁can ▁be ▁described ▁by ▁D L VO ▁theory . ▁ ▁Ad s or ption ▁theory ▁ ▁According ▁to ▁D L VO ▁theory , ▁particles ▁are ▁held ▁in ▁susp ension ▁by ▁a ▁balance ▁of ▁attract ive ▁and ▁rep uls ive ▁forces . ▁This ▁theory ▁can ▁be ▁applied ▁to ▁the ▁ad s or ption ▁of ▁small ▁mole cules ▁like ▁mu co ad hes ive ▁polym ers , ▁on ▁surfaces , ▁like ▁m uc us ▁layers . ▁Part icles ▁in ▁general ▁experience ▁attract ive ▁van ▁der ▁Wa als ▁forces ▁that ▁promote ▁co ag ulation ; ▁in ▁the ▁context ▁of ▁ad s or ption , ▁the ▁particle ▁and ▁m uc us ▁layers ▁are ▁naturally ▁attract ed . ▁The ▁attract ive ▁forces ▁between ▁particles ▁increases ▁with ▁decre asing ▁particle ▁size ▁due ▁to ▁increasing ▁surface - area - to - volume ▁ratio . ▁This ▁increases ▁the ▁strength ▁of ▁van ▁der ▁Wa als ▁interactions , ▁so ▁smaller ▁particles ▁should ▁be ▁easier ▁to ▁ad s orb ▁onto ▁m uc ous ▁memb ran es . ▁ ▁D L VO ▁theory ▁also ▁explains ▁some ▁of ▁the ▁challeng es ▁in ▁establish ing ▁contact ▁between ▁particles ▁and ▁m uc us ▁layers ▁in ▁mu co ad hes ion ▁due ▁to ▁their ▁rep uls ive ▁forces . ▁Sur faces ▁will ▁develop ▁an ▁elect rical ▁double ▁layer ▁if ▁they ▁are ▁in ▁a